Physical Geography of the Sea and its Meteorology/Chapter 20

by Matthew Fontaine Maury

CHAPTER. XX.

§ 811-542. — THE WINDS OF THE SOUTHERN HEMISPHERE.

811. Repetition often necessary.—A work of this sort, which is progressive, must necessarily bear with it more or less of repetition. It embodies the results of the most extensive system of philosophical observations, physical investigation, and friendly co-operation that has ever been set on foot. As facts are developed, theories are invented or expanded to reconcile them. As soon as this is done, or in a short time thereafter, some one or more of the fleets that are out reconnoitering the seas for us, returns with additional facts for our storehouse of knowledge. Whether these tend to confirm or disprove the theory a restatement is often called for; hence the repetition, of which the case before us is an example.

812. The S.E. and N.E. trade-winds put in a balance.—The facts stated in Chap. XV. go to show that the south-east trade-winds are stronger than the north-east. The barometer tells us (§ 643) that between the parallels of 5° and 20° the south-east trade-winds bear a superincumbent pressure upon the square foot of nearly 4 pounds greater than that to which the north-cast trades are subjected. Such an excess of superincumbent pressure upon a fluid so elastic and subtle as air, ought to force the south-east trade- winds from under it more rapidly than the lighter pressure forces the north-east. Observations showing that such is or is not the case should not be ignored.

813. Observations by 2235 vessels.—I have the separate and independent evidence from every vessel in a fleet numbering no less than 2235 sail to show that the S.E. are stronger than the N.E. trades. All of these vessels passed through both systems of trade-winds. The knots run -per hour by each one of them, as they passed through the south-east trades of the Indian Ocean and through both systems of the Atlantic, have been measured and discussed from crossing to crossing. The average result in knots is expressed in the annexed table, P. 433. The comparison is confined to the rate of sailing between the parallels of 10° and 25°, because this is the belt of steadiest trades.

814. Ships used as anemometers.—It is well to observe, that on each of these three oceans, though the direction of the wind is the same, the course steered by each fleet is different; consequently, these sailing anemometers are at different angles with the wind; through the south-east trades, the wind is nearly aft in the Atlantic, and quartering in the Indian Ocean, giving an average sailing speed of 7 knots an hour in the latter, and of 6 in the former; while through the north-east trades the average speed is 6¼ knots an hour one way (N.W. ¼ W.), with the wind just abaft the beam, and 5¾ of the other (S.S.E.), with the wind at a point not so favourable for speed. Indeed, most of the ships which average a S.S.E. course through this part of the northeast trade-wind belt are close hauled; therefore the average strength of the trades here cannot be fairly compared with the

Average Speed of Vessels sailing through the Trade-winds of the North Atlantic and South Indian Oceans.

Month.	Knots per Hour from
	10° to 15°.		15° to 20°,		20° to 25°.		Average.		Course steered through.
	N.E. Trades.	S.E. Trades.	N.E. Trades.	S.E. Trades.	N.E. Trades.	S.E. Trades.	N.E. Trades.	S.E. Trades.	N.E. Trades. (N. Atlantic)	S.E. Trades. (I. Ocean)
January	8	6½	7½	7.	6	6	7	6½	N. 49° W.	S. 69° W.
February	7½	6	6	6¾	5	6	6	6	N. 46 AV.	,,
March	8	7	7	7	5¼	6½	6¾	6¾	N. 47 W.	,,
April	7¼	7	5¾	7¾	4	6	6	6¾	N. 48 W.	S. 70° W.
May	8¼	8	6¾	7½	6½	6	7	7	N. 46 W.	,,
June	9	7½	8	7¾	5¼	7	7½	7½	N. 43 W.	,,
July	5½	8	8	8¼	6¼	7	6½	7¾	N. 46 W.	,,
August.	4¾	7¾	6	8¼	4¾	7½	5	7¾	N. 40 W.	S. 69° W.
September.	5½	8½	6	8	4	6¾	5	7¾	N. 50 W.	,,
October	7½	8¼	6¾	8	6	5¾	6¾	7¼	N. 45 W.	,,
November	6	8	6	7	4½	5¼	5½	6¾	N. 49 W.	,,
December	6	6½	6¾	6¾	5½	5	6	6	N. 48 W.	,,
Means	7	7¾	6¾	7½	5¼	6¼	6¼	7	N. 47° W.	S. 69¼° W.

Average course steered throug-h the N.E. trades (N. Atlantic Ocean N.W. ¼ W.
,,,,S.E. trades (South Indian,,), W.S.W.

Average ditto through the Trade-winds of the North and South Atlantic Oceans.

Month.	Knots per Hour from
	25° to 20°		20° to 15°.		15° to 10°.		Average.		Course steered through.
	N.E. Trades.	S.E. Trades.	N.E. Trades.	S.E. Trades.	N.E. Trades.	S.E. Trades.	N.E. Trades.	S.E. Trades.	S.E. Trades. (S. Atlantic)	N.E. Trades. (N. Atlantic)
January	6	5	5¾	5½	5¾	7½	6	6	N. 54° W.	S. 21° E.
February	4½	5	5½	6½	6	7	5¼	6	N. 55 W.	S. 25 E.
March	6½	4¾	6	6½	6	7½	6	6¼	N. 55 W.	S. 22½ E.
April	5½	5½	5½	6½	6¾	7¾	6	6½	N. 53 W.	S. 22½ E.
May	5¾	5	5¾	6½	6½	7	6	6	N. 55 W.	S. 24¾ E.
June	5¾	6	6	6	7	5	6¼	5¾	N. 55 W.	S. 28 E.
July	5¼	7	6	7½	6¼	4½	6	6	N. 55 W.	S. 27 E
August.	5	5½	5¾	6½	6	4	5¾	5¼	N. 55 W.	S. 28½ E.
September	5½	3¼	6¼	5½	7	5	6¼	4½	N. 55 W.	S. 22½ E.
October	5¼	4¼	6	4½	5¼	4¾	5½	4½	N. 56 W.	S. 20 E.
November	6½	4	6½	5¼	6¼	5¾	6½	5	N. 55 W.	S. 22½ E.
December	5½	4¾	5¾	6	6¼	7¾	5¾	5	N. 55 W.	S. 23 E.
Means	5½	5	5¾	6	6¼	6	6	5¾	N. 55° W.	S. 24° E.

