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and which sustains their pressure, absorbs the same volume of these gases as if they were subjected to the simple pressure of a single atmosphere; so that the weight absorbed becomes proportionably greater. If, then, the single fact of a uniform absorption, propagated from one bed to another throughout the whole mass of waters, be sufficient to account for the presence of a considerable volume of air, how greatly may the quantity be increased if it should be in proportion to the pressure due to each depth! As this saturation must have been in gradual operation ever since the seas were formed, it must also have modified gradually the preexisting atmosphere, and perhaps continues to affect the present one if the affinity which produces the saturation is not satisfied. The influence of these phenomena on the state of the atmosphere, and, consequently, on the conditions of existence of the living beings on the surface of the globe, is amply sufficient to induce us to examine them, and to measure the extent of their operation.
For this purpose, 'it is desirable to obtain sea-water from great depths, far from land, and to bring it to the surface with all the air which it contains. This air must then be disengaged by boiling, its volume measured under the ordinary pressure of the atmosphere, and finally subjected to chemical analysis. In these operations, the only difficult one is that of drawing the water from the desired depth, and bringing it to the surface with all its contents. First, care must be taken not to employ vessels which are exhausted, or filled with air only, designed to open and admit the water at the assigned depth; for the pressure to which they must be subjected before they are deep enough, will cause the water to filter through the joints of the most perfect plugs, or crush the vessel if these resist. And, secondly, if the gaseous mixture contained in the deep-lying beds is subjected to the same pressure which they undergo, it will expand inversely when the apparatus is brought near the surface, and will either escape by the plug, or burst the vessel containing it. In order to avoid these contrary effects, a hollow glass cylinder ought to be employed, closed at one extremity by a solid plate of metal, thus forming a complete bucket provided with a handle, to which a cord is attached to let it down to the depths of the sea. This bucket being empty, and open to the surrounding water, descends into the different beds without being injured by their pressure. When it has reached the required depth, another cord is pulled, this is attached by an inverted handle to its lower part, and serves to invert it. This second cord is then employed to draw up the apparatus, and in order that it may not get entangled with the first, it is worked from the other end of the ship. The cylinder of glass has two bottoms, one fixed, the other moveable. The latter is in reality the piston of an air-pump, which descends by its own weight, when the bucket is drawn up; at the same time, the fixed bottom has a small hole, furnished with a valve, opening inwards by the pressure of the surrounding water, and allowing it to enter into the empty space made by the descending piston. When this has completed its descent, and the space is filled, the valve in the fixed bottom closes by its own spring, and the admitted water is thus separated from all other during the drawing up. But if this water contain compressed air, nothing can counteract its expansive tendency, or that of the air it contains,
when it is brought to the surface, where the pressure of water externally is removed; it will then either escape or burst the apparatus. To guard against this, a free issue must be provided for all possible expansion either of the water or air. For this purpose the fixed bottom is furnished with a lateral tube which leads to a gas-bladder; the latter having been first filled with water, then emptied and pressed together before the sinking of the apparatus. This bladder will receive all the air which may be disengaged from the water on approaching the surface; and, if any be so disengaged, will return more or less inflated. Then by closing the stopcocks with which the tube is provided, the bladder may be separated from the vessel containing the water, its volume measured, and the enclosed air analyzed ; after this, the air which may still remain in the water may be examined, and also all the substances which the water may hold in solution. Such is the apparatus which has been intrusted to the commander of the Bonite ; and the zeal, as well as the intelligence, of that officer, affords us the assurance, that, under his directions, it will be usefully employed to solve the various questions indicated above, relating to terrestrial physics ; questions which, besides their purely scientific interest, have an additional importance attached to them, by the knowledge which their solution would supply respecting the permanence or variability of our atmosphere, and the conditions of existence of the animated beings which exist in the depths of the sea.
The Atlantic and Pacific Oceans, and the Mediterranean Sea, are traversed by numerous currents, which are the more dangerous, as they carry vessels out of their proper course, without the pilot suspecting it; in cloudy weather, particularly, he has no means of ascertaining their influence. Among the phenomena of the sea, and considered in their twofold connexion with theory and practice, there are certainly none more deserving than these of a high degree of attention by navigators of every country. The numerous memoirs and works specially appropriated to the subject, such as those of Ducoudray, Romme, and even the posthumous and scientific treatise of Major Rennell, which has recently appeared, are very far, in my opinion, from having exhausted the subject. Of this the reader will, finally, be able to judge.
