Physical Geography of the Sea and its Meteorology
AND ITS METEOROLOGY.
ELEVENTH EDITION, REVISED,
BEING THE SECOND EDITION OF THE AUTHOR'S RECONSTRUCTION OF THE WORK.
ILLUSTRATED WITH NUMEROUS CHARTS AND DIAGRAMS.
SAMPSON LOW, SON & MAESTON, 14 LUDGATE HILL.
[The right of Translation is reserved.]
LONDON; PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET
AND CHARING CROSS.
The Physical Geography of the Sea is a new department of human knowledge. It has resulted from that beautiful and admirable system of physical research, in which all the maritime nations have agreed to unite; and for the furtherance of which bureaux have been established, especially in Holland, England, France, and the United States.
Consequently, research has become very active in this field; it is diligent, too; and in proportion to that activity and that diligence has been the advancement of our knowledge concerning the PHYSICAL GEOGRAPHY OF THE SEA AND ITS METEOROLOGY. It may be doubted whether progress in any department of science has been more rapid than it has been in this.
The first treatise upon this subject appeared in America six years ago. Since then such has been the richness of the harvest of facts gathered, that the work has undergone frequent amendments and improvements; indeed, within that time it has been almost entirely re-written thrice. This re-writing was necessary because it is a main motive with the author to have the work keep pace with the science itself. The consequence has been, that each re-cast has really made a new work of it.
The present edition is not only greatly enlarged above its predecessors, but it is believed to be greatly enriched and improved also. It may even be doubted whether in the variety, extent, and value of the information now for the first time presented touching the sea and air, this edition is not so far in advance of former editions as really to make this a new work. Where error has been found in previous editions, it has been corrected in this,—where further research has confirmed opinions that in them were ventured as such, the confirmation is here given.
The present edition contains a number of chapters entirely new and not to be found in any of its predecessors. Most, if not all the chapters contained in them, have also been enlarged, amended, and improved.
In short, the author desires here to state to the friends and students of this beautiful and elevating science, that it is progressive—that occupying with regard to it somewhat the relation of a pioneer, his object has been, is, and shall be, truth.
The primary object of the researches connected with "the Wind and Current Charts," out of which has grown this Treatise, was to collect the experience of every navigator as to the winds and currents of the ocean, to discuss his observations upon them, and then to present the world with the results on charts for the improvement of commerce and navigation.
By putting down on a chart the tracks of many vessels on the same voyage, but at different times, in different years, and during all seasons, and by projecting along each track the winds and currents daily encountered during the voyage, it was plain that navigators hereafter, by consulting such a Record, would have for their guide the results of the combined experience of all whose tracks were thus pointed out.
Perhaps it might be the first voyage of a young navigator to the given port, when his own personal experience of the winds to be expected, the currents to be encountered by the way, would itself be blank. If so, there would be the wind and current chart for reference. It would spread out before him the tracts of a thousand vessels that had preceded him on the same voyage, wherever it might be, and that, too, at the same season of the year. Such a chart, it was held, would show him not only the tracks of the vessels, but the experience also of each master as to the winds and currents by the way, the temperature of the ocean, and the variation of the needle. All this could be taken in at a glance, and thus the young mariner, instead of groping his way along until the lights of experience should come to him by the slow teachings of the dearest of all schools, would here find, at once, that he had already the experience of a thousand navigators to guide him on his voyage. He might, therefore, set out upon his first voyage with as much confidence in his knowledge, as to the winds and currents he might expect to encounter, as though he himself had already been that way a thousand times before.
Such a chart could not fail to commend itself to intelligent ship masters, and such a chart was constructed for them. They took it to sea, they tried it, and to their surprise and delight they found that, with the knowledge it afforded, the remote corners of the earth were brought closer together, in some instances, by many days' sail. The passage hence to the equator alone was shortened ten days. Before the commencement of this undertaking, the average passage to California was 183 days; but with these charts for their guide, navigators have reduced that average, and brought it down to 135 days.
