Physical Geography of the Sea and its Meteorology/Chapter 9

by Matthew Fontaine Maury

CHAPTER IX.

§ 420-460.—THE SPECIFIC GRAVITY OF THE SEA, AND THE OPEN WATER IN THE ARCTIC OCEAN.

420. Interesting physical inquiries.—The crust of the planet upon which we live, with the forces that have been and are at work upon it, is the most interesting subject of physical inquiry and study that can claim the attention of diligent students. Precisely as the progress of man has been upward and onward, precisely has he looked more earnestly and with deeper longings towards the mysteries that encircle this crust. It is but a shell, and at most we can reach only a little way either above or below its very surface, and jet upon the tablets of this thin shell are the records of all that he may ever know concerning this his cosmical hearthstone.

421. Voyages of discovery to the North Pole.—Researches have been carried on from the bottom of the deepest pit to the top of the highest mountain, but these have not satisfied. Voyages of discovery, with their fascinations and their charms, have led many a noble champion of human progress both into the torrid and frigid zones; and notwithstanding the hardships, sufferings, and disasters to which many northern parties have found themselves exposed, seafaring men, as science has advanced, have looked with deeper and deeper longings towards the mystic circles of the polar regions: there icebergs are framed and glaciers launched: there the tides have their cradle, the whales their nursery: there the winds complete their circuits, and the currents of the sea their round in the wonderful system of oceanic circulation: there the Aurora Borealis is lighted up and the trembling needle brought to rest; and there too, in the mazes of that mystic circle, terrestrial forces of occult power and of vast influence upon the well-being of man are continually at play. Within the arctic circle is the pole of the winds and the poles of the cold; the pole of the earth and of the magnet. It is a circle of mysteries; and the desire to enter it, to explore its untrodden wastes and secret chambers, and to study its physical aspects, has grown into a longing. Noble daring has made arctic ice and snow-clad seas classic ground. It is no feverish excitement nor vain ambition that leads man there. It is a higher feeling, a holier motive—a desire to look into the works of creation, to comprehend the economy of our planet, and to grow wiser and better by the knowledge. Soon after the discovery of America, John Cabot and his sons, with five ships, sailed upon the first arctic expedition. Between that year and the present no less than 155 vessels, besides boat and land parties, have at various periods, and with divers objects in view, been sent from Europe and America, up into those inhospitable regions. Whatever may have been the immediate object of these various expeditions, whether to enlarge the fields of commerce, to carry the Bible, to spread civilization, to push conquests, or to bring back contributions to science, they have never lost sight of the promise made by Columbus of a western route to India.

422. The first suggestions of an open sea in the Arctic Ocean.—Like the air, like the body, the ocean must have a system of circulation for its waters. No other hypothesis will explain the fact which observations reveal concerning the saltness of the sea, the constituents of sea-water, and many other phenomena. An attentive study of the currents of the sea, and a close examination of the laws which govern the movements of the waters in their channels of circulation through the ocean, will lead any one irresistibly to the conclusion that always, in summer and winter, there must be, somewhere within the arctic circle, a large body of open water. This open water must impress a curious feature upon the physical aspects of those regions. The whales had taught us to suspect the existence of open water in the arctic basin, and in their mute way told of a passage there, at least sometimes. It is the custom among whalers to have their harpoons marked with date and the name of the ship; and Dr. Scoresby, in his work on arctic voyages, mentions several instances of whales that have been taken near the Behring's Strait side with harpoons in them bearing the stamp of ships that were known to cruise on the Baffin's Bay side of the American continent; and as, in one or two instances, a very short time had elapsed between the date of capture in the Pacific and the date when the fish must have been struck on the Atlantic side, it was argued therefore that there was a north-west passage by which the whales passed from one side to the other, since the stricken animal could not have had the harpoon in him long enough to admit of a passage—even if that were possible—around either Cape Horn or the Cape of Good Hope.

423. Harpoons—habits of the whales.—The whale-fishing is, among the industrial pursuits of the sea, one of no little importance; and when the system of investigation out of which the "Wind and Current Charts" have grown was commenced, the haunts of this animal did not escape attentive examination. The log-books of whalers were collected in great numbers, and patiently examined, co-ordinated, and discussed, in order to find out what parts of the ocean are frequented by this kind of whale, what parts by that, and what parts by neither. (See Plate IX.) Log-books containing the records by different ships for hundreds of thousands of days were examined, and the observations in them co-ordinated for this chart. And this investigation, as Plate IX. shows, led to the discovery that the tropical regions of the ocean arc to the right whale as a sea of fire, through which he cannot pass, and into which he never enters. The fact was also brought out that the same kind of whale that is found off the shores of Greenland, in Baffin's Bay, &c., is found also in the North Pacific, and about Behring's Strait, and that the right whale of the northern hemisphere is a different animal from that of the southern. Thus the fact was established that the harpooned whales did not pass around Cape Horn or the Capo of Good Hope, for they were of the class that could not cross the equator. In this way we were furnished with circumstantial evidence affording the most irrefutable proof that there is, at times at least, open-water communication through the Arctic Sea from one side of the continent to the other, for it is known that the whales cannot travel under the ice for such a great distance as is that from one side of this continent to the other. But this did not prove the existence of an open sea there; it only established the existence—the occasional existence, if you please—of a channel through which whales had passed. Therefore we felt bound to introduce other evidence before we could expect the reader to admit our proof, and to believe with us in the existence of an open sea in the Arctic Ocean.

424. The under current into the Arctic Ocean—its influences.—There is an under current setting from the Atlantic through Davis' Strait into the Arctic Ocean, and there is a surface current setting out. Observations have pointed out the existence of this under current there, for navigators tell of immense icebergs which they have seen drifting rapidly to the north, and against a strong surface current. These icebergs were high above the water, and their depth below, supposing them to be parallelepipeds, was at least seven times greater than their height above. No doubt they were drifted by a powerful under current. Now this under current comes from the south, where it is warm, and the temperature of its waters is perhaps not below 30°; at any rate, they are comparatively warm. There must be a place somewhere in the arctic seas where this under current ceases to flow north, and begins to flow south as a surface current; for the surface current, though its waters are mixed with the fresh waters of the rivers and of precipitation in the polar basin, nevertheless bears out vast quantities of salt, which is furnished neither by the rivers nor the rains. These salts are supplied by the under current; for as much salt as one current brings in, other currents must take out, else the polar basin would become a basin of salt; and where the under current transfers its waters to the surface, there is, it is supposed, a basin in which the waters, as they rise to the surface, are at 30°, or whatever be the temperature of the under current, which we know must be above the freezing-point, for the current is of water in a fluid, not in a solid state. An arrangement in nature, by which a basin of considerable area in the frozen ocean could be supplied by water coming in at the bottom and rising up at the top, with a temperature not below 30°, or even 27°.2—the freezing-point of sea water—would go far to mitigate the climate in the regions round about.

425. Indications of a milder climate.—And that there is a warmer climate somewhere in that inhospitable sea, the observations of many of the explorers who have visited it indicate. Its existence may be inferred also from the well-known fact that the birds and animals are found at certain seasons migrating to the north, evidently in search of milder climates. The instincts of these dumb creatures are unerring, and we can imagine no mitigation of the climate in that direction, unless it arise from the proximity, or the presence there of a large body of open water. It is another furnace (§ 151) in the beautiful economy of Nature for tempering climates there.