Average course steered through the S.E. trades (S. Atlantic), N.W. by W.
,,,,N.E. trades (N. Atlantic-), S.S.E.

average strength where the fleet have free winds. What is the difference in the strength of such winds, which impinging upon the sails, each at the particular angle indicated above, imparts the aforesaid velocities? Moderate winds, such as these are, give a ship her highest speed generally when they are just abaft the beam, as they are for a north-west course through the north-east trades of the North Atlantic. So, to treat these ships as anemometers that will really enable us to measure the comparative strength of the winds, we should reduce the average knots per hour to the average speed of a mean ship sailing through average "trades" in each ocean, with the wind impinging upon her sails at the same angle for all three, as, for example, just abaft the beam, as in the North Atlantic.

815. Velocity of the trade-winds.—Let us apply to the average speed through the South Atlantic and Indian Oceans such a correction. Through the former the wind is aft; through the latter, quartering. If we allow two knots as a correction for the one and one as a correction for the other, we shall not be greatly out. Applying such corrections, we may state the speed of a mean ship sailing with average trades just abaft the beam to be as follows:

Through the	N.E.	of the	N. Atlantic	6¼	knots per hour.[1]
,,	S.E.	,,	S. Atlantic	8	,,
,,	S.E.	,,	S. Atlantic	8	,,

I do not take into this comparison the force of the N.E. trades on a S.S.E. course (§ 813), because the winds along this route are known not to be as steady as they are farther away from the African coast. Thus it is clearly established that the S.E. trades are stronger than the N.E., and so they should be if there be a crossing of winds in the calm belt of Capricorn.

816. Ditto of the counter-trades.—The counter-trades of the southern hemisphere move, as before stated, towards their pole more steadily and briskly than do the counter-trades of the northern hemisphere. To give an idea of the difference of the strength of these two winds, I cite the fact that vessels sailing through the latter, as from New York to England, average 150 miles a day. Along the corresponding latitudes through the former, as on a voyage to Australia, the average speed is upwards of 200 miles a day. Consequently, the counter-trades of the southern hemisphere transport in given times larger volumes of air towards the south than our counter-trades do towards the north. This air returns to the tropical calm belts as an upper current. If, descending there, it feeds the trade-winds, then, the supply being more abundant for the S.E. trades than for the N.E., the S.E. trades must be the stronger; and so they are; observations prove them so to be. Thus the crossing of the air at the tropical calm belts, though it may not be proved, yet it is shown to be so very probable that the onus of proof is shifted. It now rests with those who dispute the crossing to prove their theory the true one.

817.The waves they get up.—Arrived at this point, another view in the field of conjecture is presented, which it is proper we should pause to consider. The movements of the atmosphere on the polar side of 40° N. are, let it be repeated, by no means so constant from the west, nor is the strength of the westerly winds there nearly so great on the average as it is in the corresponding regions of the south. This fact is well known among mariners. Every one who has sailed in that southern girdle of waters which belt the earth on the polar side of 40°, has been struck with the force and trade-like regularity of the westerly winds which prevail there. The waves driven before these winds assume in their regularity of form, in the magnitude of their proportions, and in the stateliness of their march, an aspect of majestic grandeur that the billows of the sea never attain elsewhere. No such waves are found in the trade-winds; for, though the S.E. trades are quite as constant, yet they have not the force to pile the water in such heaps, nor to arrange the waves so orderly, nor to drive them so rapidly as those "brave" winds do. There the billows, chasing each other like skipping hills, look, with their rounded crests and deep hollows, more like mountains rolling over a plain than the waves which we are accustomed to see. Many days of constant blowing over a wide expanse of ocean are required to get up such waves. It is these winds and waves which, on the voyage to and from Australia, have enabled the modern clipper-ship to attain a speed, and, day after day, to accomplish runs which at first were considered, even by the nautical world, as fabulous, and are yet regarded by all with wonder and admiration.

818. A meteorological corollary.—Seeing, therefore, that we can bring in such a variety of facts and circumstances, all tending to show that the S.E. trade-winds are stronger than the N.E., and that the westerly winds which prevail on the polar side of 40° S. are stronger and more constant than their antœcian fellows of the north, we may consider it as a fact established, independently of the conclusive proof afforded by Plate XIII., that the general system of atmospherical circulation is more active in the southern than it is in the northern hemisphere. And, seeing that it blows with more strength and regularity from the west in the extra-tropical regions of the southern than it does in the extra-tropical regions of the northern hemisphere, we should deduce, by way of corollary, that the counter-trades of the south are not so easily arrested in their course, or turned Lack in their circuits, as are those of the north. Consequently, moreover, we should not, either in the trades or the counter-trades of the southern hemisphere, look for as many calms as in those of the northern systems.

