Low water,

apply to the explanation of the rapid current which precipitates itself from the Gulf of Mexico into the Ocean by the Straits of Bahama, would prove, hypothetically, that the Pacific and Atlantic Oceans, viewed as a whole, form a surface of the same level. We shall escape from this difficulty by relating the results of some observations made in Florida a few years since, by the French officers appointed by the American Congress, to survey the line of a canal designed to unite the river St. Marie, on the Atlantic, with the bay of Appalachicola, on the Gulf of Mexico.

According to the first result of the measurements, low water in the Gulf of Mexico would be higher than the low water of the Atlantic, by 3.73 feet. A second result gave a difference of the same nature between the two low waters, of 2.8 feet. The mean is 3.28 feet.

But even this slight inequality of level is greater than the real one. In fact, when we compare two seas subject to tides, it is evidently the mean levels, that is to say, the surfaces, equally remote from high and low water, that ought to be compared. In this instance, although I can perceive no reason for it, the comparison was made between two low waters. In order to state the matter accurately, therefore, it is necessary to elevate the surface taken for comparison in the Gulf of Mexico to half the height of the tide observed in that gulf. The same thing must be done in regard to the eastern or Atlantic side of the Floridas. In the gulf, near the point where the level was terminated, the tide does not rise more than about ten inches. On the other side of the Floridas, near the mouth of the river St. Marie, the tide is about 6:56 feet. therefore, is 2:63 feet more removed from the mean tide at St. Marie than in the gulf. If, then, the mean levels are referred to, as must be done to obtain the real result, instead of 3.28 feet, it will be found that the difference of the level of the two seas is 3.28 minus 2.63 feet, that is to say, 0.65 feet, or about eight inches.

This quantity is evidently within the limits of error, which observations embracing the whole breadth of the Floridas must be liable to. But even though the difference alleged were real, it may be doubted whether any one would now be inclined to regard an inequality so unimportant as a sufficient explanation of the cause of a current which, issuing from the Straits of Bahama, at a rate of not less than five miles an hour, con tinues its progress nearly in a straight line into the very middle of the Atlantic, to a distance of about 1200 miles, without its rapidity abating during so long a course.

Let us now consider the Mediterranean. In this sea the alleged lowness of the level,—the presumed cause of the current flowing from the Ocean towards the Straits of Gibraltar, is said to be the result of an enormous annual evaporation, which the mass of waters contributed by the Nile, the Rhone, the Po, &c. is insufficient to compensate. Direct and demonstrative proofs of this want of compensation are, it is true, completely wanting. But if this objection be advanced, a new form is immediately given to the argument, and then it is said (which is in reality the case), that in summer, at equal latitudes, the waters of the Mediterranean are about 5]° to 6° Fahr. warmer than those of the Ocean, from which it inevitably follows that the first undergo more evaporation than the others, and that nothing more is required to explain the current of the Strait.

And this, it must be confessed, would be sufficient, if the cause indicated were to produce a very sensible difference of level in the two seas. Thus, whatever may have been said of it, the problem will be found to be reduced to one of numbers, or to a question of fact. It must be ascertained, either by calculation or experiment, to what extent the Atlantic Ocean is higher than the Mediterranean. The calculation, I have already stated, will be difficult to be made with precision, owing to the want of sufficient data. With regard to experiment, the results of that which I am about to present, seem to me calculated to satisfy the most scrupulous minds.

Delambre had already found, by the great chain of triangles on the meridian which extends through France, from Dunkirk to Barcelona, the means of directly connecting the level of the two seas. The triangles comprehended between Rhodez and the Mediterranean, gave him for the vertical height of that town, a result which agreed to a fraction of a metre with the height, referred to the Ocean, that was obtained from that portion of the chain placed between Rhodez and Dunkirk.

It has already been stated in opposition to this result, that the observations from which it was deduced, were not always made under favourable circumstances; that they should have been many times repeated, if intended to be decisive of a difference of level ; and that,

; moreover, the necessary calculations had neither been made with care, nor by methods sufficiently accurate. These objections were not without weight: and for this reason the officers of the corps of Ingénieurs-géographes endeavoured to take advantage of the chains of the primordial triangles which stretch in different directions and cover the whole surface of France, in order to submit the question of the levels of the two seas to a new examination. M. Delcros, among others, devoted himself to the subject. He made extensive investigations, but which, however, are still in MS., and I have therefore to regret that I am unable to state the results in this paper. But the observations which M. Corabæuf has presented to the Academy of Sciences, are also as directly to the point as could be desired, and were conducted with a precision which it would appear difficult to exceed.

This operation, carried on along the southern frontier of France, during the years 1825, 1826, and 1827, embraced, in the line of the shortest distance, the whole of the interval lying between the Ocean and the Mediterranean. Forty-five primordial triangles, many of which have their vertices on the highest peaks of the Pyrenees, join the fort of Socoa, near St. Jean de Luz, to several points of the plain of Perpignan, the small elevation of which above the sea was obtained by two secondary triangles. All the angles were measured by M. Gambey's repeatingcircle, and were repeated three times at the least. The same was the case with the zenith distances. Care was taken, also, to make the observations between 10 A. M., and 3 or 4 P. M., only, in order to avoid the effects of irregular refraction, which takes place near the horizon some hours after sun-rising, and before his setting. The amount of atmospheric refraction, between each couple of stations, was deduced from a comparison of reciprocal zenith distances. As assistants in these important operations, M. Corabæuf had Captain Peytier and Lieutenants Hossard and Testu of the corps of Ingénieurs-géographes.