426. How the littoral waters, by being diluted from the rivers and the rains, serve as a mantle for the Salter and warmer sea water below.—The hydrographic basin of the Arctic Ocean is large, and it delivers into that sea annually a very copious drainage. Such an immense volume of fresh water discharged into so small a sea as the Arctic Ocean is, must go far towards diluting its brine. Fig. 2, Plate X. (§ 433), shows the extent to which the brine of our littoral seas is diluted by the drainage from the Atlantic slopes of the United States. It will be observed by that figure that suddenly after crossing the parallel of 34° N. the water begins to grow cooler and lighter. The observations for the two curves are a part of the celebrated series made by Captain Rodgers in the U.S. ship "Vincennes" all the way from Behring's Straits by the way of Cape Horn to New York. He cleared the inner edge of the Gulf Stream in 34°, where the waters began to grow cooler and lighter, and so continued to do as he approached the shore. The remarkable and sudden approach of the thermal and specific gravity curves after crossing 34° N. can be explained by no other hypothesis than this, viz.: the surface water of the sea was so diluted with the fresh water from the Chesapeake, the Delaware, and New York Bays, that, notwithstanding the temperature decreased as Rodgers approached the shore, yet the specific gravity decreased also, because the saltness decreased by reason of the increasing proportion of river water as he neared the shore. And thus we have in our own waters an illustration and an example of how water that is cool and light—because not so salt—may be made to cover and protect as with a mantle, a sheet of warmer, but Salter and heavier water below.

427. An under current of warm but salt and heavy water.—The mean specific gravity of the Arctic Ocean water as observed by Rodgers, and reduced to the freezing-point (27°.2) of sea water, was 1.0263. The specific gravity of the Gulf Stream water, as observed by him, and reduced to the same temperature (27°.2), was 1.0303. If these be taken as fair specimens of the water of the torrid and frigid zones, it would appear that the waters of intertropical seas have 15 per cent, more salt in them than the surface water of the Arctic Ocean has. It is to be regretted that the hydrometer has not been more frequently used in the Arctic Ocean, for a careful series of observations upon the specific gravity of the water there at the surface and at various depths would indicate to us not only the extent to which the water there is diluted by the rivers and the rains, but it would yield other highly interesting results. Now this salt and heavy water, whose specific gravity at 27°.2 would have been 1.0303, is the very water which Rodgers observed in the Gulf Stream on its way to the arctic regions. This is the water which, after passing the Grand Banks and meeting the diluted water as an ice-bearing current from the north, dips down, but continues its course as an under current. It is protected from farther loss of heat, after the manner of our own littoral waters, by the colder but lighter and upper current from the north, until it enters the Arctic Ocean, there to rise up like a boiling spring in the centre of an open sea.

428. De Havens water sky.—Relying upon a process of reasoning like this, and the deductions flowing therefrom. Lieutenant De Haven, when he went in command of the American expedition in search of Sir John Franklin and his companions, was told in his letter of instructions, to look, when he should get well up into Wellington Channel, for an open sea to the northward and westward. He looked, and saw in that direction a "water sky." Captain Penny afterwards went there, found open water, and sailed upon it. The open sea in the Arctic Ocean is probably not always in the same place, as the Gulf Stream (§ 126) is not always in one place. It probably is always where the waters of the under currents are brought to the surface; and this, we may imagine, would depend upon the freedom of ingress and egress for the currents. Their course may perhaps be modified more or less by the ice on the surface, by changes, from whatever cause, in the course or velocity of the surface current, for obviously the under current could not bring more water into the frozen ocean than the surface current would carry out again, either as ice or water. Exploring parties may have been near this open sea without perceiving the warmth of its climate, for every winter, an example of how very close warm water in the sea and a very severe climate on the land or the ice may be to each other is afforded to us in the case of the Gulf Stream and the Labradorlike climate of New England, Nova Scotia, and Newfoundland. In these countries, in winter, the thermometer frequently sinks far below zero, notwithstanding that the tepid waters of the Gulf Stream may be found with their summer temperature within one day's sail of these very, very cold places.

429. Dr. Kane.—Dr. Kane reports an open sea north of the parallel of 82°. To reach it, his party crossed a barrier of ice 80 or 100 miles broad. Before gaining this open water, he found the thermometer to show the extreme temperature of—60°. Passing this ice-bound region by travelling north, he stood on the shores of an iceless sea, extending in an unbroken sheet of water as far as the eye could reach towards the pole. Its waves were dashing on the beach with the swell of a boundless ocean. The tides ebbed and flowed in it, and I apprehend that the tidal wave from the Atlantic can no more pass under this icy barrier to be propagated in the seas beyond, than the vibrations of a musical string can pass with its notes a fret upon which the musician has placed his finger. The swell of the sea cannot pass wide fields or extensive barriers of ice; for De Haven, during his long imprisonment and drift (§ 475), found the ice so firm that he observed regularly from an artificial horizon placed upon it, and found the mercury always "perfectly steady." These tides, therefore must have been born in that cold sea, having their cradle about the North Pole. If these statements and deductions be correct, then we infer that most, if not all the unexplored regions about the pole are covered with deep water; for, were this unexplored area mostly land or shallow water, it could not give birth to regular tides. Indeed, the existence of these tides, with the immense flow and drift which annually take place from the polar seas into the Atlantic, suggests many conjectures concerning the condition of these unexplored regions. Whale-men have always been puzzled as to the place of breeding for the right whale. It is a cold-water animal, and, following up this train of thought, the question is prompted, Is not the nursery for the great whale in this polar sea, which has been so set about and hemmed in with a hedge of ice that man may not trespass there? This providential economy is still farther suggestive, prompting us to ask, Whence comes the food for the young whales there? Do the teeming waters of the Gulf Stream (§ 160) convey it there also, and in channels so far down in the depths of the sea that no enemy may waylay and spoil it on the long journey? Seals were sporting and water-fowl feeding in this open sea of Dr. Kane's. Its waves came rolling in at his feet, and dashing with measured tread, like the majestic billows of old ocean, against the shore. Solitude, the cold and boundless expanse, and the mysterious heavings of its green waters, lend their charm to the scene. They suggested fancied myths, and kindled the ardent imagination of the daring mariner's many longings. The temperature of its waters was only 36°! Such warm water could get there from the south only as a current far down in the depths below. The bottom of the ice of this eighty miles of barrier was no doubt many—perhaps hundreds of—feet below the surface level. Under this ice there was also doubtless water above the freezing-point.