819. Facts established.—Wherefore, holding to this corollary, we may consider the following as established facts in the meteorology of the sea: That the S.E. trade-winds are stronger than the N.E.; that the N.W. passage-winds—the counter-trades of the south—are stronger and less liable to interruption in their circuits than the S.W., the counter-trades of the north; that the atmospherical circulation is more regular and brisk in the southern than it is in the northern hemisphere; and, to repeat: since the wind moves in its circuit more briskly through the southern than it does through the northern hemisphere, it consequently has less time to tarry or dally by the way in the south than in the north; hence the corollary just stated. But observations, also, as well as mathematically-drawn inferences, show that calms are much less prevalent in the southern hemisphere. For this inference observations are ample; they are grouped together by thousands and tens of thousands, both on the Pilot and the Storm and Rain Charts. These charts have not yet been completed for all parts of the ocean, but as far as they have been constructed the facts they utter are iii perfect agreement with the terms of this corollary.

820. Atmospherical circulation more active in the southern than in the northern hemisphere.—These premises being admitted, we may ascend another round on this ladder, and argue that, since the atmosphere moves more briskly and in more constant streams through its general channels of circulation in the southern than it does through them in the northern hemisphere; and that, since it is not arrested in its courses by calms as often in the former as it is in the latter, neither should it be turned back by the way, so as to blow in gales from the direction opposite to that in which the general circulation carries it. The atmosphere, in its movements along its regular channels of circulation, may be likened, that in the southern hemisphere to a fast railway train; that of the northern to a slow The slow train may, when "steam is up," run as fast as the fast train, but it is not obliged to get through so quick; therefore it may dally by the way, stop, run back, and still be through in time. Not so the fast; it has not time to stop often or to run back far; neither have the counter-trades of the south time to blow backward; consequently, such being the conditions, we should also expect to find in the extra-tropical south a gale with easting in it much more seldom than in the extra-tropical north.

821. Gales in the two hemispheres.—We shall appeal to observations for the correctness of this conjecture, and claim for it, also, as presently will appear, marine meteorology.

Average Number {to the 1000 Observations) of Gales, with Easting and with Westing in them, between the corresponding Parallels in the North and South Atlantic, as shown by the Storm and Rain Charts.

			North.	South.
Between 40° and 45°,	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Number of Observations	17,274	8,756
		Gales in 1000 do., with easting	23	12
		,,,,,,westing	66	82
Between 45° and 50°,	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Number of Observations .	11,425	5,548
		Gales in 1000 do., with easting	24	1
		,,,,,,westing	106	61
Between 50° and 55°	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Number of Observations	4,816	5,169
		Gales in 1000 do., with easting	24	10
		,,,,,,westing	144	97

Thus the Storm and Rain Charts show that between the parallels of 40° and 55° there were in the northern hemisphere 33,515 observations, and that for every 1000 observations there were 24 gales with easting and 105 with westing. That in the southern, there were 19,473 observations, and for every 1000 of these there were 5 gales with easting and 80 with westing in them. Those for the southern hemisphere are only for that part of the ocean through which vessels pass on their way to and fro around Cape Horn. That part of this route which lies between 40° and 55° S., is under the lee of South America; and Patagonia, that lies east of the Andes, is almost a rainless region; consequently, we might expect to find more unsteady winds and fewer rains in that part of the ocean where the observations for the southern part of the tables were made than we should expect to meet with well out to sea, as at the distance of two or three thousand miles to the eastward of Patagonia. So that the contrast presented by the above statement would probably be much greater did our observations extend entirely across the South, as they do across the North Atlantic. But as it is, the contrast is very striking. In some aspects, the meteorological agents of the two hemispheres, especially those forces which control the winds and the weather, differ very much. The difference is so wide as to suggest greater regularity and rapidity of circulation on one side of the equator than on the other.

822. Calms in the two hemispheres.—Average Number of Calms to the 1000 Observations between the Parallels of 30° and 55°, in the North and South Atlantic, and between the Parallels 30° and 60° in the North and South Pacific Oceans, as shown by the Pilot Charts.

	Atlantic.		Pacific.
Between the Parallels of	North.	South.	North.	South.
30° and 35°, No. of Observations	12,935	15,842	22,730	44,846
Calms to the 1000 do	46	26	34	35
35° and 40°, No. of Observations.	22,136	23,439	13,939	66,275
Calms to the 1000 do	37	24	31	23
40° and 45°, No. of Observations	16,363	8,203	12,400	31,889
Calms to the 1000 do	45	27	53	23
45° and 50°, No. of Observations	8,907	4,183	15,897	4,940
Calms to the 1000 do	38	25	35	21
50° and 55° No. of Observations	3,519	3,660	32,804	9,728
Calms to the 1000 do	40	16	32	17
55° and 60°, No. of Observations	..	..	15,470	9,111
Calms to the 1000 do	..	..	43	21
Total No. of Observations	63,050	55,327	113,240	166,829
Average Calms to the 1000 do	41	24	39	25

Each one of these observations embraces a period of eight hours; the grand total, if arranged consecutively, with the observations drawn out each to occupy its period separately, would be equal to 373 years. They exhibit several curious and suggestive facts concerning the difference of the atmospherical stability in the two hemispheres.