ON THE CAUSE OF CURRENTS.-The most remarkable currents observed by navigators are, in the Atlantic :
The current, which, having gone round the Bank of the Agulhas and
the Cape of Good Hope, proceeds from south to north along the
western side of Africa, as far as the Gulf of Guinea. The current, termed equinoctial, which runs invariably from east to
west on both sides of the equator, between Africa and America. The current which, after having issued from the Gulf of Mexico by the Straits of Bahama, runs at a certain distance from the coasts of the United States in the direction of N.E. as far as the Bank
of Nantucket, where its direction is changed. Lastly, the current, by the action of which the waters of the
Ocean which wash the coasts of Spain, Portugal, and Africa,
from Cape Finisterre as far as the parallel of the Canaries, are all
directed towards the Straits of Gibraltar. What is the cause of these currents ?
The trade-winds, according to some, by continually blowing from east to west in the Indian Ocean, must produce a liquid intumescence on the eastern coast of Africa near the equator. This accumulated water flows continually from north to south through the Straits of Mosambique. When it reaches the parallel of the Cape of Good Hope, the eastern wall or mound which had hitherto maintained it being discontinued, the water necessarily flows westward. It is thus that it forms the current of the Agulhas.
The equinoctial current of the Atlantic is attributed to the constant impulsion of the trade-wind on the waters in the vicinity, and to the north and south, of the equator.
The Atlantic equinoctial current, in this respect resembling the equatorial current of the Indian Ocean, must produce a great accumulation of water along the first coast which presents itself as a barrier; this coast is America. Hence results a general movement of the Caribbean sea towards the strait which separates the eastern point of Yucatan from the western point of Cuba; this produces an elevation of the level of the sea in the Gulf of Mexico ; and this elevation, finally, is the cause of the rapid formed by the accumulated water in the Gulf, at its escape from the Strait of Bahama, the prolongation of which becomes the Gulfstream.
With respect to the current in the Straits of Gibraltar, it might be caused by a depression of the level of the Mediterranean, and this depression might be occasioned by an excessive evaporation, which the influx of the tributary rivers is insufficient to compensate.
These explanations are simple; they appear to rest on physical causes the action of which must take place in the direction that is assumed; and the most intelligent observers, Franklin, Rennell, &c., have adopted them ; yet, I am about to prove that they are not so completely established by observation, measurement, and experiment, as to prevent us from entertaining legitimate doubts on the subject.
A continued and strong wind raises the level of the sea along the coast towards which it tends to direct the water; thus, at Brest, Lorient, Rochefort, &c., the tide is always highest, all other circumstances being equal, during a west wind. So on the opposite shores of the Atlantic, and along the coasts of the United States, it is, on the contrary, the east wind which produces this effect. So it is by south winds that the waters of the Mediterranean are accumulated in the ports of Genoa, Toulon, and Marseilles, and by north winds in those of Algiers, Bugia, and Tunis. These facts are not disputed, nor do they admit of being called in question. It only remains to determine the value of the accidental changes of level which winds may produce.
Franklin relates that, in an extensive piece of water three leagues broad, and about three English feet in depth, a strong wind caused the whole of one of the sides to become dry, while it raised the water on the opposite side three feet above its former level, the depth there being six feet instead of three. In our own seas, I do not think that in general,
a higher number than this should be stated as the maximum effect produced by the most violent tempests*.
The Trade-winds are certainly constant, but their strength is extremely moderate. The depressions of the sea-surface which they occasion must, therefore, be inconsiderable. It seems difficult to admit, that the vertical fall of a yard, for example, or even two yards, can produce currents which are not entirely annihilated after a passage of many hundreds of leagues.
I have stated that the trade-winds, on account of their feeble intensity, seem little likely to produce any considerable intumescence in the waters of the ocean. I shall even go further, and prove, that, in point of fact, the very seas from which currents appear to emanate, are exactly, or very nearly, of the same level as those which these currents afterwards traverse.
It has been indisputably proved by M. Lepère, by observations made during the Egyptian expedition, that the level of the Mediterranean near Alexandria, is lower by 26.5 feet than the low water level of the Red Sea near Suez, and by 32:5 feet than the high water level at the same place.
