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feet a mass, which, in melting, must necessarily add one tenth to the level of the ocean, that is, sixty feet, which we will reduce to fifty, to make allowance for the difference between the volumes of ice, and of ice when melted into snow. At the equinox in September, the greatest part of the ice at our poles, which has been worked upon during the whole summer, is melted; and it is at the same time also, that the ice at the southern pole begins to thaw. You will observe, however, that the tides at the equinox in March are higher than those of September; and this proceeds from its being the end of summer at the southern pole, which has a greater mass of ice than the northern, and consequently gives a greater volume of water to the ocean. The reason of its having more ice is, that the sun continues six days longer in the northern, than in the southern hemisphere. The intermissions, however, between these polar effusions, and which of course, must be supposed to cause the flux and reflux of our tides, twice in the course of the four and twenty hours, may probably depend on the melting of the ice during the course of the day, and the cessation, or the inconsiderableness of thaw, during the night. And the difference of three quarters of an hour in

Physique du Monde.

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the tide may also be occasioned by the difference of quantity in the body of the ice, as it progressively recedes towards the pole, and as, in its daily thaw, the line of congelation gets farther from us.

From all this then it appears, it is not from the exclusive action of the sun and moon between the tropics, that the tides are to be deduced; but that it is to the influence of these luminaries at the poles of the world, we are to look for their principal source. The polar effusions may be, in a considerable degree, the real causes; and as the phases of the moon are co-ordinate with the course of the sun, which creates those effusions, the reason is clear, why the tide and the phases of the moon, should happen at the same time. In a word, the tides are semi-diurnal polar effusions, as the general currents of the ocean are semi-annual. I say, the two general currents in the year, because the sun in turn, warms each of the two hemispheres during the progress of our annual revolution.

"Providence thus," says Saint Pierre," has probably ordained, that the effusions from the polar regions should be in proportion to the evaporations from the ocean. And hence, we

may consider the two hemispheres of the earth, as two mountains; the two poles, as the icy summits of these mountains; and the seas, as the rivers which majestically flow from them. And if we compare the proportions of the glaciers of Switzerland with their mountains and with their rivers, we shall be able readily to form an idea of the polar glaciers, and of the oceanic rivers, which they send forth. The. tides, indeed, are twice in each day. But, this is to be accounted for, as I have already said, from the sun, in his progress during the twenty four hours, heating both sides of the polar ice. In lakes, in the day time, there is a sort of flux and reflux. Nor is it to be doubted, that if the sun, during the course of the night, could heat the opposite sides of the mountains whence lakes are derived, they would also have a similar flux and reflux to that of the sea. In all this, however, we are not to suppose that each day's tide is the same day's effusion at the pole. It is only the effect of a course of such effusions, which perpetually succeed one to another; and the time, which any diurnal effusion at the poles takes in passing to us, may be estimated at six weeks.

LET

LETTER XXVIII.

THIS simple and plain deduction of the tides of the ocean, from the glacial effusions at the poles, is too novel, and too unfriended, I am afraid, to be generally adopted. Yet it bears itself up, with a wonderful appearance of probability. The origin of the flow of rivers from masses of ice, support it by analogy; and the phænomena of the spring and neap tides are accounted for, both by reasoning and by calculation. In the main of the ocean, for instance, it is proved that there are in general no tides. In the Caspian Sea, which is about 860 miles long, and and in one part 260 miles broad, there are no tides, though there are strong currents. In the Baltic, there is no regular flux nor reflux. There is no tide in False Bay at the Cape of Good Hope, which has the aggregated masses of the Indian, Southern, and Atlantic Oceans. The same may be said of the Mediterranean, excepting in a few particularly situated spots, although in the Adriatic there is a flux and reflux. Why are the first of these considerable masses of the

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general body of the waters never raised, and the others always raised? And why is it, as Addison says, and as I have before remarked, that in summer, the lake of Geneva should have something like an ebb and flow, which arises, as it should seem, says he, "from the melting of the snow, that falls into it more copiously at noon, than at other times of the day?"

These striking deviations from, or rather contradictions to the received theory of the tides, argue with me, strongly I confess, against the infallibility of the doctrine. I cannot but think we may have imposed upon ourselves, when we have abstractedly given the phænomena of tides, to the operating energy of causes arbitrarily imagined. However the phases of the moon may accord with the swellings of the water, may not both one and the other be looked upon as harmonious and co-ordinate effects of another distinct but powerful principle? What is the augmented quantity, or rather the increased elevation of the main body of the ocean, at the moment of a spring tide? Nothing compara tively speaking. Does not the fact then seem to be, that it is not the mass of the ocean, which is diurnally raised, but, that it is the volume of the waters which is diurnally augmented? An

increase