823. The propelling power of the winds.—If we would discover the seat of those forces which produce this difference in the dynamical status of the two great aerial oceans that envelop our planet, we whould search for them in the unequal distribution of land and water over the two hemispheres. In one the wind is interrupted in its circuits by the continental masses, with their wooded plains, their snowy mantles in winter, their sandy deserts in summer, and their mountain ranges always. In the other there is but little land and loss snow. On the polar side of 40° S. especially, if we except the small remnant of this continent that protrudes beyond that parallel in the direction of Cape Horn, there is scarcely an island. All is sea. There the air is never dry; it is always in contact with a vapour-giving surface; consequently, the winds there are loaded with moisture, which, with every change of temperature, is either increased by farther evaporation or diminished by temporary condensation. The propelling power of the winds in the southern hemisiphere resides chiefly in the latent heat of the vapour which they such up from the engirdling sea on the polar side of Capricorn.

824. Lieut. Van Gough's Storm and Main Charts.—The Storm and Rain Charts show that within the trade-wind regions of both hemispheres the calm and rain curves are symmetrical; that in the extra-tropical regions the symmetry is between the calm and fog curves; and also, especially in the southern hemisphere, between the gale and rain curves. Lieutenant Van Gough, of the Dutch Navy, in an interesting paper on the connection between storms near the Cape of Good Hope and the temperature of the sea,^[2] presents a storm and rain chart for that region. It is founded on 17,810 observations, made by 500 ships, upon wind and weather, between 14° and 32° E., and 33° and 37° S. By that chart the gale and rain curves are so symmetrical that the phenomena of rains and gales in the extra- tropical seas present themselves suggestively as cause and effect. The general storm and rain charts of the Atlantic Ocean, prepared at the National Observatory, Washington, hold out the same idea. Let us examine, expand, and explain this fact.

825. The "brave west winds" caused by rarefaction in the antarctic regions.—We ascribe the trade-winds to the equatorial calm-belt. But to what shall we ascribe the counter-trades, particularly of the southern hemisphere, which blow with as much regularity towards the pole as the north-east trades of the Atlantic do towards the equator? Shall we say that those winds are drawn towards the south pole by heat, which causes them to expand and ascend in the antarctic regions? It sounds somewhat paradoxical to say that heat causes the winds to blow towards the poles as well as towards the equator; but, after a little explanation, and the passing in review of a few facts and circumstances, perhaps the paradox may disappear. It is held as an established fact by meteorologists that the average amount of precipitation is greater in the northern than in the southern hemisphere; but this, I imagine, applies rather to the land than the sea. On the polar side of 40° it is mostly water in the southern, mostly land in the northern hemisphere. It is only now and then, and on rare occasions, that ships carry rain-gauges to sea. We can determine by quantitive measurements the difference in amount of precipitation on the land of the two hemispheres, and it is the result of this determination, I imagine, that has given rise to the general remark that the rain-fall is greater for the northern than it is for the southern hemisphere. But we have few hyetographic measurements for quantity at sea; there the determinations are mostly numerical. Our observers report the "times" of precipitation, which, whether it be in the form of rain, hail, or snow, is called by the charts, and in this discussion, rain. Among such a large corps of observers, rain is sometimes, no doubt, omitted in the log; so that, in all probability, the charts do not show as many "times" with rain as there are "times" actually with rain at sea. This omission, however, is as likely to occur in one hemisphere as in the other. Still, we may safely assume that it rains oftener in all parts of the sea than our observations, or the rain charts that are founded on them, indicate.

826. Relative frequency of rains and gales at sea.—With the view of comparing the rains at sea between the parallels of 55° and 60°, both in the North and South Atlantic, we have taken from the charts the following figures:

South Atlantic—Observations,	8410;	gales,	1228;	rains,	1105
North Atlantic—,,	526;	,,	135;	,,	64
Gales to the 1000 observations	. .	S. Atlantic,	146;	N. Atlantic,	256
Rains,,,,,	. .	S. Atlantic,	131;	N. Atlantic,	121

That is, for every 10 gales, there are in the southern hemisphere 9 rains, and in the northern 4.7. In which hemisphere does most water fall on the average during a rain at sea? Observations do not tell, but there seems to be a philosophical reason why it should rain not only oftener, but more copiously at sea, especially in the extra-tropical regions, in the southern hemisphere than in those of the northern. On the polar side of 40° N., for example, the land is stretched out in continental masses, upon the thirsty bosom of which, when the air drops down its load of moisture, only a portion of it can Lo taken up again; the rest is absorbed by the earth to feed the springs. On the polar side of 40° S. we have a water instead of a land surface, and as fast as precipitation takes place there, the ocean replenishes the air with moisture again. It may consequently be assumed that a high dew-point,—at least one as high as the ocean can maintain in contact with winds blowing over it, and going from warmer to cooler latitudes all the time—is the normal condition of the air on the polar side of 40° S., whereas on the polar side of 40° N. a low dew-point prevails. The rivers to the north of 40°, I reckon, could not, if they were all converted into steam, supply vapour enough to make up this average difference of dew-point between the two hemispheres. The symmetry of the rain and storm curves on the polar side of 40° S. suggests that it is the condensation of this vapour which, with the liberation of its latent heat, gives such activity and regularity to the circulation of the atmosphere in the other hemisphere.