430. Under currents change temperature slowly.—Now need the presence of warm water within the arctic circle excite suprise, when we recollect that the cold waters of the frigid zone are transferred to the torrid without changing their temperature perhaps more than 7° or 8° by the way. This transfer of cold waters for a part of the way may take place on the surface, and until the polar flow (§ 89) dips down and becomes submarine. At any rate, officers on the Coast Survey have found water at the bottom of the Gulf Stream, in latitude 25° 30' X., as low in temperature as 35°. Now, if water flowing out of the polar basin at the temperature of 28° may, by passing along the secret paths of the sea, reach the Gulf of Mexico in summer at a temperature of only 3° above the freezing-point of fresh water, why may not water leaving the torrid zone at a temperature of 82°, and travelling by the same hidden ways, reach the frigid zone without losing more than the cold currents gained in temperature, viz., 7°? In 1840, Sir James C. Ross, being in the antarctic regions with the surface water at 32°, found the temperature in depth to be 38°.8 at 400 fathoms, and 39°.8 at 600. At a greater depth there is a greater pressure; and there ought to be (§ 404) a certain temperature, that after passing a certain depth in the deep sea grows higher and higher as the depth increases. The thermal laws of "deep-sea" temperatures for fresh and for salt water are very different. In September, when the surface water of Loch Lomond and Loch Katrine—Scottish lakes—which are between 500 and 600 feet deep, is 58°, that at the bottom is uniformly 41°, which is very near the point of maximum density for fresh water. Saussure has shown the same for the Italian lakes: only, at the depth of 1000 feet in the Lake of Geneva, it was a little warmer, probably on account of pressure (§ 404), than it was at less depth in Lakes Lucerne and Thun. In these it was 41°, or 1° colder than the bottom of Geneva, their surface water being about 60°. In Lago Sabatino, near Rome, with the surface water at 77°, Barlocci reports.44° at the depth of 490 feet. The winter in Rome is not severe enough to cool such a mass of water below 44°. But with the exception of the Lake of Geneva, which is deep enough to have the temperature of its water somewhat influenced by pressure (§ 404), the law is uniform: as you descend in fresh-water lakes, the temperature decreases to that of maximum density. Saussure extended his experiments to the Gulfs of Nice and Genoa—salt-water bays in the neighbourhood of his fresh-water lakes. Here, with the surface temperature of 69°, he found even at the depth of 1720 feet, the water no cooler than 55°. 8. This salt water might have been cooled 30° lower before it would have reached the maximum density (25°. 6) of average sea water. We see that the severest winters are not sufficient to bridge our deep fresh-water lakes over with ice, though their waters being cooled below 39°. 5, grow light, and remain on the surface to be frozen. On the contrary, sea water contracts, grows heavy, and sinks, until the whole basin, from the bottom to the top, be reduced to 27°. 2. Yet many confess no surprise at the open water in fresh-water lakes that are comparatively shallow, while they can conceive of no such thing in the Arctic Ocean, though it be very much deeper than the deepest fresh-water lakes!

431. Solid matter annually drifted out of the polar basin.—At the very time that the doctor was gazing with longing eyes upon these strange green waters (§ 429), there is known to have been a powerful drift setting out from another part of this Polar Sea, and carrying with it from her moorings the English exploring ship "Resolute," which her officers and men had abandoned fast bound in the ice several winters before. This drift carried a field of ice that covered an area not less than 300,000 square miles, through a distance of a thousand miles to the south. The drift of this ship was a repetition of De Haven's celebrated drift (§ 474); for in each case the ice in which the vessel was fastened floated out and carried the vessel along with it; by which I mean to be understood as wishing to convey the idea that the vessel was not drifted through a line or an opening in the ice, but, remaining fast in the ice, she was carried along with the whole icy field or waste. This at least was the case with De Haven, A field of ice covering to the depth of seven feet an area of 300,000 square miles, would weigh not less than 18,000,000,000 tons. This, then, is the quantity of solid matter that is drifted out of the polar seas through one opening—Davis' Straits—alone, and during a part of the year only. The quantity of water which was required to float and drive this solid matter out was probably many times greater than this. A quantity of water equal in weight to these two masses had to go in. The basin to receive these inflowing waters, i. e., the unexplored basin about the North Pole, includes an area of a million and a half square miles; and as the outflowing ice and water are at the surface, the return current must be submarine. A part of the water that it bears probably flows in beneath Dr. Kane's barrier of ice (§ 429).

432. Volume of water kept in motion by the arctic flow and reflow.—These two currents therefore, it may be perceived, keep in motion between the temperate and polar regions of the earth a volume of water, in comparison with which the mighty Mississippi, in its greatest floods, sinks down to a mere rill. On the borders of this ice-bound sea Dr. Kane found subsistence for his party—another proof of the high temperature and comparative mildness of its climate.

433. The hydrometer at sea.—The Brussels Conference recommended the systematic use of the hydrometer at sea. Captain Pledgers, Lieutenant Porter, and Dr. Ruschenberger, all of the United States Navy, with Dr. Raymond, in the American steamer "Golden Age," and Captain Toynbee, of the English East Indiaman the "Gloriana," have each returned to me valuable observations with this instrument. Rodgers, however, has afforded the most extended series. It embraces 128° of latitude, extending from 71° in one hemisphere to 57° in the other. And here I beg to remark that those navigators who use the hydrometer systematically and carefully at sea are quietly enlarging for us the bounds of knowledge; and they are gleaning in our field of research. These observations have already led to the discovery of new and beneficent relations in the workshops of the sea. In the physical machinery of the universe there is no compensation to be found that is more exquisite or beautiful than that which, by means of this little instrument, has been discovered in the sea between its salts, the air, and the sun. The observations made with it by Captain Rodgers, on board the U. S. ship "Vincennes," have shown that the specific gravity of sea water varies but little in the trade-wind regions, notwithstanding the change of temperature. The temperature was a little greater in the south-east trade-wind region of the Pacific; less in the Atlantic. But, though the sea at the equatorial borders of the trade-wind belt is some 20° or 25° warmer than it is on the polar edge, yet the specific gravity of its waters at the two places in the Atlantic differs but little. Though the temperature of the water was noted, his observations on its specific gravity have not been corrected for temperature. The object which the Brussels Conference had in view when the specific gravity column was introduced into the sea-journal was, that hydrographers might find in it data for computing the dynamical force which the sea derives for its currents from the difference in the specific gravity of its waters in different climes. The Conference held, and rightly held, that a given difference as to specific gravity between the water in one part of the sea and the water in another would give rise to certain currents, and that the set and strength of these currents would be the same, whether such difference of specific gravity arose from difference of temperature or difference of saltness, or both.

434. Specific gravity of average sea water.—According to Rodgers' observations, the average specific gravity of sea water, as it is taken from the sea on the parallels of 34° north and south, at a mean temperature of 64°, is just what, according to saline and thermal laws, it ought to be; but its specific gravity when taken from the equator, at a mean temperature of 81°, is much greater than, according to the same laws, it ought to be. The observed difference of its specific gravity at 64° and 81° is .0015; whereas it ought to be .0025. Now as we approach the equator, the water is warmer, and it should therefore, were it of equal saltness, be proportionably lighter; but instead of the specific gravity of equatorial water being .0025 lighter—as by thermal laws it ought to be—than sea water at the temperature of 01° in latitude 34°, it is only .0015. What makes the equatorial water of the sea so much heavier than according to thermal laws it ought to be? Let us inquire:

435. An anomaly.—The anomaly is in the trade-wind region, and is best developed (Plate X., Fig. 2) in the North Atlantic, between the parallel of 40° and the equator. Though it is sufficiently apparent both in the North and South Pacific {Fig. 1)—it is masked by the Gulf Stream in the North Atlantic—commencing at the polar borders of these winds, the anomaly is developed as you approach the equator. The water grows warmer, but not proportionably lighter: this is in the trade-wind region. These winds evaporate as they go; but can it be possible that they are so regulated and adjusted, counterpoised and balanced, that the salt which they, by evaporation, leave behind, is just sufficient to counterbalance the dilatation due to the increasing warmth of the sea?