The station at Crabère is nearly in the middle of the interval which separates the Ocean from the Mediterranean. The eastern part of the chain of triangles served to determine its height above the Mediterranean; the other part gave the height above the Ocean. It is important to remark, that the calculations could have been made by a variety of distinct combinations, among which M. Corabeuf made choice of three. ascended, in the first place, from the Ocean and the Mediterranean to Crabère, following the single series of vertices which bounded the chain on the south ; then, secondly, by taking exclusively the northern vertices; then, thirdly and fourthly, by travelling diagonally, that is to say, by visiting alternately a northern vertex and a southern one. The following is the result of these various combinations :

The mean difference, 0",73 (2:4 Eng. ft.), is so small, particularly when we recollect the extent of the line which was levelled, that it cannot prevent the conclusion that, in a state of repose, the waters of the Ocean, and those of the Mediterranean, have a surface of the same level. At all events, there can be scarcely a doubt, that if any difference in this respect does exist, it is too small to be appreciated.

In this article I wish merely to prove that the subject of currents is far from being exhausted ; that differences of level, to which hydrographers have recourse for an explanation of them, are either completely nugatory, or insignificant; and that there is still room for ample investigation. This object I conceive I have attained. I shall still, however, add a few reflections.

The theory of currents has made little progress hitherto, because those phenomena have chiefly been considered which affect the surface of the sea. Currents produced by differences of saltness and of temperature exist at all depths. There are currents, for example, in contact with the very bed of the sea, which transport the cold waters of the polar zones as far as the equator. Near the poles these waters, like the solid part of the earth that supports them, move at a very slow rate, from west to east. As they pass by degrees to temperate and warm regions, they arrive at greater terrestrial parallels, which thenceforth move quicker than they, and hence the relative currents which run from the east to the west, and of which the volume is equal to that of the polar currents.

It is, if I am not deceived, by placing them in this point of view; by descending, in imagination, to the profoundest depths of the ocean ; and by applying to the sea the theory which has already given a satisfactory explanation of the trade-winds, that we shall succeed in un

ravelling the subject under consideration. It will thus, in my opinion, be equally possible to conceive how currents of very inconsiderable velocity cross such immense extents of sea; how they are inflected and reflected in their course, by the coasts of continents and islands while yet at a distance; and how they deviate when they approach banks, such as those of the Agulhas or Newfoundland, on which there is not less than fifty-five fathoms of water !

SEA OF WEEDS (Varec).- Among the phenomena of the ocean, which, although so long known to us, may yet become the subject of curious investigation, I should be inclined to place that of the Weedy Sea, or the Sea of Wrack.

These names are applied to a portion of the Atlantic Ocean, situated to the west of the Azores. It is, on an average, from forty to fifty leagues in width; its extent in latitude is 25°: and the space which it occupies is nearly equal in area to the surface of France*. It is entirely covered with plants (Fucus natans). The Portuguese call it Mar de Sargasso; Oviedo, Praderias (Prairies) de Yerva. In 1492 the companions of Christopher Columbus were greatly alarmed by it,—they conceived that they had reached the remotest limits of the navigable ocean, and expected to be stopped by the weed, as their fabulous St. Barandan had formerly been by the ice of the polar regions.

By examining a multitude of observations on the subject, deposited in the archives of the English Admiralty, in order to determine the limits of the Sea of Sargasso, Major Rennell found that this great bank of fucus had undergone no change of place between the years 1776 and 1819, either in longitude or latitude.

This remarkable permanency of position M. von Humboldt has shown to have existed so far back as the end of the fifteenth century, by discussing the observations of Columbus.

Three different explanations have been advanced to account for the existence of Fucus natans in this sea. Some are of opinion that there are, in these latitudes, numerous banks at the bottom of the ocean on which the fuci grow, and from which they are accidentally detached ; others, that these plants vegetate, and develop themselves even on the surface of the water ; but the opinion most generally received is, that the Sea of Weeds is merely the site where the Gulf-stream continually deposits the plants with which it is loaded on issuing from the Gulf of Mexico.

This last-mentioned hypothesis has been adopted by Major Rennell, although it is very far from explaining why a great proportion of the floating weeds in the Sea of Sargasso are, instead of being faded or decayed, in a state of great freshness. Indeed, English navigators never fail, when they speak of these regions, to mention the fresh weed, and the weed much decayed. Christopher Columbus himself, as M. von Humboldt has remarked, was likewise much struck with the mixture of yerba muy vieja y otra muy fresca.

Y The floating fuci of the Sea of Sargasso are always destitute of roots and fruit. If we suppose them to be developed in the same region

It would therefore be 140 to 170 miles from east to west, and about 1700 miles from north to south; an area of about 264,000 square miles.-Ed.

where they are found, we must consider them to be, as M. Meyen has done, similar to fresh-water algæ, many of which multiply only by new branches. It will likewise remain to be explained by what means it is that the waters over such a great extent of sea escape so completely from the action of winds and currents, that centuries have not been sufficient to disperse the plants which were found collected there at the end of the fifteenth century, when the galleys of Columbus ploughed through them for the first time.

It doubtless appears more natural to suppose, that, in proportion as the winds and currents drive the floating fuci beyond the ordinary limits of the Sea of Sargasso, their places at the surface are occupied by others detached from the bottom. According to this hypothesis the fuci are stationary in appearance only; the sea would always appear alike covered over the region which produces them, yet the individuals would be continually regenerated.

What, then, is necessary at the present time to throw light on this curious point in terrestrial physics ? A few experiments, which, though extremely simple, are still wanting to science,-soundings, sufficiently deep, made along the borders, and towards the centre, of the Sea of Sargasso.

[To be concluded in our next. ]

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