827. The rain-fall of Cape Horn and Cherraponjie.—On the polar side of 40° S., near Cape Horn, the gauge of Captains King and Fitzroy showed a rain-fall of 153.75 inches in 41 days. There is no other place except Cherraponjie where the precipitation approaches this in amount. Cherraponjie (§ 299) is, it has already been stated, a mountain station in India, 4500 feet high, which, in latitude 25° N., acts as a condenser for the monsoons fresh from the sea. But on the polar side of latitude 45°, in the northern hemisphere, it is, except along the American shores of the North Pacific, a physical impossibility that there should be a region of such precipitation as King and Fitzroy found on the western slopes of Patagonia—a physical impossibility, because that peculiar combination of conditions required to produce a Patagonian rain-fall is wanting on the polar side of 45° N. There is in the North Atlantic, water surface enough to afford vapour for such an amount of precipitation. In the North Pacific the water surface may be broad and ample enough to afford the vapour, but in neither of these two northern sheets of water are the winds continuous enough from the westward to bring in the requisite quantities of vapour from the sea. Moreover, if the westerly winds of the extra-tropical north were as steady and as strong as are those of the south, there is lacking in the north that continental relief—mountain ranges rising abruptly out of the sea, or separated from it only by lowlands—that seems to be necessary to bring down the rain in such floods. Colonel Sykes^[3] quotes the rain-fall of Cherraponjie at 605.25 inches for the 214 days from April to October, the season of the south-west monsoons. Computing the Cape Horn rains according to the ratio given by King and Fitzroy for their 41 days of observations, we should have a rain-fall in Patagonia of 825 inches in 214 days, or a yearly amount of 1368.7 inches. Neither the Cape Horn rains, nor the rains anywhere at sea on the polar side of 45° S., are periodical. They are continuous; more copious, perhaps, at some seasons of the year than at others, but abundant at all.

828. Influence of highlands upon precipitation.—Now, considering the extent of water surface on the polar side of the south-east trade-wind belt, we see no reason why, on these parallels, the engirdling air of that great watery zone of the south should not, entirely around the earth, be as heavily charged with vapour as was that which dropped this flood upon the Patagonian hills. If those mountains had not been there, the condensation and the consequent precipitation would probably not have been as great, because the conditions at sea are less apt to produce rain; but the quantity of vapour in the air would have been none the less, which vapour was being borne in the channels of circulation towards the antarctic regions for condensation and the liberation of its latent heat; and we make, as we shall proceed to show, no violent supposition, if, in attempting to explain this activity of circulation south of the equator, we suppose a cloud region, with a combination of conditions in the antarctic circle peculiarly favourable to heavy and almost incessant precipitation. But, before describing these conditions, let us turn aside to inquire how far precipitation in the supposed cloud region of the south may assist in giving force and regularity to the winds of the southern hemisphere.

829. The latent heat of vapour.—If we take a measure, as a cubic foot, of ice at zero, and apply heat to it by means of a steady flame that will give off heat at a uniform rate, and in such quantities that just enough heat may be imparted to the ice to raise its temperature 1° a minute, we shall find that at the end of 32 minutes the ice will be at 32°. The ice will now begin to melt; but it and its water, the heat being continued, will remain at 32° for 140 minutes, when all the ice will have become water at 32°^[4]. This 140° of heat, which is enough to raise the temperature of 140 cubic feet of ice one degree from any point below 32°, has been rendered latent in the process of liquefaction. Freeze this water again, and this latent heat will become sensible heat, for heat no more than ponderable matter can be annihilated. But if, after the cubic foot of ice has been converted into water at 32°, we continue the uniform supply of heat as before and at the same rate, the water will, at the expiration of 180 minutes more, reach the temperature of 212°—the boiling-point—and at this temperature it will remain for 1030 minutes, notwithstanding the continuous supply of heat during the interval. At the expiration of this 1030 minutes of boiling heat, the last drop of water will have been converted into steam; but the temperature of the steam will be, that only of the boiling water; thus, in the evaporation of every measure of water, heat enough is rendered latent during the process to raise the temperature of 1030 such measures one degree. If this vapour be now condensed, this latent heat will be set free and become sensible heat again. Hence we perceive that every rain-drop that falls from the sky has, in its process of condensation, evolving heat enough to raise one degree the temperature of 1030 rain-drops. But if instead of the liquid state, as rain, it come down in the solid state, as hail or snow, then the heat of fluidity, amounting to enough to raise the temperature of 140 additional drops one degree, is also set free.

830. The cause of the boisterous weather off Cape Horn.—We have in this fact a clew to the violent wind which usually accompanies hail-storms. In the hail-storm congelation takes place immediately after condensation, and so quickly that the heat evolved during the two processes may be considered as of one evolution. Consequently, the upper air has its temperature raised much higher than could be done by the condensing only. So also the storms which have made Cape Horn famous are no doubt owing, in a great measure, to this heavy Patagonian rainfall. The latent heat which is liberated by the vapour as it is condensed into rain there, has the effect of producing a great intumescence in the air of the upper regions round about them, which in turn produces commotion in the air below. But this is digressive. Therefore let us take up the broken thread, and suppose, merely for illustration, such a rain-fall as King and Fitzroy encountered in Patagonia to have taken place under the supposed cloud region of the antarctic circle, and to have been hail or snow instead of rain, then the total amount of caloric set free among the clouds, in those 41 days of such a flood, would be enough to raise from freezing to boiling six and a half times as much water as fell. But if the supposed antarctic precipitation come down in the shape of rain, then the heat set free would be sufficient only to raise from freezing to boiling about of as much water as the flood brought down. We shall have, perhaps, a better idea of the amount of heat that would be set free in the condensation and congelation in the antarctic regions of as much vapour as it took to make the Patagonian rain-fall, if we vary the illustration by supposing this rain-fall of 153.75 inches to extend over an area of 1000 square miles, and that it fell as snow or hail. The latent heat set free among the clouds during these 41 days would have been sufficient to raise from the freezing to the boiling point all the water in a lake 1000 square miles in area and 83i feet in depth. The unknown area of the antarctic is eight millions of square miles. We now see how the cold of the poles, by facilitating precipitation, is made to react and develop heat to expand the air, and give force to the winds.