436. Influence of the trade-winds upon the specific gravity of sea water.—It is the trade-winds, then, which prevent the thermal and specific gravity curves from conforming with each other in intertropical seas. The water they suck up is fresh water, and the salt it contained, being left behind, is just sufficient to counter-balance, by its weight, the effect of thermal dilatation upon the specific gravity of sea water between the parallels of 34° north and south. As we go from 34° to the equator, the water grows warm and expands. It would become lighter, but the trade-winds, by taking up vapour without salt, make the water Salter, and therefore heavier. The conclusion is, the proportion of salt in sea water, its expansibility between 62° and 82° (for its thermal dilatability varies with its temperature), and the thirst of the trade-winds for vapour are, where they blow, so balanced as to produce perfect compensation; and a more beautiful compensation cannot, it appears to me, be found in the mechanism of the universe than that which we have here stumbled upon. It is a triple adjustment: the power of the sun to expand, the power of the winds to evaporate, and the quantity of salts in the sea—these are so proportioned and adjusted that when both the wind and the sun have each played with its forces upon the intertropical waters of the ocean, the residuum of heat and of salt should be just such as to balance each other in their effects, and so the aqueous equilibrium of the torrid zone is preserved.

437. Compensating influences.—Nor are these the only adjustments effected by this exquisite combination of compensations. If all the intertropical heat of the sun were to pass into the seas upon which it falls, simply raising the temperature of their waters, it would create a thermo-dynamical force in the ocean capable of transporting water scalding hot from the torrid zone, and spreading it, while still in the tepid state, around the poles. The annual evaporation from the trade-wind region of the ocean has been computed, according to the most reliable observation, to be as much as 15 feet, which is at the rate of half an inch per day. The heat required for this evaporation would raise from the normal temperature of intertropical seas to the boiling-point a layer of water covering the entire ocean to the depth of more than 100 feet. Such increase of temperature, by the consequent change which it would produce upon the specific gravity of the sea, would still further augment its dynamical power, until, even in the Atlantic, there would be force enough to put in motion and feed with boiling-hot water many Gulf Streams. But the trade-winds and the seas are so adjusted that this heat, instead of penetrating into the depths of the ocean to raise inordinately the temperature of its waters, is sent off by radiation or taken up by the vapour, or borne away by under currents, or carried off by the winds, and dispensed by the clouds in the upper air of distant lands. Nor does this exquisite system of checks and balances, compensations and adjustments, end here. In equatorial seas the waters are dark blue, in extra- tropical they are green. This difference of colour bears upon their heat-absorbing properties,^[1] and it comes in as a make-weight in the system of oceanic climatology, circulation, and stability. Now, suppose there were no trade-winds to evaporate and to counteract the dynamical force of the sun; this hot and light water, by becoming hotter and lighter, would flow off in currents with almost mill-tail velocity, towards the poles, covering the intervening sea with a mantle of warmth as with a garment. The cool and heavy water of the polar basin, coming out as under currents, would flow equatorially with equal velocity. How much, if to any extent, the former warm climates of the British. Islands and Northern Asia may be due to such a warm covering of the sea, may perhaps, at some future time, be considered worthy of special inquiry. We have already seen (§ 434) that there is something else besides temperature that is at work in effecting changes in the specific gravity of sea water. Whatever increases or diminishes its saltness, increases or diminishes its specific gravity; and the agents that are at work in the sea doing this are sea shells, the rivers, and the rains, as well as the winds. Between 35° or 40° and the equator evaporation is in excess of precipitation; at any rate, there is but little precipitation except under the equatorial cloud-ring (see Storm and Rain Chart, Plate XIII.); and though, as we approach the equator on either side from these parallels, the solar ray warms and expands the surface water of the sea, the winds, by the vapour they carry off and the salt they leave behind, prevent it from making that water lighter.

438. Nicely adjusted.—Thus two antagonistic forces are unmasked, and, being unmasked, we discover in them a most exquisite adjustment—a compensation—by which the dynamical forces that reside in the sunbeam and the trade-wind are made to counterbalance each other; by which the climates of intertropical seas are regulated; and by which the set, force, and volume of oceanic currents are measured. This compensation is most beautiful; it explains the paradox (§ 434), gives volume to the harmonies of the sea, and makes them louder in their song of Almighty praise than the noise of many waters. Philosophers have admired the relations between the size of the earth, the force of gravity, and the strength of fibre in the flower-stalks of plants (§ 303), but how much more exquisite is the system of counterpoises and adjustments here presented between the sea and its salts, the winds and the heat of the sun! The capacity of the sun to warm, of the sea water to expand, the quantity of salts these contain, and the power of the wind to suck up vapour, are all in such nice adjustment the one with the other, that there is the most perfect compensation. By it they make music in the sea, and the harmony that comes pealing thence, though not of so lofty a strain, is nevertheless, like the songs of the stars, divine.

439. A thermal tide.—Suppose there were no winds to suck up fresh water from the brine of the ocean; that its average depth were 3000 fathoms; that the solar ray were endowed with power to penetrate with its heat from the top to the bottom; and that. from bottom to top, the seas of each hemisphere, in thermal alternation with the seasons, were raised to summer heat and lowered to winter temperature: the change of sea level from summer to winter, and from winter to summer, in one hemisphere, would, from this cause alone, be upwards of 125 feet; and in its rise and fall we should have, from pole to pole, the ebb and flow of a great thermal tide that would turn with the sun in the ecliptic, and tell the equinoxes by the march on the tide staff of its rising and falling waters. But difference of level would not be all that would give strength and volume to this tide; difference of specific gravity would lend its weight as so much dynamical force, which difference would create an upper and under annual tide from one hemisphere to the other. This double disturbance of equilibrium would not give rise to a tidal wave—not mere motion without translation—but to a tidal flow and reflow of water from one hemisphere to the other in volumes of vast magnitude, power, and majesty. This is an exaggerated view of the dynamical force of the sunbeam; but it is presented to show the origin of the thermal tide shown on Plate IV. The difference between the actual and the supposed thermal tides is one of degree merely; for the sea water that is liable to any considerable change of temperature, instead of reaching from the bottom to the top, is scarcely more than a "pellicle" to the ocean. Nevertheless, there is a regular periodical flow and reflow between the poles and the equator. It is the annual ebb of this tide which fills the upper half of the North Atlantic with icebergs every spring and summer. The heated portion forms a stratum or layer which is thickest at the equator, and which comes to the surface near the polar edge of the temperate zones; it then dips again as it recedes towards the region of perpetual winter.

440. The isothermal floor of the ocean.—The observations of Kotzebue, Admiral Beechey, and Sir James C. Ross first suggested the existence in the ocean of this isothermal floor. Its temperature, according to Kotzebue, is 36°. The depth of this bed of water of invariable and uniform temperature is 1200 fathoms at the equator. It gradually rises thence to the parallel of about 50 N. and S., when it crops out, and there the temperature of the sea, from top to bottom, is conjectured to be permanently at 36°. The place of this outcrop, no doubt, shifts with the seasons, vibrating up and down, i.e., north and south, after the manner of the calm belts. Proceeding, in our description, onward to the frigid zones, this aqueous stratum of an unchanging temperature dips again, and continues to incline till it reaches the poles at the depth of 750 fathoms. So that on the equatorial side of the outcrop the water above this floor is the warmer, but on the polar side the supernatant water is the colder. By this floor with its waters of one uniform and permanent temperature, "the ocean," says Sir John Herschel, "is divided into three great regions—two polar basins in which the surface temperature is below, and one medial zone in which it is above 39°. 5,^[2] being 80° at the equator; and at the poles, of course, the freezing-point of sea water. It will be very readily understood that in this statement there is nothing repugnant to hydrostatical laws, the compressibility of water insuring an increase of density in descending within much wider limits of temperature than here contemplated."