This is certainly a very great difference of level between two seas which may be considered as communicating with each other; for, on the one hand, the Mediterranean opens into the Atlantic by the Straits of Gibraltar ; on the other, the Red Sea joins the Indian Ocean by the Straits of Bab-el-mandel ; and, thirdly, the Atlantic and Indian Ocean blend with each other at the Cape of Good Hope. It is very far from my intention to depreciate what is curious or interesting in such a result as this, but I must be allowed to say, that it throws no light on the disputed question of currents, for, to render the explanation admissible, there ought to be a sensible difference between the level of two contiguous seas, between that from which the current issues, and that into which it flows.
Further, has a difference of level been clearly proved to exist between the Gulf of Mexico in which the Gulf-stream originates, and that part of the Atlantic Ocean which washes the eastern side of the Floridas and Georgia?
The inhabitants of the Isthmus of Panama believe, but without proof, that the Pacific is higher than the Atlantic Ocean. Franklin, Rennell, &c. likewise admit a difference of elevation, but of an opposite nature. M. von Humboldt confirmed this latter opinion by barometrical observations made at Cumana, Carthagena, and Vera Cruz, compared with others made at Acapulco and Callao. At the three places first mentioned, the waters appeared to him to be about ten feet above the level of the Pacific, as taken on the western shores of Mexico and Peru. Now, as no one doubts that the general mass of the Pacific and Atlantic Oceans has the same level, that portion of the latter near the Antilles, and that which is enclosed in the Gulf of Mexico will thus form a local elevation or intumescence of about ten feet.
* Places are mentioned in the Mediterranean, where gusts of wind from the south-west (called Labeschades) have
raised the waters twenty-three feet above their ordinary level ; but these effects are entirely local,
Before citing a work which does not confirm this result, I ought to mention that my illustrious friend has himself remarked, with his usual caution, that his observations were not sufficiently numerous to place the fact of so small a difference of level beyond doubt.
Two engineers have lately crossed America at its narrowest part, in order to settle definitively the relative elevation of the two oceans. We may add that their object was not purely of a scientific nature, but had a direct reference to one of the grandest problems which commerce ever proposed,--the possibility of a communication between the Atlantic and the Pacific, across the Isthmus of Panama. Such was the object of the investigation, the results of which I am about to state, and which was intrusted by General Bolivar to Mr. Lloyd, an English engineer, and to a Swedish captain, named Falmarc.
This survey was made in 1828 and 1829. The level used was constructed by Carey, of London. The line commenced at Panama, on the Pacific Ocean, at the level of the highest tides of the equinox, corresponding to the third day of the full or of the new moon. Its termination was at a place named Bruja, to which the influence of the tide extends. Bruja is on the river Chagres, about twelve miles from the place where that river enters the sea of the Antilles.
At Panama, the mean difference of the levels of high and low water, during spring tides, is 21.2 feet. At Chagres, on the Atlantic, this difference does not exceed 1.1 foot.
By assuming, in each place, for the mean level of the ocean, a surface equally remote from the successive levels of high and low water, it follows, from the survey of Messrs Lloyd and Falmarc :
1st. That the mean level of the Pacific Ocean, at Panama, is 3.52 feet higher than the mean level of the Atlantic Ocean at Chagres.
2nd. That at the instant of high water, the Ocean on the western coast of the Isthmus, is 13.55 feet higher than on the eastern coast.
3rd. That, on the contrary, at the instant of low water on the same coasts, the Pacific Ocean is lower than the Atlantic.
These observations, seem, then, to confirm the opinion long since adopted, that the mean level of the Pacific is more elevated than the mean level of the Atlantic; but the difference, instead of being enormous, as was supposed, is only 3.52 feet. It may even be permitted to suppose, without injustice to the merits of Messrs. Lloyd and Falmarc, that, in carrying their operations through a wild country abounding with difficulties, in running a line, whose total extent, including sinuosities, is eighty-two miles, and that, in observing at 935 stations, they may have erred to the small extent of a yard and a half. It would then follow, that there is nothing to prove that there is any sensible difference between the mean levels of the two great seas which communicate with each other by the Straits of Magellan and Cape Horn*.
The labours of Messrs Lloyd and Falmarc, so far at least as they
* If, after the scientific memoirs of M. , they again assume their full majesty in von Humboldt, it is still necessary to re- Mexico, I would remark, that the most turn to the truly astonishing depression elevated point of the transverse line surthat the Cordilleras of South America pre- veyed by Messrs. Lloyd and Falmarc, is sent in the Isthmus of Panama, before only 633 feet above the level of the sea.