Between England and Australia, the average time going, without these charts, is ascertained to be 124 days, and coming, about the same; making the round voyage one of about 250 days on the average.
These charts, and the system of research to which they have given rise, bid fair to bring that colony and the mother country nearer by many days, reducing in no small measure the average duration of the round voyage.
At the meeting of the British Association of 1853, it was stated by a distinguished member—and the statement was again repeated at its meeting in 1854—that in Bombay, whence he came, it was estimated that this system of research, if extended to the Indian Ocean, and embodied in a set of charts for that sea, such as I have been describing, would produce an annual saving to British commerce, in those waters alone, of one or two millions of dollars; and in all seas, of ten millions. A system of philosophical research which is so rich with fruits and abundant with promise could not fail to attract the attention and commend itself to the consideration of the seafaring community of the whole civilized world. It was founded on observation; it was the result of the experience of many observant men, now brought together for the first time, and patiently discussed. The results tended to increase human knowledge with regard to the laws and phenomena of both sea and air; and therefore the system of research could not be wanting in attractions to right-minded men.
The results of the first chart, however, though meagre and unsatisfactory, were brought to the notice of navigators; their attention was called to the blank spaces, and the importance of more and better observations than were generally contained in the old sea-logs was urged upon them.
They were told that if each one would agree to co-operate in a general plan of observations at sea, and would send regularly, at the end of every cruise, an abstract log of his voyage to the National Observatory at Washington, he should, for so doing, be furnished, free of cost, with a copy of the charts and sailing directions that might be founded upon those observations.
The quick, practical mind of the enterprising ship-master seized the proposition at once. To him the field was inviting, for he saw in it the promise of a rich harvest and of many useful results.
So, in a little while, there were more than a thousand navigators engaged day and night, and in all parts of the ocean, in making and recording observations according to a uniform plan, and in furthering this attempt to increase our knowledge as to the winds and currents of the sea, and other phenomena that relate to the safe navigation of its waters, and to its physical geography.
To enlist the service of such a large corps of observers, and to have the attention of so many clever and observant men directed
aggregate of $2,250,000 saved per annum. This is on the outward voyage alone, and the tonnage trading with all other parts of the world is also left out of the calculation. Take these into consideration, and also the fact that there is a vast amount of foreign tonnage trading between these places and the United States, and it will be seen that the annual sum saved will swell to an enormous amount."—Extract from Hunt's Merchant's Magazine, May, 1854. to the same subject, was a great point gained: it was a giant stride in the advancement of knowledge, and a great step towards its spread upon the waters.
Important results soon followed, and valuable discoveries were made. These attracted the attention of the commercial world, and did not escape the notice of philosophers generally.
The field was immense, the harvest was plenteous, and there was both need and room for more labourers. Whatever the reapers should gather, or the merest gleaner collect, was to insure to the benefit of commerce and navigation—the increase of human knowledge—the good of all.
Therefore, all who use the sea were equally interested in the undertaking. The government of the United States, so considering the matter, proposed a uniform system of observations at sea, and invited all the maritime states of Christendom to a conference upon the subject.
This conference, consisting of representatives from France, England, and Russia, from Sweden and Norway, Holland, Denmark, Belgium, Portugal, and the United States, met in Brussels, August 23, 1853, and recommended a plan of observations which should be followed on board the vessels of all friendly nations, and especially of those there present in the persons of their representatives.
Prussia, Spain, Sardinia, Oldenberg and Hanover, the Holy See, the free city of Hamburg, the republics of Bremen and Chili, and the empires of Austria and Brazil, have since offered their co-operation also in the same plan.
Thus the sea has been brought regularly within the domains of philosophical research, and crowded with observers.
In peace and in war these observations are to be carried on, and, in case any of the vessels on board of which they are conducted may be captured, the abstract log—as the journal which contains these observations is called—is to be held sacred.