441. Thermal dilatation of the water.221 The temperature of 39°.5 was assigned to this floor probably under the supposition that sea water follows fresh in its laws of thermal dilatation. Not so; while fresh water attains its maximum density at 39°.5, average sea water does not arrive at its degree of maximum density until it passes its freezing-point (27°.2) and reaches the temperature of 25°.6. In the winter of 1858 a very elaborate series of observations was conducted at the National Observatory, by Professor Hubbard, upon the thermal dilatation of sea water, and with the following results, 60° being the standard temperature :

Thermal Dilatation of Sea Water^[3]

Temp.	Dilatation,	Temp.	Dilatation.	Temp.	Dilatation.	Temp.	Dilatation.
o		o		o		o
22	0.99807	32	0.99795	50	0.99895	110	0950
23	801	33	797	55	943	120	1218
24	798	34	800	60	1.00000	130	1506
25	795	35	803	65	067	140	1804
26	793	36	806	70	142	150	2118
27	792	37	810	75	221	160	2460
28	791	38	814	80	309	170	2823
29	791	39	819	85	402	180	3192
30	792	40	.99823	90	503	190	3588
31	793	45	0.99856	100	0716	200	1.03993

442. Experiments on the freezing-point.—The dilatation of the glass tube is included in this table. To determine the freezing-point of average sea water I filled a glass jar 18 inches high, and 3 inches in diameter, with specimens of average sea water obtained in mid-ocean and near the equator. On the 12th of February, 1858, the thermometer in the shade being 23°, I exposed this jar of water, with a standard thermometer immersed, to the out-door temperature. When the thermometer in the jar reached 27°, small crystals of ice, like macles of snow, were observed to form near the bottom, to rise, and to increase as they rose. In truth, the phenomenon presented most beautifully in miniature a snow-storm reversed, for the flakes appeared literally to "fall upward;" and while it was "snowing up "in the jar, covering the top with ice, the water in it rose in temperature from 27°.2 to 28°, thus showing the maximum density of the water to be not above 27°.2. As soon, and invariably as soon, as the first crystals of ice began to appear, the water immediately rose to 28°, and there remained as long as the process of congelation was going on. In some instances the water was brought down, as in a confined vessel, to 18° before freezing; but as soon as freezing commenced, the thermometer would mount up to 28°. The same water was used for the following series of observations upon the thermal changes of the specific gravity of sea water, fresh water being the unit:

Temperature	27°.1	Spec. Grav.	1.0290	Temprature	38°.0	Spec. grav	1.0287
"	28°.3	"	89	"	43°.5	"	86
"	28°.8	"	91	"	54°.7	"	775
"	29°.0	"	885	"	55°.5	"	77
"	29°.5	"	906	"	62°.5	"	69
"	30°.0	"	885	"	63°.5	"	675
"	32°.0	"	88	"	64°.5	"	665
"	34°.0	"	88	"	8O°.5	"	43
"	34°.4	"	89	"	88°.3	"	30
"	35°.2	"	89	"	"[4]93°.3	"	1.0221

443. Sea water at summer more expansible than sea water at winter temperature.—All these experiments unite in showing that sea water at equatorial temperatures is many times more expansible than sea water at polar temperatures; that is, sea water, according to its rate of dilatation (§ 441), will expand about seventeen times as much for 5°, when its temperature is raised from 85°, as it will when raised from 28°; and yet, according to Plate X., the curves of temperature and specific gravity are symmetrical in.polar, non-symmetrical in equatorial seas. These. experiments, and the compressibility of sea water (§ 404), show that we have not yet data sufficient to establish the depth, or even the existence of such an isothermal floor all the way from pole to pole.

444. Data for Plate X.—"The physical consequences of this great law, should it be found completely verified by farther research, are in the last degree important." The observations which furnished the data for Fig. 1 were made in the North Pacific between the months of August, 1855, and April, 1856, and in the South Pacific during April and May; whereas for Fig. 2 the southern observations were made in May and June, the northern in June and July.

445. A thermal tide: it ebbs and flows once a year.—It is well to bear this difference as to season north and south in mind, and to compare these curves with those of the thermal charts; for the two together indicate the existence in the ocean of the thermal tide, which, as before stated, ebbs and flows but once a year. By this. figure the South Atlantic appears to be cooler and heavier than the northern. The season of observation, however, is southern fall and winter vice northern summer. In January, February, and March, the waters of the southern ocean are decidedly warmer, as at the opposite six months the}' are decidedly cooler, parallel for parallel, than those of the northern oceans. Thus periodically differing in temperature, the surface waters of the two hemispheres vary also in specific gravity, and give rise to an annual ebb and flow—an upper and an under tide—not from one hemisphere to the other, but between each pole and equator. In contemplating the existence and studying the laws of this thermal tide we are struck with the compensations and adjustments that are allotted to it in the mechanism of the sea; for these feeble forces in the water remind one of the quantities of small value—residuals of compensation—with which the astronomer has to deal when he is working out the geometry of the heavens. He finds that it is these small quantities which make the music of the spheres; and so, too, it is the gentle forces like this in the waters which preserve the harmony of the seas. Equatorial and polar seas may be of an invariable temperature, but in middle latitudes the sunbeam has power to wrinkle and crumple the surface of the sea by alternate expansion and contraction of its waters. In these middle latitudes is the cradle of the tiny thermal tide here brought to light; feeble, indeed, and easily masked are its forces, but they surely exist. It may be that the thermometer and hydrometer are the only instruments which are nice enough to enable us to detect it. Its footprints, nevertheless, are well marked in our tables showing the thermal dilatation of sea water. The movements of the isothermal lines, marching up and down the ocean, show by signs not to be mistaken its rate and velocity. These movements are well represented on the thermal charts. The tiny ripplings of this feeble tide have, we may be sure, their office to perform in the general system of aqueous circulation in the sea. Their influence may be feeble, like small perturbations in the orbits of planets; but the physicist is no more at liberty to despise these than the astronomer is to neglect those.

446. Sea water of the southern cooler and heavier, parallel for parallel, than sea water of the northern hemisphere.—The problem that we now have in hand, and which is represented by the diagrams of Plate X., is to put the seas in scales, the ocean in a balance, and to weigh in the specific-gravity bottle, the waters of the northern with the waters of the southern hemisphere. By Fig. 2 it would appear that both the water and the air of the south Atlantic are decidedly both cooler and heavier, parallel for parallel, than the waters of the north Atlantic; but this difference may be more apparent than real; for the observations were made in the northern summer on this side, and in the southern fall and winter on the other side of the equator. Had we a series of observations the converse of this, viz., winter in the north Atlantic, summer in the south, perhaps the latter would then appear to be specifically the lighter; at any rate, the mean summer temperature of each Atlantic, north and south, is higher than its mean winter temperature, and consequently the specific gravity of the waters of each must change with the seasons. A diagram—had we the data for such a one—to show these changes, would be very instructive; it would show beautifully, by its marks, the ebb and flow of this born tide of the ocean. By Fig. 1 the south Pacific also outweighs the north in specific gravity; but here again the true difference, whatever it be, is somewhat masked by the time of year when the observations were made. Those north were made during the fall, winter, and spring; those south, during the fall and first winter months of that hemisphere. Nevertheless, the weight of the observations presented on Plate X. does, as far as they go, indicate that the seas of the southern do outweigh in specific gravity the seas of the northern hemisphere in the proportion of 1.0272 to 1.0262 of specific gravity.^[5] Daubeny, Dove, et al., have pointed out an excess of salt contained in sea water south of the equator, as compared with that contained in sea water north.