831. Offices of icebergs in the meteorological machinery.—Thus we obtain another point of view from which we may contemplate, in a new aspect, the icebergs which the antarctic regions send forth in such masses and numbers. They are a part of the meteorological machinery of our planet. The offices which they perform as such are most important, and oh, how exquisite! While they are in the process of congelation the heat of fluidity is set free, which, whether it be liberated by the freezing of water at the surface of the earth, or of the rain-drop in the sky, helps in either case to give activity and energy to the southern system of circulation by warming and expanding the air at its place of ascent. Thus the water, which by parting with its heat of liquefaction, has expended its meteorological energy in giving dynamical force to the air, is like the exhausted steam of the engine; it has exerted its power and become inert. It is, therefore, to be got out of the way. In the grand meteorological engine which drives the wind through his circuits, and tempers it to beast, bird, and plant, this waste water is collected into antarctic icebergs, and borne away by the currents to more genial climes, where the latent heat of fluidity which they dispensed to the air in the frigid zone is restored, and where they are again resolved into water, which, approaching the torrid zone in cooling streams, again joins in the work and helps to cool the air of the trade-winds, to mitigate climate, and moderate the gale. For, if the water of southern seas were warmer, evaporation would be greater; then the S.E. trade-winds would deliver vapour more abundantly to the equatorial calm belts; this would make precipitation there more copious, and the additional quantity of heat set free would give additional velocity to the inrushing trade-winds. Thus it is, as has already been stated, that, parallel for parallel, between 40° or 50° north and south, trans-equatorial seas are cooler than cis-equatorial; thus it is that icebergs are employed to push forward the winds in the polar regions, to hold them back in the equatorial; and thus it is that, in contemplating the machinery of the air, we perceive how icebergs are "coupled on," and made to perform the work of regulator, with adjustments the most beautiful, and compensations the most exquisite, in the grand machinery of the atmosphere.

832. The antarctic calm place a region of constant precipitation.—With this illustration concerning the dynamical force which the winds derive, from the vapour taken up in one climate and transported to another, we may proceed to sketch those physical features which, being found in the antarctic circle, would be most favourable to heavy and constant precipitation, and, consequently, to the development of a system of aerial circulation peculiarly active, vigorous, and regular for the aqueous hemisphere, as the southern in contrast with the northern one may be called. These vapour-bearing winds which brought the rains to Patagonia are—I wish to keep this fact in the reader's mind—the counter-trades (§ 257) of the southern hemisphere. As such they have to perform their round in the grand system of aerial circulation, and as, in every system of aerial circulation there must be some point or place at which motion ceases to be direct and commences to be retrograde, so there must be a place somewhere on the surface of our planet where these winds cease to go forward, stop, and commence their return to the north; and that place is, in all probability, within the antarctic regions. Its precise locality has not been determined, but I suppose it to be a band or disc—an area—within the polar circle, which, could it be explored, would be found, like the equatorial calm belt, a place of light airs and calms, of ascending columns of air,—a region of clouds, of variable winds, and constant precipitation.

833. Also of a low barometer.—But, be that as it may, the air which these vapour-bearing winds—vapour-bearing because they blow over such an immense tract of ocean—pour into this stopping-place has to ascend and flow off as an upper current, to make room for that which is continually flowing in below. In ascending it expands and grows cool, and, as it grows cool, condensation of its vapour commences; with this, vast quantities of latent heat, which converted the water out at sea into vapour for these winds, are set free in the upper air. There it reacts by warming the ascending columns, causing them still farther to expand, and so to rise higher and higher, while the barometer sinks lower and lower. This reasoning is suggested not only by the facts and circumstances already stated as well known, but it derives additional plausibility for correctness by the low barometer of these regions. In the equatorial calm belts the mean barometric pressure is about 0.25 inch less than it is in the trade-winds, and this diminution of pressure is enough to create a perpetual influx of the air from either side, and to produce the trade-winds. Off Cape Horn the mean barometric pressure^[5] is 0.75 inch less than in the trade-wind regions. This is for the parallel say of 57°—8° S. According to the mean of 2,472 barometric observations made along that part only of the route to Australia which lies between the meridians of the Cape of Good Hope and Melbourne, the mean barometric pressure on the polar side of 42° S. has been shown by Lieutenant Van Gough, of the Dutch Navy, to be 0.33 inch less than it is in the trade-winds. The mean pressure in this part of the South Indian Ocean is, under winds with easting in them, 29.8 inches: ditto, under winds with westing, 29.6 inches. Plate I. shows a supposed mean pressure in the polar calms of not more than 28.75 inches. The barometric curve, page 468, shows with a higher degree of probability that the mean pressure there is about 28.14 inches.