The illustrious Humboldt, several years before his death, expressed the opinion that the results already obtained from this system of research had given rise to a new department of science, which he called the physical geography of the sea.
Rarely before has there been such a sublime spectacle presented to the scientific world: all nations agreeing to unite and cooperate in carrying out according to the same plan, one system of philosophical research with regard to the sea. Though they may be enemies in all else, here they are to be friends. Every ship that navigates the high seas with these charts and blank abstract logs on board may henceforth be regarded as a floating observatory, a temple of science. The instruments used by every co-operating vessel are to be compared with standards that are common to all; so that an observation that is made anywhere and in any ship may be referred to and compared with all similar observations by all other ships in all parts of the world.
But these meteorological observations which this extensive and admirable system includes will relate only to the sea. This is not enough. The plan should include the land also, and be universal. Other great interests of society are to be benefited by such extension no less than commerce and navigation have been. A series of systematic observations, directed over large districts of country, nay, over continents, to the improvement of agricultural and sanitary meteorology, would, I have no doubt, tend to the development of many interesting, important, and valuable results.
With proper encouragement, this plan of research is capable of great expansion. With the aid of the magnetic telegraph, and by establishing a properly devised system of daily weather reports by telegram, sentinels upon the weather may be so posted that we may have warning in advance of every storm that traverses the country. Holland, France, and England, have recently established such a plan of daily weather reports from certain stations. And Admiral Fitzroy, at the head of the Meteorological Department of the Board of Trade in London, informs me that already, though the plan went into operation only in the month of September, 1860, yet it is most rich with the promise of a fine harvest of practical results.
The agricultural societies of many states of America have addressed memorials to the American Congress, asking for such extension over that continent.
This plan contemplates the co-operation of all the states of Christendom, at least so far as the form, method, subjects of observations, time of making them, and the interchange of results are concerned. Great good is to come of it—shipwrecks and disasters are to be prevented by it—the public weal is to be promoted by it, the convenience of society is to be enhanced by it, the bounds of human knowledge are to be enlarged by it, and it is hoped that the friends of meteorology, and all who may find interest or pleasure in a perusal of these passages, will lend their assistance to the carrying out of this plan, by advocating it among their friends. These researches for the land look not only to the advancement of the great interests of sanitary and agricultural meteorology, but they involve also a study of the laws which regulate the atmosphere, and call for a careful investigation of all its phenomena.
Another beautiful feature in this system is, that it costs nothing additional. The instruments that these observations at sea call for are such as are already in use on board of every well-conditioned ship, and the observations that are required are precisely those which are necessary for her safe and proper navigation.
As great as is the value attached to what has been accomplished by these researches in the way of shortening passages and lessening the dangers of the sea, a good of higher value is, in the opinion of many seamen, yet to come out of the moral, the educational influence which they are calculated to exert upon the seafaring community of the world. A very clever English ship-master, speaking recently of the advantages of educational influences among those who intend to follow the sea, remarks:
"To the cultivated lad there is a new world spread out when he enters on his first voyage. As his education has fitted, so will he perceive, year by year, that his profession makes him acquainted with things new and instructive. His intelligence will enable him to appreciate the contrasts of each country in its general aspect, manners, and productions, and in modes of navigation adapted to the character of coast, climate, and rivers. He will dwell with interest on the phases of the ocean, the storm, the calm, and the breeze, and will look for traces of the laws which regulate them. All this will induce a serious earnestness in his work, and teach him to view lightly those irksome and often offensive duties incident to the beginner." 
And that these researches do have such an effect many noble-hearted mariners have testified. Captain Phinney, of the American ship "Gertrude," writing from Callao, January, 1855, thus expresses himself: "Having to proceed from this to the Chincha Islands and remain three months, I avail myself of the present opportunity to forward to you abstracts of my two passages over your southern routes, although not required to do so until my own return to the United States next summer; knowing that you are less amply supplied with abstracts of voyages over these regions than of many other parts of the ocean, and, such as it is, I am happy to contribute my mite towards furnishing you with material to work out still farther towards perfection your great and glorious task, not only of pointing out the most speedy routes for ships to follow over the ocean, but also of teaching us sailors to look about us, and see by what wonderful manifestations of the wisdom and goodness of the great God we are continually surrounded.