447. Testimony of the hydrometer in favour of the air crossings at the calm belts.—These indications, as far as they go, and this view of the subject, whatever future investigations may show to be its true worth, seem to lean in support of the idea advanced and maintained by facts and arguments in Chapter IV., viz., that the southern seas are the boiler and the northern hemisphere the condenser for the grand atmospherical engine, which sucks up vapour from the south to feed the northern hemisphere with rains. If it be true,—and Dove also thinks it is—that the clouds which supply our fountains with rain for the great American lakes, and with rains for the majestic water-courses of Europe and Asia, Northern Africa and America, are replenished from seas beyond the equator, then the waters of the ocean south should be a little Salter, and therefore specifically a little heavier, parallel for parallel, and temperature for temperature, than the waters of cis-equatorial seas. We begin to find that the hydrometer is bearing testimony in support of the evidence adduced in Chapters IV. and VII., to show that when the trade-winds meet and rise up in the equatorial calm belt, the atmosphere which came there as south-east trade-winds passes with its vapour over into the northern hemisphere. We had not anticipated that this little instrument could throw any light upon this subject; but if, as it indicates, the sea water of the other hemisphere be Salter and heavier than the sea water of this, what makes it so but evaporation, and what prevents currents from restoring its equilibrium but the winds, which are continually sucking up from the brine of trans-equatorial seas and pouring it down as fresh water upon cis-equatorial seas and land? It is taking out of one scale of the balance and putting into the other; and the difference of specific gravity between the sea water of the opposite hemisphere may give us a measure for determining the amount of fresh water that is always in transitu. Certainly, if evaporation and rains were to cease, if the rivers were to dry up, and the sea-shells to perish, the waters of the ocean would, in the course of time, become all of the same saltness, and the only difference of specific gravity in the sea would be due to thermal agencies. After having thus ceased, if evaporation were then to commence only in the other hemisphere, and condensation take place only in this, half the difference, as to saltness of the sea water in opposite hemispheres, would express the ratio in volumes of fresh water, whether as vapour or liquid, that would then be kept in transitu between the two hemispheres. But it evaporates on both sides and precipitates on both; nevertheless, more on one side than on the other, and the difference of saltness will still indicate the proportion in transitu. If we follow the thermal and specific gravity curves from the parallels of 30°—34° to the equator (Figs. 1 and 2, Plate X.), we see, as I have said, that sea water in this part of the ocean does not grow lighter in proportion as it grows warmer. This is accounted for on the supposition that the effects of the thermal dilatation on the specific gravity is counteracted by evaporation. Now, if we knew the thickness of the stratum which supplies the fresh water for this evaporation, we should not only have a measure for the amount of water which as vapour is sucked up and carried off from the trade-wind regions of the sea, to be deposited in showers on other parts of the earth, but we should be enabled to determine also the quantity which is evaporated in one hemisphere and transported by the clouds and the winds to be precipitated in the other. These are questions which are raised for contemplation merely; they cannot be answered now; they grow out of some of the many grand and imposing thoughts suggested by the study of the revelations which the hydrometer is already beginning to make concerning the wonders of the sea. Returning from this excursion towards the fields of speculation, it will be perceived that these observations upon the temperature and density of sea water have for their object to weigh the seas, and to measure in the opposite scales of a balance the specific gravity of the waters of one hemisphere with the specific gravity of the waters of the other. This problem is quite within the compass of this exquisite system of research to solve. But, in order to weigh the seas in this manner, it is necessary that the little hydrometric balance by which it is to be done should be well and truly adjusted.

448. Amount of salt in, and mean specific gravity of sea water.—From these premises it would not be difficult to show that the saltness of the sea is a physical necessity. In some of the aspects presented, the salts of the sea hold the relation in the terrestrial mechanism that the balance-wheel does to the machinery of a watch. Without them the climates of the earth could not harmonize as they do; neither could the winds, by sucking up vapour, hold in check the expansive power of tropical heat upon the sea; nor counteract, by leaving the salts behind, the thermal influence of the sun in imparting dynamical force to marine currents; nor prevent the solar ray from unduly disturbing the aqueous equilibrium of our planet. As evaporation goes on from a sea of fresh water, the level only, and not the specific gravity, of the remaining water is changed. The waters of fresh intertropical seas would, instead of growing heavy by reason of evaporation between the tropics, become lighter and lighter by reason of the heat; while the water of fresh polar seas would grow heavier and heavier by reason of the cold—a condition which, by reason of evaporation and precipitation, is almost the very reverse of that which nature has ordained fur the salt sea, and which, therefore, is the wisest and the best. The average amount of salts in sea water is not accurately known. From such data as I have, I estimate it to be about 4 per cent. (.039), and the mean specific gravity of sea water at 60° to be about 1.0272. Supposing these conditions to be accurate—and they are based on data which entitle them to be considered as not very wide of the mark—the hydrometer and thermometer, with the aid of the table (§ 441), will give us a direct measure for the amount of salt in any specimen of sea water into which the navigator will take the trouble to dip these two instruments.

449. Light cast by Plate X. on the open sea in the Arctic Ocean.—These specific gravity and thermal curves, as they are presented on this Plate (X.), throw light also on the question of an open sea in the Arctic Ocean. That open sea is like a boiling spring (§ 427) in the midst of winter, which the severest cold can never seal up; only it is on a larger scale than any spring, or pool, or lake, and it is fed by the under currents with warm water from the south, which, by virtue of its saltness (see Fig. 2), is heavier than the cool and upper current which runs out of the polar basin, and which is known as an ice-bearing current. It is the same which is felt by mariners as far down as the Grand Banks of Newfoundland, and recognized by philosophers off the coast of Florida. This upper current, though colder than its fellow below, is lighter, because it is not so salt. Figure 2 reveals to us a portion of sea between the parallels of 34° and 40° north, exactly in such a physical category as that in which this theory presents the Arctic Ocean. Here, along our own shores, the thermal curve loses 12° of heat; and what does the specific gravity curve gain in the same interval? Instead of increasing up to 1.027, according to the thermal law, it decreases to 1.023 for the want of salt to sustain it. Now recollect that the great American chain of fresh-water lakes never freezes over. Why? Because of their depth and their vertical circulation. The depths below are continually sending water above 32° to the surface, which, before it can be cooled down to the freezing-point, sinks again. Now compare the shallow soundings in these lakes with the great depths of the Arctic Ocean; compute the vast extent of the hydrographic basin which holds this polar sea; gauge the rivers that discharge themselves into it; measure the rain, and hail, and snow that the clouds pour down upon it; and then contrast its area, and the fresh-water drainage into it, with the like of Long Island Sound, Delaware Bay, and the Chesapeake; consider also the volume of diluted sea water between our shore-line and the Gulf Stream; strike the balance, and then see if the arctic supply of fresh water be not enough to reduce its salts as much as our own fresh-water streams are diluting the brine of the sea under our own eyes. The very Gulf Stream water, which the observing vessel left as she crossed 34° and entered into those light littoral waters, was bound northward. Suppose it to have flowed on as a surface current until it, with its salts, was reduced to the temperature of 40°. Its specific gravity at that temperature would have been 1.030, or specifically 30 per cent, heavier than the sea water of our own coasts. Could two such currents of water meet anywhere at sea, except as upper and under currents? If water that freezes at 32°, that grows light and remains on the surface as you cool it below 39°, is prevented from freezing in our great fresh-water lakes by vertical circulation, how much more would both vertical and horizontal circulation prevent congelation in the open polar sea, that is many times deeper and larger than the lakes, and the water of which contracts all the way down to its freezing-point of 27°.2'!