834. Aqueous vapour the cause of both.—To what, if not to the effects of the condensation of vapour borne by those surcharged winds, and to the immense precipitation in the austral regions, shall we ascribe this diminution of the atmospherical pressure in high south latitudes? It is not so in high north latitudes, except about the Aleutian Islands of the Pacific, where the sea to windward is also wide, and where precipitation is frequent, but not so heavy. The steady flow of "brave" winds towards the south would seem to call for a combination of physical conditions about their stopping-place in the antarctic regions, exceedingly favourable to rapid, and heavy, and constant precipitation there. The rain-fall at Cherraponjie and on the slopes of the Patagonian Andes reminds us what those conditions are. There mountain masses seem to perform in the chambers of the upper air the office which the jet of cold water does for the exhausted steam in the condenser of the engine. The presence of land, not water, about this south polar stopping-place is therefore suggested; for the sea is not so favourable as the mountains are for aqueous condensation.

835. The topographical features of the antarctic hands.—By the terms in which our proposition has been stated, and by the manner in which the demonstration has been conducted, the presence in the antarctic regions of land in large masses is called for; and if we imagine this land to be relieved by high mountains and lofty peaks, we shall have in the antarctic continent a most active and powerful condenser. If, again, we tax imagination a little farther, we may, without transcending the limits of legitimate speculation, invest that unexplored land with numerous and active volcanoes. If we suppose this also to be the case, then we certainly shall be at no loss for sources of dynamical force sufficient to give that freshness and vigour to the atmospherical circulations which observations have abundantly shown to be peculiar to the southern hemisphere. Neither under such physical aspects need it be any longer considered paradoxical to ascribe the polar tendency of the "brave west winds" to rarefaction by heat in the antarctic circle. This heat is relative, and though it be imparted to air far below the freezing-point, raising its temperature only a few degrees, its expansive power for that change is as great when those few degrees are low down as it is when they are high up on the scale. If such condensation of vapour do take place, then liberation of heat and expansion of air must follow, and consequently the oblateness of the atmospherical covering of our planet will be altered; the flattening about the poles will be relieved by the intumescence of the expanded and ascending air, which, protruding above the general level of the aerial ocean, will receive an impulse equatorially, as well from the mere derangement of equilibrium as from the centrifugal forces of tho revolving globe. And so this air, having parted with its moisture, and having received the expansive force of all the latent heat evolved in the process of vaporous condensation, will commence its return towards the equator as an upper current of dry air.

836. A perpetual cyclone.—Arrived at this point of the investigation, we may contemplate the whole system of these "brave west winds" in the light of an everlasting cyclone on a gigantic scale. The antarctic continent is in its vortex, about which the wind, in the great atmospherical ocean all around the world, from the pole to the edge of the calm belt of Capricorn, is revolving in spiral curves, continually going with the hands of a watch, and twisting from left to right.

837. Discovery of design in the meteorological machinery.—In studying the workings of the various parts of the physical machinery that surrounds our planet, it is always refreshing and profitable to detect, even by glimmerings never so faint, the slightest tracings of the purpose which the Omnipotent Architect of the universe designed to accomplish by any particular arrangement among its various parts. Thus it is in this instance: whether the train of reasoning which we have been endeavouring to follow up, or whether the arguments which we have been adducing to sustain it be entirely correct or not, we may, from all the facts and circumstances that we have passed in review, find reasons sufficient for regarding in an instructive, if not in a new light, that vast waste of waters which surrounds the unexplored regions of the antarctic circle. It is a reservoir of dynamical force for the winds—a regulator in the grand meteorological machinery of the earth. The heat which is transported by the vapours with which that sea loads its superincumbent air is the chief source of the motive power which gives to the winds of the southern hemisphere, as they move through their channels of circulation, their high speed, great regularity, and consistency of volume. And this insight into the workings of the wonderful machinery of sea and air we obtain from comparing together the relative speed of vessels as they sail to and fro upon intertropical seas!

838. Indications which the winds afford concerning the unexplored regions of the south.—Such is the picture which, after no little labour, much research, and some thought, the winds have enabled us to draw of certain unexplored portions of our planet. As we have drawn the picture, so, from the workings of the meteorological machinery of the southern hemisphere, we judge it to be. The evidence which has been introduced is meteorological in its nature, circumstantial in its character, we admit; but it shows the idea of land in the antarctic regions—of much land, and high land—to be plausible at least. Not only so: it suggests that a group of active volcanoes there would by no means be inconsistent with the meteorological phenomena which we have been investigating. True, volcanoes in such a place may not be a meteorological necessity. We cannot say that they are; yet the force and regularity of the winds remind us that their presence there would not be inconsistent with known laws. According to these laws, we may as well imagine the antarctic circle to encompass land as to encompass water. We know, ocularly, but little more of its topographical features than we do of those of one of the planets; but, if they be continental, we surely may, without any unwarrantable stretch of the imagination, relieve the face of nature there with snow-clad mountains, and diversify the landscape with flaming volcanoes. None of these features are inconsistent with the phenomena displayed by the winds. Let us apply to other departments of physics, and seek testimony from other sources of information. None of the evidence to be gathered there will appear contradictory—it is rather in corroboration. Southern explorers, as far as they have penetrated within the antarctic circle, tell us of high lands and mountains of ice; and Ross, who went farthest of all, saw volcanoes burning in the distance.

