your HYPOTHESIS; though I had not thought of that it may be true, for aught I know to the contrary: but, as you had alleged in its favour fome phenomena which appeared to me dependant on other caufes, I gave the reafons that made me think fo. On the other hand, you feem to think, that, in giving these reasons, I had in view your THEORY OF RAIN; but they have nothing to do with it; and the less so, as in treating of it, I have granted your HYPOTHESIS: I had even formed it myself, before I knew it was yours (Id. fur la Met. S$ 529 and 580); and it was to fhew, that, admitting every poffible fuppofition (not yet contradicted by phenomena) in refpect of any confequence of the cooling of moisture exifting in the atmosphere before the formation of clouds, the phenomena of rain were not explained at all. This I did from the very experiments and obfervations that you ftill wait for; whose results I am going to repeat, to fix your attention upon them.

mum.

16. I grant your hypothefis, as I know nothing against it: I will alfo grant you, but only for a moment, that the atmofpheric ftrata, which you muft fuppofe to be mixed together for the production of rain, contain both moisture to its maxiThat quantity, according to the experiments above quoted (6), the temperature being about 60°, is only of the mafs of the air, when the barometer ftands at about 30 inches: it will be a greater part of it in higher ftrata; but as the heat will be lefs, I fuppofe, for the fake of brevity, that it will bring it again to the above aliquot part of the mafs of the air. I cannot make any calculation from this data, upon what would refult from your hypothefis; as you have not given it any determination but if you pleafe to calculate yourself, not forgetting, that, according to that hypothefis, the mixed mass must retain moisture to its maximum, befides the fuppofed precipita tion; you will find, how short this will be, even of a fingle fummer shower. All that you could fo fqueeze out of your firata, would be evaporated in the air under them, commonly very dry in thefe cafes; and it would not reach the ground.

17. But, Sir, what I was granting there, to fhew you à fortiori, the infufficiency of your hypothefis (in cafe even that it was proved), nature refufes. And, here, there is no want of calculation; for the facts them felves are ftriking. From the time that regular hygrometers have been obferved upon high mountains, there is not, to my knowlege, one fingle obfervation, where the transparent air has not been found very dry, even in the night. And, moreover, I have related (ld. fur la Met. $563), an obfervation, in which, while on the top of an high mountain, the dryness of the air was to a degree unknown in the plain, clouds were forming verv rapidly in the fame ftraAPP. Rev. VOL, LXXXI.

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tum of air, by which the very spot where I had obferved a little before (from which I had retired precipitately, foreseeing the event) was enveloped. The beat of the air increased in the mean time, without (as yet) any wind; and foon after, a vio lent ftorm of rain, hail, thunder and wind, was the product of clouds formed in a very dry air. It should feem, that the air was to remain very moist, at the moment the clouds came to be diffipated; especially, as the ground of all the neighbouring mountains and valleys was foaked with water. But this will fhew you still more how fmall is our knowlege upon these phenomena. Happening to be still in an high valley, at a moment when the clouds, that filled it before, were difperfed, and obferving my hygrometer in the open air at that moment, I found this as dry as it was on the fame fpot before the ftorm; which did not prevent, very foon after, that valley, with all the others at the fame height, from being filled again with clouds attended with rain.. (Ibid.)

18. These are the facts which I had oppofed; not to your hypothefis (for they have nothing to do with it), but to your theery of rain, admitting that hypothefis; the infufficiency of which, appears to me thereby completely demonftrated. Nevertheless, you fay now, fpeaking of that theory," M. De Luc has indeed difputed it; but any perfon who has read his laft publications, will hardly expect that, with bis meteorological ideas, our author fhould, on this occafion, be altogether free from partiality." Let us, however, examine this. When I publifhed my Rech. fur les Mod. de l'Atm. I did not doubt, that rain proceeded from the precipitation of a certain quantity of moisture, being part of that which exifted in the atmosphere, in the fame form under which it had been fpread in it by evaporation. On that idea, I founded an explanation of the variations of the barometer, and of their variable connection with rain and clear weather. For that theory indeed I might have had fome partiality: for it was new; I fubftituted it for those which had moft attracted the attention of natural philofophers, after having fhewn either their contradiction to phenomena, or their infufficiency; it had obtained fuch approbations as might fatisfy me; and it was fo intimately united with the then generally admitted hypothefis on the fource of rain, that I could maintain it even now with the fame advantage as at that time, and even with more, if rain proceeded, in any manner, from a moiflure exifting in the atmosphere before the formation of the clouds. But, at the time I publifhed that theory, I was at work to obtain, if poffible, fome fort of regular bygrometer; having already many reafons to think, that nothing, in meteorology, could be certain to any fatisfactory degree, without the concurrence of obfervations made with fuch an inftrument.

The

The firft obfervations of that kind which I made upon high mountains (those I have juft now mentioned), were in the year 1772; and from that time I fufpected my own theory; which the union of all the experiments, and obfervations, mentioned above, has entirely overturned. I did not refift fuch admonitions; there is no fatisfaction in ideas that we fufpect of error; and a great part of the publications you allude to, has been intended for afcertaining that I then was wrong. It is true, that the confequences I have drawn against my own theory from thefe new facts, prefent meteorology under an entirely new afpect, which I was to explain. This afpect then (on which I hardly pretend to have thrown a twilight), with fome confiderations arifing from it, relating to other meteors, and all the particular facts connected with them, compose the reft of that work, which I have offered to the attention of natural and experimental-philofophers. Thefe, I fhould fuppofe, are the meteorological ideas from which you conclude that I cannot be free from partiality: but, as they envolved my own theory in the fame doom as yours, I think you might have exempted me from that trivial accufation.

I think, Sir, I have now replied to all your anfwers; and fince thefe are given to the public, I take the first opportunity* of likewife publifhing this letter; as it may contribute to fix the attention of natural-philofophers upon one of the moft important objects of inquiry, in the prefent ftate of our knowlege

*By the favour of the Monthly Reviewers, who, contrary to their ufual custom, have permitted this letter to be printed in their Appendix, on account of my friendship with one of them.

To the READERS of the REVIEW.

A tedious and fevere indifpofition of one of our corps, has unfortunately delayed our conclufion of the account of the Berlin Edition of the late King of Pruflia's Works. To every difpenfation of this kind (the common lot of human nature!) all muft fubmit, and fhould, with humble refignation.-We are, however, at length, in reasonable hope of the fpeedy recovery of our learned and worthy affociate. As foon as he is able to refume the pen, we may expect that his yet unfinished Extracts will be completed; and we are perfuaded that they will be worth waiting for.

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INDEX

To the REMARKABLE PASSAGES in this Volume.

N. B. To find any particular Book, or Pamphlet, see the Table of Contents, prefixed to the Volume.