450. The heaviest water.—The heaviest water in the sea, uncorrected for the temperature, as shown by the observations before us, is 1.028. This water was found (Figs. 1 and 2) off Cape Horn. Let us examine a little more closely into the circumstances connected with the heaviest water on our side of the equator. It was a specimen of water from the Sea of Okotsk, which is a sea in a riverless region, and one where evaporation is probably in excess of precipitation—thus fulfilling the physical conditions for heavy-water. The Red Sea is in a riverless and rainless region. Its waters ought to be heavier than those of any other mere arm of the ocean, and the dynamical force arising from the increase of specific gravity acquired by its waters after they enter it at Babel-mandeb is sufficient to keep up a powerful inner and outer current through those straits. At the ordinary meeting of the Bombay Geographical Society for November, 1857, the learned secretary stated that recent observations then in his possession, and which were made by Mr. Ritchie and Dr. Giraud (§ 381), go to show that the saltest water in the Red Sea is where theory (§377) makes it, viz., in the Gulf of Suez; and that its waters become less and less salt thence to its mouth, and even beyond, till you approach the meridian of Socotra; after which the saltness again increases as you approach Bombay.

451. Chapman's experiments.—Its waters, from the mouth of the straits for 300 or 400 miles up, have been found as high in temperature as 95° Fahrenheit—a sea at blood heat! The experiments of Professor Chapman, of Canada, which indicated as law—the Salter the water the slower the evaporation, seem to suggest an explanation of this, at least in part. Evaporation ought to assist in keeping the surface of intertropical seas cool in the same way that it helps to cool other wet surfaces. And if the waters of the Red Sea become so salt that they cannot make vapour enough to carry off the excessive heat of the solar ray, we may be sure that nature has provided means for carrying it off. But for the escape which these highly heated waters are, by means of their saltness, enabled to make from that sea, its climate, as well as the heat of its waters, would be more burning and blasting than the sands of Sahara. Even as it is, the waters of this sea are hotter than the air of the desert.

452. The hydrometer indicates the rainy latitudes at sea.—There is another indication which this little instrument has afforded concerning the status of the sea, and which deserves notice. We are at first puzzled with the remarkably light water between 9° and 16° S., Fig. 1, and in Fig. 2 between 7° and 9° N., as well as in 19° N. But after a little examination, we are charmed with the discovery that the hydrometer points out the rainy regions at sea. Rodgers' observations on his homeward passage from San Francisco to Cape Horn furnish the data for the curves {Fig. 1) between 37° N. and 57° S. Now Plate VIII. shows that the equatorial calm belt lies south of the line where it is intersected by the homeward route from California. It also shows that when he crossed the "Doldrums" in the Atlantic, that belt was in north latitude about 7°-10°, and that when he was in 18°-20° N. (Fig. 2) he was then passing through the offings of what are called the "Leeward Islands" of the West Indies, and that these are rainy latitudes at sea—the first two being under the cloud ring, the last being near the land in the trade-wind region, and confirming the remark so often made concerning the influence of islands at sea upon vapour, clouds, and precipitation.

453. Astronomical view.—The most comprehensive view that we are permitted to take of cosmical or terrestrial arrangements and adaptations is at best narrow and contracted. Nevertheless, in studying the mechanism which Wisdom planned and the Great Architect of nature designed for the world, we sometimes fancy that we can discover a relation between the different parts of the wonderful machinery, and perceive some of the reasons and almost comprehend the design which Omnipotent Intelligence had in view when those relations were established. Such fancies, rightly indulged, are always refreshing, and the developments of the hydrometer which we have been studying point us to one of them. This fancied discovery is, that a sea of fresh water instead of salt would not afford the compensations that are required in the terrestrial economy, and we also fancy that we have almost discovered a relation between the orbit of the earth and the arrangement of land and water on its surface and their bearing upon climate. Our planet passes its perihelion during the. southern summer, when it is nearer the centre and source of light and heat by more than three millions of miles than it is at its winter solstice, so that, on the 1st of January, the total amount of heat received by the earth is about 1/15 more than it receives during a day in July, when it is in aphelion.^[6] January is the midsummer month of the southern hemisphere, consequently that half of the globe receives more heat in a day of its summer than tho other half receives in a day of the northern summer. But the northern summer is a week the longer, by the reason of the ellipticity of the earth's orbit. What becomes of this diurnal excess of southern summer heat, be it in its aggregate never so small, and why does it not accumulate in trans-equatorial climes? So far from it the southern hemisphere is the cooler.

454. The latent heat of vapour.—In the southern hemisphere there is more sea and less land than in the northern. But the hydrometer indicates that the water in the seas of the former are Salter and heavier than the waters of seas cis-equatorial; and man's reasoning faculties suggest, in explanation of this, that this difference of saltness or specific gravity is owing to the excess of evaporation in the southern half, excess of precipitation in the northern half of our planet. "When water passes, at 212° Fahrenheit, into steam it absorbs 1000° of heat, which becomes insensible to the thermometer, or latent; and conversely, when steam is condensed into water, it gives out 1000° of latent heat, which thus becomes free, and affects both the thermometer and the senses. Hence steam of 212° Fahrenheit will, in condensing, heat five and a half times its own weight of water from the freezing to the boiling point."—M'Culloch. Now there is in the southern a very much larger water surface exposed to the sun than there is in the northern hemisphere, and this excess of heat is employed in lifting up vapour from that broad surface, in transporting it across the torrid zone and conveying it to extra-tropical northern latitudes, where the vapour is condensed to replenish our fountains, and where this southern heat is set free to mitigate the severity of northern climates.

455. Its influence upon climates.—In order to trace a little farther, in our blind way, the evidences of wisdom and design, which we imagine we can detect in the terrestrial arrangement of land and water, let us fancy the southern hemisphere to have the land of the northern, and the northern to have the water of the southern, the earth's orbit remaining tho same. Is it not obvious to our reason that by this change the whole system of climatology in both hemispheres would be changed? The climates of our planet are as obedient to law as the hosts of heaven. They are as they were designed to be; and all those agents which are concerned in regulating, controlling, and sustaining them are "ministers of His." Johnston, in the chapter to Plate XV-III. of his great Physical Atlas, thus alludes to the seas, land, and climates of the two hemispheres: "The mild winter of the southern hemisphere, plus the contemporaneous hot summer of the northern hemisphere, necessarily gives a higher sum of temperature than the cool summer of the southern, plus the cold winter of the northern hemisphere. The above-described relations appear to furnish the motive power in the machinery of the general atmosphere of the earth in the periodical conversion of the aqueous vapours into liquid form. In this manner the circuit of the fluid element, the essential support of all vegetable and animal life, no longer appears to depend on mere local coolings, or on the intermixture of atmospheric currents of different temperatures; but the unequal distribution of land and sea in the northern and southern hemispheres supplies an effectual provision, from whence it necessarily follows that the aqueous vapour, which from the autumnal to the vernal equinox is developed to an immense extent over the southern hemisphere, returns to the earth, in the other half of the year, in the form of rain or snow. And thus the wonderful march of the most powerful steam-engine with which we are acquainted, the atmosphere, appears to be permanently regulated. The irregular distribution of physical qualities over the earth's surface is here seen to be a preserving principle for terrestrial life. Professor Dove considers the northern hemisphere as the condenser in this great steam-engine, and the southern hemisphere as its water reservoir; that the quantity of rain which falls in the northern hemisphere is, therefore, considerably greater than that which falls in the southern hemisphere; and that one reason of the high temperature of the northern hemisphere is that the larger quantity of heat which becomes latent in the southern hemisphere in the formation of aqueous vapour is set free in the north in great falls of rain and snow."