"For myself, I am free to confess that for many years I commanded a ship; and, although never insensible to the beauties of nature upon the sea or land, I yet feel that, until I took up your work, I had been traversing the ocean blindfolded. I did not think; I did not know the amazing and beautiful combination of all the works of Him whom you so beautifully term 'the Great First Thought.'
"I feel that, aside from any pecuniary profit to myself from your labours, you have done me good as a man. You have taught me to look above, around, and beneath me, and recognize God's hand in every element by which I am surrounded. I am grateful for this personal benefit. Your remarks on this subject, so frequently made in your work, cause in me feelings of the greatest admiration, although my capacity to comprehend your beautiful theory is very limited.
"The man of such sentiments as you express will not be displeased with, or, at least, will know how to excuse, so much of what (in a letter of this kind) might be termed irrelevant matter. I have therefore spoken as I feel, and with sentiments of the greatest respect."
Sentiments like these cannot fail to meet with a hearty response from all good men, whether ashore or afloat. Admiral Fitzroy, admitting the value of the practical results already derived by commerce and navigation from these researches, is of opinion that their influence in improving and elevating the mind of the British seaman also, can scarcely be of less importance.
Never before has such a corps of observers been enlisted in the cause of any department of physical science as is that which is now about to be engaged in advancing our knowledge of the Physical Geography of the Sea, and never before have men felt such an interest with regard to this knowledge.
Under the term "Physical Geography and its Meteorology," will be included a philosophical account of the winds and currents of the sea; of the circulation of the atmosphere and ocean; of the temperature and depth of the sea; of the wonders that lie hidden in its depths; and of the phenomena that display themselves at its surface. In short, I shall treat of the economy of the sea and its adaptations—of its salts, its waters, its climates, and its inhabitants, and of whatever there may be of general interest in its commercial uses or industrial pursuits;—for all such things pertain to this department of science.
The object of this work, moreover, is to show the present state, and, from time to time, the progress of this new and beautiful system of research, as well as of the advancement made in this interesting department of science; and the aim of the author is to present the gleanings from this new field in a manner that may be interesting and instructive to all, whether old or young, ashore or afloat, who desire a closer look into "the wonders of the great deep," or a better knowledge as to its winds, its adaptations, or its Physical Geography. The results that are embodied in Plate I. alone of this edition would, had the data for it been collected by a force specially employed for the purpose, have demanded constant occupation from a fleet of ten sail for more than one hundred years. The coordinating of these observations after they were made, and the bringing of them to the present condensed form, has involved a vast amount of additional labour. Officers here have been engaged upon the work for many years. This patient industry has been rewarded with the discovery of laws and the development of truths of great value in navigation and very precious to science.
It would be presumptuous to claim freedom from error for a work like this: true progress consists in the discovery of error as well as of truth. But I may be pardoned for saying that the present edition of this work will be found to contain more of truth and less of error than any of its predecessors, simply because it is founded on wider research, and based on the results of more abundant observations than they. Indeed, it could not, or, rather, it should not be otherwise; for, as long as we are making progress in any field of physical research, so long must the results continue to increase in value; and just so long must what at first was conjecture grow and gain as truth, or fade and fall as error.
The fact seems now to be clearly established that the atmosphere is very unequally divided on opposite sides of the equator, and that there is a mild climate in the unknown regions of the antarctic circle. Over the extra-tropical regions of our planet, the atmosphere on the polar side of 40° N. and 40° S. is so unequally divided as to produce an average pressure, according to the parallel, of from 10 to 50 lbs. less upon the square foot of sea surface in southern than upon the square foot of sea surface in northern latitudes. These, and many other developments not
Mr. Trembley  to conclude that there are, "at the bottom of the water, mountains, plains, valleys, and caverns, just as upon the land."