839. 'Their extent; Plate XIV.—The unexplored area around the south pole is about twice as large as Europe. This untravelled region is circular in shape, the circumference of which does not measure less than 7000 miles. Its edges have been penetrated here and there, and land, whenever seen, has been high and rugged. Plate XIV. shows the utmost reach of antarctic exploration. The unexplored area there is quite equal to that of our entire frigid zone. Navigators on the voyage from the Cape of Good Hope to Melbourne, and from Melbourne to Cape Horn, scarcely ever venture, except while passing Cape Horn, to go on the polar side of 55° S. The fear of icebergs deters them. These may be seen there drifting up towards the equator in large numbers and large masses all the year round. I have encountered them myself as high up as the parallel of 37°—8° S. The belt of ocean that encircles this globe on the polar side of 55° S. is never free from icebergs. They are found in all parts of it the year round. Many of them are miles in extent and hundreds of feet thick. The area on the polar side of the 55th parallel of south latitude comprehends a space of 17,784,600 square miles. The nursery for the bergs, to fill such a field, must be an immense one; such a nursery cannot be on the sea, for icebergs require to be fastened firmly to the shore until they attain full size. They therefore, in their mute way, are loud with evidence in favour of antarctic shore lines of great extent, of deep bays where they may be formed, and of lofty cliffs whence they may be launched.

840. A physical law concerning the distribution of land and water.—There is another physical circumstance which obtains generally with regard to the distribution of land and water over the surface of the earth, and which, as far as it goes, seems to favour the hypothesis of much land about the south pole; and that circumstance is this : It seems to be a physical necessity that land should not be antipodal to land. Except a small portion of South America and Asia, land is always opposite to water. Mr. Gardner has called attention to the fact that only one twenty-seventh part of the land is antipodal to land. The belief is, that on the polar side of 70° north we have mostly water, not land. This law of distribution, so far as it applies, is in favour of land in the opposite zone. Finally, geographers are agreed that, irrespective of the particularized facts and phenomena which we have been considering, the probabilities are in favour of an antarctic continent rather than of an antarctic ocean.

841. Dr. Jilek.—"There is now no doubt," says Dr. Jilek, in his Lehrbuch der Oceanographie, "that around the south pole there is extended a great continent mainly within the polar circle, since, although we do not know it in its whole extent, yet the portions with which we have become acquainted, and the investigations made, furnish sufficient evidences to infer the existence of such with certainty. This southern or antarctic continent advances farthest northward in a peninsula S.S.E. of the southern end of America, reaching in Trinity Land almost to 62° south latitude. Outwardly these lands exhibit a naked, rocky, partly volcanic desert, with high rocks destitute of vegetation, always covered with ice and snow, and so surrounded with ice that it is difficult or impossible to examine the coast very closely. * * *

842. Antarctic expeditions.—"The principal discoverers of these coasts are (Wilkes), D'Urville, and Ross (the younger), of whom the latter, in 1842, followed a coast over 100 miles between 72° and 79° south latitude, and 160° and 170° east longitude, to which he gave the name Victoria Land, and on which he discovered a volcano (Erebus) 10,200 feet high in 167° east longitude and 77° south latitude, as well as another extinct one (Terror) 10,200 feet high, and then discovered the magnetic south pole." ^[6]

1.  That this correction is not too large is indicated not only by the experiments which Admiral Chabaunes, in command of the French fleet on the coast of Brazil, has kindly caused to be made by the brig " Zebra," but by experiments which he has subsequently made with the frigate "Alceste," as per the following extracts of a letter, just received from that distinguished officer, dated—
   "Frigate ' L'Alceste,' at Monte Video, May 10, 1860.

   "Dear Sir,—I am replying to your letter of the 30th of January last, which I have received with the duplicates of that dated January 15th, 1851), which you have had the goodness to send me through M. de Montholon, as well as the interesting nautical monograph No. 1, and, finally, the manuscript passage from an article relative to the force of the winds to the height and velocity of clouds and waves. I regret exceedingly that I have not under my orders a squadron of sailing vessels with which I could put to sea, and devote myself to all these researches. Unfortunately, I have only steamers in my division, with the exception of the frigate which carries my flag, and political circumstances have retained me almost constantly in the La Plata. I am now approaching the end of my command, and in a few days must leave Monte Video to engage in some hydrographical works on the coast of Brazil, and afterwards I shall return to France, where I expect to arrive in the beginning of September. I shall not fail during this voyage to make a series of experiments such as you  have indicated, in order to establish the relative force of the trade-winds in the southern and in the northern hemisphere—determining the differences of velocity of the frigate with wind aft and wind abeam.

   "In my last letter, of the 25th of January, I gave you the results obtained by the brig 'Zebra,' but they are very incomplete; and I wish, with the 'Alceste,' to determine the comparative velocities obtained with a perfectly regular wind in the different rates of going from the nearest the wind to wind full aft. It is evident that the differences observed in this manner will be dependent on the special qualities of each vessel used in the experiments, and that it will be also necessary to take full account of the state of the sea; but by multiplying experiments, one will certainly arrive at an average in which confidence may be placed. I think with you, that for a ship sailing 6 knots, with wind full aft, there will be an increase of speed of 2-5 to 3 knots, with the wind a little abaft the beam. I have already had occasion to remark that for the 'Alceste' this difference sometimes exceeds 3 knots. My next letter will give you the details of the experiments which I shall have made in this respect, both south and north of the Line, which will necessarily lengthen my voyage a little. I am too anxious to co-operate in my feeble way in the great works you are carrying on, not to sacrifice willingly some hours daily whenever circumstances will permit."

2. De Stormen nabij de Kaap de Goede Hope in verband beschouwd met de Temperatuur der Zee.
3. Report of the British Association for 1852, p. 256.
4. See Espy's Philosophy of Storms.
5. Maury's Sailing Directions, 6th ed., lSo4, p. 692; ditto, 8th ed., 1859, vol. ii., p. 450.
6. Text-book of Oceanography for the Use of the Imperial Naval Academy, by Dr. August Jilek, Vienna, 1857.