456. The results of the marine hydrometer.—In this view of what our little hydrometer has developed or suggested, we trace the principles of compensation and adjustment, the marks of design, the evidence of adaptation between the orbit of the earth and the time from the vernal to the autumnal, and from the autumnal to the vernal equinox; between the arrangement of the land in one hemisphere and the arrangement of the water in the other; between the rains of the northern and the winds of the southern hemisphere; between the vapour in the air and the salts of the sea; and between climates on opposite sides of the equator. And all this is suggested by merely floating a glass bubble in sea water during a voyage to the Pacific! Thus even the little hydrometer, in its mute way, points the Christian philosopher to the evidences of design in creation. That the arrangements suggested above are adapted to each other, this instrument affords us evidence as clear as that which the telescope and the microscope bear in proof that the eye, in its structure, was adapted to the light of heaven. The universe is the expression of one thought, and that it is so every new fact developed in the progress of our researches is glorious proof.

457. Barometer indications of an open sea.—In the course of our investigations into the physics of the sea, 100,000 observations of the barometer, and more than a million on the direction of the winds have been discussed. They indicate an open water in the Arctic Ocean. They show that about the poles there is a high degree of aërial rarefaction—higher, indeed, than there is about the equator; for the barometer not only stands lower in this place of polar calms than it does in the equatorial calm belt, but the inrushing air comes from a greater distance to the cold than to the warm calms.^[7]

458. Polar rarefaction.—The question may be asked, Whence comes the heat that expands and rarefies the atmosphere in these polar places? The answer is, it comes from the condensation of vapour. The south pole is surrounded by water, the north pole by land. But the unexplored regions within the arctic basin are (§ 429) for the most part probably sea, within the antarctic, land. The rarefaction produced in the latter by the latent heat of vapour is such that the mean height of the barometer there is about 28 inches, while that in the arctic calm place is such as to reduce the barometer there to a mean not far from 29.5 inches. In the equatorial calm its mean height is about 29.9 inches. The hypothesis of an open sea in the Arctic Ocean becomes necessary to supply a source for this vapour; for the winds, entering the Arctic Ocean as they do after passing over land and mountain heights of America, Europe, and Asia, must be robbed of much of their moisture ere they reach that ocean; it will require an abundant supply of vapour to create there by precipitation and the liberation of latent heat a degree of rarefaction sufficient to cause a general movement of the air polarward for the distance of 40° of latitude all round. That there is an immense volume of comparatively warm water going into the Arctic Ocean is abundantly shown by observation, and the rising up there of this water to the surface would afford heat and vapour enough for a vast degree of rarefaction.

459. The middle ice.—The records of arctic explorations, together with the whalemen's accounts of "middle ice" in Baffin's Bay and Davis' Straits, go to confirm this view, which is further elaborated in the next chapter (§ 475). The facts there stated, and this "middle ice," go to show that every winter a drift takes place which brings out of the Frozen Ocean a tongue of ice a thousand miles or more in length: it is the compact and cold "middle ice." In our fresh- water streams it is the middle ice that first breaks up; that which is out of the way of the current remains longest. Not so in this bay and strait; there the littoral ice first gives way, leaving an open channel on either side in spring and early summer, while the "middle ice" remains firm and impassable. The explanation is simple enough: the middle ice was formed in the severe cold of more northern latitudes, from which it has drifted down, while that on the sides was formed in the less severe climates of the bay and straits. This winter tongue of ice, which we know by actual observation is in motion from December till May, must, during that time, be detached from the main mass of ice in the Arctic Ocean, consequently there must be water between the ice that is in motion and the ice that is at rest. Not only so. In early summer the whalemen will run up to the north in the open water at the side of the "middle ice" in Davis' Strait and Baffin's Bay, even as far sometimes as Cape Alexander in 78°, to look for a crossing-place. Here, though so far north, they will find the "middle ice" gone, or so broken up that they can cross over to the west side. They trace it up thus far, because at the south, and in spite of a higher thermometer, they find the "middle ice" compact and firm, so much so as to be impassable. In this fact we recognize another circumstance favouring the theory of an open sea at the north, and giving plausibility to the conjecture that this "middle ice" drifts out from the southern edge of the open sea as fast as it is formed during the winter. According to this conjecture, the thickest part of the "middle ice" should be that which has been exposed to the longest and severest cold, and this is probably that which began to be formed on the edge of the open sea in January. As it drifted to the south it continued to form and grow thick, and perhaps would be the last to melt: while that which began to be formed at the edge of the open sea in March or April would drift out, and not attain much thickness before it would cease to freeze and commence to thaw. It is this spring-made "middle ice" then, which, as it drifts to the south, would, being thin, be the first to break up; and experience has taught the whalemen to look north, not south, for the first breaking up and the earliest passage through the "middle ice."

460. Position of the open sea.—The open sea, therefore, is, it may be inferred, at no groat distance from the several straits, which, leading in a northwardly direction, connect Baffin's Bay with the Arctic Ocean. It is through these straits that the winter drift takes place. The ice in which the Fox, the Resolute, the Advance, and the Rescue each drifted a thousand miles or more, came down through these straits. The fact of this annual winter drift from the Arctic Ocean is a most important one for future explorers. Had Captain Franklin known of it, he might have put his vessels in the line of it, and so escaped the rigours of that second winter. It would have brought him safely to the parallel of 65° or 60°, and set him free, as it did four other vessels, in the glad waters of the Atlantic Ocean.

1. See chap. XXII. on the Actinometry of the Sea.
2. This remark was made by Sir John on the supposition, probably, that the maximum density of sea water was at the same temperature as that of fresh, but it is the same 12° or 14° lower.
3. This agrees more nearly with Despretz (p. 245) than with Dr. Marcet. The latter states that sea water decreases in weight to the freezing point until actually congealed. In four experiments Dr. Marcet cooled sea water down to between 18° and 190 Fahr., and found that it decreased in bulk till it reached 22°, after which it expanded a little, and continued to do so till the fluid was reduced to between 190 and 180, when it suddenly expanded, and became ice with a temperature of 280. It should always be recollected that a saturated solution of common salt does not become solid, or converted into ice, at a less temperature than 40 Fahr.; and, therefore, if the sea should he, as is sometimes supposed, more saline at great depths, and as it appears to be in the Mediterranean from the experiments of Dr. Wollaston, ice could not be formed there at the same temperature as it could nearer the surface.—(Vide M. de la Beche, Manual Geology, p. 22.)
4. Specific gravity at 200°=0.9908.
5. According to Dr. Marcet, the southern ocean contains more salt than tho northern in the proportion of 1.02919 to 1.02757.
6. Sir John Herschel.
7. Plate IV., Nautical Monograph No 1.