But by far the most interesting and valuable book touching the physical geography of the Mediterranean is Admiral Smyth's last work, entitled "The Mediterranean; a Memoir, Physical, Historical, and Nautical. By Rear-AdmiralWilliam Henry Smyth, K.S.F., D.C.L.," &c. London: John W. Parker and Son. 1854.
less interesting, seemed to call for a re-cast of the work. Indeed several new chapters have been added to this edition, and many new subjects have been treated of in it. New views also have been presented, and the errors of former views corrected wherever in them farther research has pointed out error. These researches have grown so wide that they comprehend not only the physics of the sea, but they relate extensively to its meteorology also; hence the present title, The Physical Geography of the Sea, and its Meteorology.
1, Albemarle Street, London,
20th November, 1860.
- The outward passage, it has since been ascertained, has been reduced to 97 days on the average, and the homeward passage has been made in 63 under canvas alone.
- See Inaugural Address of the Earl of Harrowby, President of the British Association at its 24th meeting. Liverpool, 1854.
- . . . " Now let us make a calculation of the annual saving to the commerce of the United States effected by those charts and sailing directions. According to Mr. Maury, the average freight from the United States to Rio Janeiro is 17⋅7 cts. per ton per day; to Australia, 20 cts.; to California, also, about 20 cts. The mean of this is a little over 19 cts. per ton per day; but to be within the mark, we will take it. at 15, and include all the ports of South America, China, and the East Indies. "The sailing directions have shortened the passages to California 30 days, to Australia 20, to Rio Janeiro 10. The mean of this is 20, but we will take it at 15, and also include the above-named ports of South America, China, and the East Indies. "We estimate the tonnage of the United States engaged in trade with these places at 1,000,000 tons per annum. "With these data we see that there has been effected a saving for each one of these tons of 15 cents per day for a period of 15 days, which will give an
- "The Log of a Merchant Officer; viewed with reference to the Education of young Officers and the Youth of the Merchant Service. By Robert Methren, Commander in the Peninsular and Oriental Company, and author of the 'Narrative of the Blenheim Hurricane of 1851.'" London: John Weale, 59 High Holborn; Smith, Elder & Co., Cornhill; Ackerman & Co., Strand. 1854.
- There is an old and very rare book which treats upon some of the subjects to which this little work relates. It is by Count L. F. Marsigli, an Italian, and is called Natural Description of the Seas. The copy to which I refer was translated into Dutch by Boerhaave in 1786. The learned count made his observations along the coast of Provence and Languedoc. The description only relates to that part of the Mediterranean. The book is divided into four chapters: the first, on the bottom and shape of the sea; the second, of sea water; the third, on the movements of sea water; and the fourth, of sea plants. He divides sea water into surface and deep-sea water; because, when he makes salt from surface water (not more than half a foot below the upper strata), this salt will give a red colour to blue paper; whereas the salt from deep-sea water will not alter the colours at all. The blue paper can only change its colour by the action of an acid. The reason why this acid (iodine?) is found in surface and not in deep-sea water is, it is derived from the air; but he supposes that the saltpetre that is found in sea water, by the action of the sun's rays and the motion of the waves, is deprived of its coarse parts, and, by evaporation, embodied in the air, to be conveyed to beasts or plants for their existence, or deposited upon the earth's crust, as it occurs on the plains of Hungary, where the earth absorbs so much of this saltpetre vapour Donati, also, was a valuable labourer in this field. His inquiries enabled
- Philosophical Transactions.
