frequently but a few hours above the horizon of such harbours. All islands are in the midst of currents: on looking therefore at the south pole with a bird's eye view, we should see a succession of archipelagos dispersed in a spiral line all the way to the northern hemisphere, which indicates the current of the sea, just as the projection of the two continents on the side of the north pole indi. cates the current of the Atlantic. Thus, the course of the seas from one pole to the other is in a spiral line round the globe, like the course of the sun from one tropic to the other: admitting therefore the alternate fusion of the polar ices, all the phænomena of the tides and currents of the ocean may be explained with the greatest facility.

I have then established by facts simple, clear,, and numerous, the disagreement of the tides in most seas with the moon's action on the. equator, and their perfect coincidence with the sun's action on the polar ices.

I have no doubt various objections may be urged against this hasty explanation of the course of the tides, &c. But these physical causes present themselves with a higher degree of probability, simplicity, and conformity to the general progress of nature, than the astronomical causes by which it is attempted to explain them.

Thus far St. Pierre, who complains that the prejudices of mankind are so strong in favour of received opinions, that he canuot obtain a hearing.'

To the best of my judgment I have offered a fair and candid exposition of a hypothesis which he has dressed up with some eloquence and much declamation, and ushered into the world with a solemn and imposing air of confidence and assurance, tolerably well calculated to confound the ignorance and candour of his readers. I am not conscious of having omitted any material fact or argument which tends to the support and elucidation of his theory; I have neglected much absurd reasoning, yet not without retaining some curious specimens. I did once intend to have entered into a general examination of his principles and reasoning; to have shown the fal lacy of the former, the inconclusiveness and inconsistency of the latter; but I shall now be satisfied with offering a few facts and observations extracted from the second and third voyage of Cap. tain Cook, which appear to me decisive of the question.

Captain Cook, who spent three summers as near as the ice would permit his approach towards the south pole, found on December 14, 1772, and from that date to the beginning of January 1773, in latitude from 55° to 64° south, a vast compact body of ice which prevented his further progress. The thermometer varied from 30° to 35°. Being immersed 100 fathom deep for about 20 minutes, it came up 34°; and on the 13th of January 1774, on a repetition of this experiment, the open air being 36°, the surface of the sea 8310, the thermometer came up 32°. They found water generally freeze at 33?. "We certainly had no thaw, (says he,) the mercury keeping usually below the freezing point. Being near an island of ice (December 24, 1772) 50 feet high and 400 fathom in circuit, I sent the master in the jolly boat to see if any water ran from it. He soon returned with an account there was not one drop, or any other appearances of thaw." And in the summer of 1774-75 his experience was nearly similar. On the 13th of February 1775, the thermometer stood at 29°. In his third voyage to the northwest coast of America, on the 17th of August 1778, in lat, 70° 44', they were stopped by a field of ice 10 or 12 feet high, as compact as a wall; "further north it appeared much higher; here and there we saw upon it pools of water; we tried but found no current. July 7, 1779, lat. 69°; stopped by a large field of ice, presenting a great extent of solid and compact surface not in the smallest degree thawed: the thermometer stood at 31°."

"As far as our experience went, the sea is clearer of ice in August than in July, and perhaps it may be still freer in a part of September. We tried the currents, and found them never to exceed a mile an hour; we found the month of July infinitely colder than August; the thermometer in July was once 28°, and very commonly 30; whereas it was seldom as low as the freezing point in August."

"I am of opinion (says Captain Cook) that the sun contributes very little towards reducing these vast masses of ice; for, although that luminary is a considerable time above the horizon, it seldom. shines out more than a few hours at a time, and often is not seen for several days in succession. It is the wind, or rather the waves raised by the wind, that reduces the bulk of these enormous masses, by grinding one piece against another, and by undermining and washing away those parts that lie exposed to the surge; and more

ice may be destroyed in one stormy season than is formed in several winters, and its accumulation thus prevented."

This evidence clearly proves that the sun's influence at the poles, so far from being equal to produce a constant and uniform effect, creating an impulse extending its effect to the remotest parts of our globe, and a daily elevation of several feet to the waters of the ocean, is not sufficient in the hottest period of summer to diffuse a sensible thaw; and thus we are convinced that a few plain and simple facts are of much greater avail than a multitude of fanciful conjectures.

Currents, Gulph-streams, and Temperature of the Sea.

BESIDES the common and periodical tides described and ex. plained in the preceding section, a variety of local currents are frequently met with in different seas, on different parts of the ocean, for the most part not far from land. These are usually and perhaps correctly ascribed to particular winds, but they do not always appear to issue from this cause, nor is it easy to ascertain their origin; occasionally indeed they have been traced below the surface of the water, running in a contrary direction to the stratum of water above, and in such cases undoubtedly the result of something very different from winds or monsoons. This last has often been ascribed, and at times, perhaps, correctly, to the immense masses of polar ice, producing a greater degree of cold in the under than in the upper water: whence Count Rumford suspects there is an under current of cold water flowing perpetually from the poles towards the equator, even where the superior water flows from the equator towards the poles; and he thus endeavours to account for the great inferiority of temperature which is frequently found in deep and superficial soundings of the same space of water.