|I.||The Sea and the Atmosphere||1|
|II.||The Gulf Stream||22|
|III.||Influence op the Gulf Stream upon Climates and Commerce||53|
|V.||Rains and Rivers||105|
|VI.||Red Fogs and Sea Breezes||133|
|VII.||The Easting of the Trade-Winds, the Crossing at the Calm Belts, and the Magnetism of the Atmosphere||153|
|VIII.||Currents of the Sea||179|
|IX.||The Specific Gravity of the Sea, and the Open Water in the Arctic Ocean||206|
|X.||The Salts of the Sea||235|
|XI.||The Cloud Region, the Equatorial Cloud-Ring, and Sea Fogs||270|
|XII.||The Geological Agency of the Winds||285|
|XIII.||The Depths of the Ocean||303|
|XIV.||The Basin and Bed of the Atlantic||314|
|XV.||Sea Routes, Calm Belts, and Variable Winds||334|
|XVII.||The Climates of the Sea||382|
|XVIII.||Tide-Rips and the Sea-Drift||394|
|XIX.||Storms, Hurricanes, and Typhoons||413|
|XX.||The Winds of the Southern Hemisphere||431|
|XXI.||The Antarctic Regions and their Climatology||452|
|XXII.||The Actinometry of the Sea||468|
EXPLANATION OF THE PLATES.
Plate 1.—This Plate combines in its construction the results of 1,159,353 separate observations on the force and direction of the wind, and a little upwards of 100,000 observations on the height of the barometer at sea. The wind observations embrace a period of eight hours each, or three during the twenty-four hours. Each one of the barometric observations expresses the mean height of the barometer for the day; therefore each one of the 100,000 may itself be the mean of many, or it may be only one. Suffice it to say, that 83,334 of them were obtained by Lieutenant Andrau from the logs of Dutch ships during their voyages to and fro between the parallels of 50° N. and 36° S.; that nearly 6,000 of them were made south of the parallel of 36° S., and obtained from the log-books at the Observatory in Washington; that for the others at sea I am indebted to the observations of Captain "Wilkes, of the Exploring Expedition, of Sir James Clark Ross, on board the Erebus and Terror, in high southern latitudes, and of Dr. Kane in the Arctic Ocean. Besides these, others made near the sea have been used, as those at Greenwich, St. Petersburg, Hobart Town, etc., making upwards of 100,000 in all. This profile shows how unequally the atmosphere is divided by the equator.
The arrows within the circle fly with the wind. They represent its mean, annual direction from each quarter, and by bands 5° of latitude in breadth, and according to actual observation at sea. They show by their length the annual duration of the wind in months. They are on a scale of one twentieth of an inch per month, except the half-bearded arrows, which are on a scale twice as great, or one tenth of an inch to the month. It will thus be perceived at a glance that the winds of the longest duration are the S.E. trades, between the parallels of 5° and 10° south, where the long-feathered arrows represent an annual average of ten months.
The most prevalent winds in each band are represented by full-feathered arrows; the next by half-feathered, except between the parallels of 30° and 35° N., where the N.E. and S.W. winds, and between the parallels of 35° and 40°, where the N.W. and S.W. winds contend for the mastery as to average annual duration.
The rows of arrows on each side of the axis, and nearest to it, are projected with the utmost care as to direction, and length or duration.
The feathered arrows in the shading around the circle represent the crossing at the calm belts, and the great equatorial and polar movements by upper and lower currents of air in its general system of atmospherical circulation.
The small featherless and curved arrows, n q r s, on the shading around the circle, are intended to suggest how the trade-winds, as they cross parallels of larger and larger circumference on their way to the equator, act as an undertow, and draw supplies of pure air down from the counter-current above; which supplies are required to satisfy the increasing demands of these winds: for, as they near the equator, they not only cross parallels of larger circumference, but, as actual observations show, they also greatly increase both their duration and velocity. In like manner, the counter-trades, as they approach the poles, are going from latitudes where the parallels are larger to latitudes where the parallels are smaller. In other words, they diminish, as they approach the poles, the area of their vertical section; consequently there is a crowding out—a sloughing off from the lower current, and a joining and a turning back with the upper current. This phenomenon is represented by the small featherless and curved arrows in the periphery on the polar side of the calm belts of Cancer and Capricorn.
This dotted or shaded periphery is intended to represent a profile view of the atmosphere as suggested by the readings of the barometer at sea. This method of delineating the atmosphere is resorted to in order to show the unequal distribution of the atmosphere, particularly on the polar side of lat. 40° S.; also the piling up over the calm belts, and the depression—barometrical—over the equatorial calms and cloud ring.
The engirdling seas of the extra-tropical south suggest at once the cause of this inequality in the arrangement over them of the airy covering of our planet. Excepting a small portion of South America, the belt between the parallels of 40° and 65° or 70° south may be considered to consist entirely of sea. This immense area of water surface keeps the atmosphere continually saturated with vapour. The specific gravity of common atmospheric air being taken as unity, that of aqueous vapour is about 0.6; consequently the atmosphere is expelled thence by the steam, if, for the sake of explanation, we may so call the vapour which is continually rising up from this immense boiler. This vapour displaces a certain portion of air, occupies its place, and, being one third lighter, also makes lighter the barometric column. Moreover, being lighter, it mounts up into the cloud region, where it is condensed into clouds or rain, and the latent heat that is set free in the process assists still farther to lessen the barometric column; for the heat thus liberated warms and expands the upper air, causing it to swell out above its proper level, and so flow back towards the equator with the upper current of these regions.
Thus, though the barometer stands so low as to show that there is less atmosphere over high southern latitudes than there is in corresponding latitudes north, yet, if it were visible and we could see it, we should discover, owing to the effect of this vapour and the liberation of its latent heat, and the resulting intumescence of the lighter air over the austral regions, the actual height of this invisible covering to be higher there than it is in the boreal regions.
Taking the mean height of the barometer for the northern hemisphere to be 30 inches, and taking the 100,000 barometric observations used as data for the construction of this diagram to be correct, we have facts for the assertion that in the austral regions the quantity of air that this vapour permanently expels thence is from one twelfth to one fifteenth of the whole quantity that belongs to corresponding latitudes north—a curious, most interesting, and suggestive physical revelation.
Plates II. and III. are drawings of Brooke's Deep-sea Sounding Apparatus for bringing up specimens of the bottom (§ 573).
Plate IV. (§ 723) is intended to illustrate the extreme movements of the isotherms 50°, 60°, 70°, etc., in the Atlantic Ocean during the year. The connection between the law of this motion and the climates of the sea is exceedingly interesting.
Plate V. (§ 781) is a section taken from one of the manuscript charts at the Observatory. It illustrates the method adopted there for co-ordinating for the Pilot Charts the winds as reported in the abstract logs. For this purpose the ocean is divided into convenient sections, usually five degrees of latitude by five degrees of longitude. These parallelograms are then subdivided into a system of engraved squares, the months of the year being the ordinates, and the points of the compass being the abscissa;. As the wind is reported by a vessel that passes through any part of the parallelogram, so is it assumed to have been at that time all over the parallelogram. From such investigations as this the Pilot Charts are constructed. Plate VI illustrates the position of the channel of the Gulf Stream (Chap. II.) for summer and winter. The diagram A shows a thermometrical profile presented by cross-sections of the Gulf Stream, according to observations made by the hydrographical parties of the United States Coast Survey. The elements for this diagram were kindly furnished me by the superintendent of that work. They are from a paper on the Gulf Stream read by him before the American Association for the Advancement of Science at its meeting in Washington, 1854. Imagine a vessel to sail from the Capes of Virginia straight out to sea, crossing the Gulf Stream at right angles, and taking the temperature of its waters at the surface and at various depths. The diagram shows the elevation and depression of the thermometer across this section as they were actually observed by such a vessel.
The black lines x, y, z, in the Gulf Stream, show the course which those threads of warm waters take (§ 130). The lines a, b, show the computed drift route that the unfortunate steamer San Francisco would take after her terrible disaster in December, 1853.