The following ingenious article inserted in the Philosophical Transactions for 1684, by Dr. Smith, furnishes us with various instances of under-currents, and at the same time accounts for them upon a different principle.

"In the Offing, between the North and South Foreland, it runs tide and half tide, that is, it is either ebbing water or flood on the

shore, in that part of the Downs three hours, which is, grossly speaking, the time of half a tide, before it is so off at sea. The reason of this diversity of tides I take to be from the meeting of the two seas in that narrow strait. Often when the wind has blown hard at N. E. or at W. or W. and by S. there has happened an alteration of the tides in the Thames, which ignorant people have mistakingly reckoned a prodigy. And, it is a most certain observation, that where it flows tide and half tide, though the tide of flood runs aloft, yet the tide of ebb runs under foot, that is, close by the ground; and so at the tide of ebb, it will flow under foot.

"Now, as to the Straits, there is a vast draught of water poured continually out of the Atlantic into the Mediterranean; the mouth or entrance of which between Cape Spartel or Sprat, as the seamen call it, and Cape Trafalgar, may be near seven leagues wide, the current setting strong into it, and not losing its force till it runs as far as Malaga, which is about 20 leagues within the Straits. By the benefit of this current, though the wind be contrary, if it does not overblow, ships easily turn into the Gut, as they term the narrow passage, which is about 20 miles in length. At the end of which are two towns, Gibraltar on the coast of Spain, which gives deno⚫ mination to the strait, and Ceuta on the Barbary coast; at which places Hercules is supposed to have set up his pillars. What becomes of this great quantity of water poured in this way, and of that which runs from the Euxine into the Bosphorus and Propontis, and carried at last through the Hellespont into the Ægean or Archipelago, is a curious speculation, and has exercised the ingenuity of philosophers and navigators. For there is no sensible rising of the water all along the Barbary coast, even down to Alexandria, the land beyond Tripoli, and that of Egypt lying very low, and easily to be overflowed. They observe, indeed, that the water rises three feet or three feet and in the gulf of Venice, and as much, or very near as much, all along the river of Genoa, as far as the river Arno; but this rather adds to the wonder.

"I here omit to speak at large of the several hypotheses which have been invented to solve this difficulty; such as subterraneous vents, cavities, and indraughts, exhalations by the sun-beams, the running out of the water on the African side, as if there were a kind of circular motion of the water, and that it only flowed in upon the VOL. III.

2 A

Christian shore, which latter I consider as a mere fancy, and contrary to all observation.

"My conjecture is, that there is an under-current, by which as great a quantity of water is carried out as comes flowing in. To confirm which, besides what I have said above about the difference of tides in the offing, and at the shore in the Downs, which neeessarily supposes an under-current, I shall present you with an instance of the like nature in the Baltic sound, as I received it from an able seaman, who was at the making of the trial. He told me, that being there in one of the king's frigates. they went with their pin. nace into the mid stream, and were carried violently by the current; that soon after they sunk a bucket with a large cannon ball, to a certain depth of water, which gave check to the boat's motion, and sinking it still lower and lower, the boat was driven ahead to wind. ward against the upper current: the current aloft, as he added, not being four or five fathom deep, and that the lower the bucket was let fall, they found the under current the stronger.”

Of upper currents, Mr. Rennell has particularly described a very singular one often prevailing to the westward of Scilly, and dangerous to ships that approach the British Channel. They are, however, more frequently met with about the Straits of Gibraltar, and near the Antilles. These latter are especially worthy of notice, and are thus described and accounted for by Dr. Peysennel f.

The coasts of these American islands are subject to countertides, or extraordinary currents, which render it very dangerous to chaloupes and other small craft to laud; while at the same time the boats and ships in the roads are scarcely ever sensible of them, and seldom incommoded by them; nor do those who are out at sea appear to be affected by them. It is however certain that a regular wind constantly blows, in these parts of the torrid zone, from the tropic of Cancer, to the equinoctial line, from the east ; inclining sometimes northward, and sometimes southward. This wind is called alizé, or trade-wind, for reasons admitted by philosophers, and it draws the water westward, giving a total and uniform course to that immense quantity, which comes from the great river of the Amazons, and from an infinite number of other rivers, which discharge themselves into the ocean. These currents passing

Phil. Trans. 1793. Vol. lxxxiii.

+ Ib. 1755. Vol. xlix.