Plate VII. is intended to show how the winds may become geological agents. It shows where the winds that, in the general system of atmospherical circulation blow over the deserts and thirsty lands in Asia and Africa (where the annual amount of precipitation is small) are supposed to get their vapours from; where, as surface winds, they are supposed to condense portions of it; and whither they are supposed to transport the residue thereof through the upper regions, retaining it until they again become surface winds.
Plate VIII. shows the prevailing direction of the wind during the year in all parts of the ocean. It also shows the principal routes across the seas to various places. Where the cross-lines representing the yards are oblique to the keel of the vessel, they indicate that the winds are, for the most part, ahead; when perpendicular or square, that the winds are, for the most part, fair. The figures on or near the diagrams representing the vessels show the average length of the passage in days.
The arrows denote the prevailing direction of the wind; they are supposed to fly with it; so that the wind is going as the arrows point. The half-bearded and half-feathered arrows represent monsoons (§ 630), and the stippled or shaded belts the calm zones.
In the regions on the polar side of the calms of Capricorn and of Cancer where the arrows are flying both from the north-west and the south-west, the idea intended to be conveyed is, that the prevailing direction of the wind is between the north-west and the south-west, and that their frequency is from these two quarters in proportion to the number of arrows.
Plate IX. is intended to show the present state of our knowledge with regard to the drift of the ocean, or, more properly, with regard to the great flow of polar and equatorial waters, and their channels of circulation as indicated by the thermometer (§ 742). Farther researches will enable us to improve this chart. The sargasso seas and the most favourite places of resort for the whale—right in cold, and sperm in warm weather—are also exhibited on this chart.
Plate X. (p. 208) represents the curves of specific gravity and temperature of the surface waters of the ocean, as observed by Captain John Rodgers in the U.S. ship Vincennes, on a voyage from Behring's Strait via California and Cape Horn to New York.
Plates XI. and XII. speak for themselves. They are orographic for the North Atlantic Ocean, and exhibit completely the present state of our knowledge with regard to the elevations and depressions in the bed of that sea as derived from the deep-sea soundings taken by the American and English navies from the commencement of the system to Dayman's soundings in the Bay of Biscay, 1859; Plate XII. exhibiting a vertical section of the Atlantic, and showing the contrasts of its bottom with the sea-level in a line from Mexico across Yucatan, Cuba, San Domingo, and the Cape de Verds, to the coast of Africa, marked A on Plate XI. Plate XIII.—The data for this Plate are furnished by Maury's Storm and Rain Charts, including observations for 107,277 days in the North Atlantic, and 158,025 in the South; collated by Lieutenant J. J. Guthrie, at the Washington Observatory, in 1855.
The heavy vertical lines, 5°, 10°, 15°, etc., represent parallels of latitude; the other vertical lines, months; and the horizontal lines, per cents., or the number of days in a hundred.
The continuous curve line stands for phenomena in the North, and the broken curve line for phenomena in the South Atlantic. Thus the Gales' Curve shows that in every hundred days, and on the average, in the month of January of different years, there have been observed, in the northern hemisphere, 36 gales (36 per cent.) between the parallels of 50° and 55°; whereas during the same time and between the same parallels in the southern hemisphere, only 10 gales on the average (10 per cent.) have been reported.
The fact is here developed that the atmosphere is in a more unstable condition in the North than in the South Atlantic; that we have more calms, more rains, more fogs, more gales, and more thunder in the northern than in the southern hemisphere, particularly between the equator and the 55th parallel. Beyond that, the influence of Cape Horn becomes manifest.
Plate XIV. (§ 839) shows the limits of the unexplored area about the south pole.
Plate XV. shows by curves the prevalence of winds with northing as compared with winds with southing in them in each of the two hemispheres, north and south.
Plate XVI. shows the Barometric Curve projected according to actual observations at sea, from the parallel of 78° north to the parallel of 56° south, and carried thence to the poles, by conjecture and in conformity with indications.