caufe, and in bygrofcopic bodies that of an effect. These two circumftances furnish us alfo with a fixed module for determining correfpondent degrees.

Moiflure is totally abfent, firft, in the medium when it contains no vapour, and then as a confequence in hygrofcopic bodies, because they contain no more water that can evaporate, without a decompofition of their component parts. The case here fuppofed is, that when, by fome adequate caufe, no fenfible quantity of vapour is permitted to remain in the medium, as in the lime veffel ufed by Mr. de Luc to obtain the point of extreme drynefs.

Moiflure is extreme, firft, in the medium, whether air or a space free from air, when no more vapour can be introduced therein, without a part being decompofed; and then, as a confequence in hygrofcopic bodies, because no more water can be admitted in their pores.

Here it is to be obferved, that from the nature of the last of thefe maxima the quantity of water which produces it (i.e. extreme moisture) in a given body is fixed, because it is determined by the actual capacity of it's pores; but the quantity of water which produces extreme moisture in a medium of a given extent is as variable as the temperature.

The equilibrium therefore between the medium and hygrofcopic bodies in different ftages of moisture, which equilibrium is the object of hygrometry as a fcience, does not depend on certain quantities of water contained in the medium of which bodies may receive their fhare; it depends on different aptitudes of the vapour contained in the medium to communicate water to those bodies; which aptitudes vary not only with the different denfities

denfities of that fluid, but also in vapour of the fame denfity according to the temperature.*

From the hygrometer we have learnt, that in the phenomenon of dew the grafs often begins to be wet, when the air a little above it is ftill in a middle state of moisture; and that extreme moisture is only certain in that air, when every folid expofed thereto is wet. It has taught us, that the maximum of evaporation in a clofe fpace, is far from being identical with the maximum of moifture; this depending confiderably, though with the constant exiftence of the other, on the temperature common to the Space and the water that evaporates. It has fhewn, that the cafe of extreme moisture exifting in the open tranfparent air, in the day, even when it rains, is extremely rare: Mr. de Luc has only found it once in this ftate, the temperature being 39°. Meffrs. de Sauffure and de Luc have proved by the hygrometer, that the air is dryer and dryer as we afcend in the atmofphere, fo that in the upper attainable regions, it is conftantly very dry, except in the clouds. Mr. de Sauffure has fhewn, that if the whole atmosphere paffed from extreme dryness to extreme moisture, the quantity of water thus evaporated would not raise the barometer half an inch. Laftly, in chemical operations on the air, the greatest quantity of evaporated water that may be fuppofed in them, at the common temperature of the atmosphere, even if they were at extreme moisture, is not fo much as the one hundredth part of their mafs. The two laft very important propofitions have been demonftrated by M. de Sauffure.†

LECTURE.

* See Mr. de Luc's paper on Evaporation, from which the remarks on Hygrometers, &c. is an extract, Phil. Trans. for 1791, part 9.

See Mr. de Luc's fecond paper on Hygrometry, Phil. Trans.

LECTURE LII.

OF RAIN.

Na fcience fo very difficult as that of the weather, it is not to be fuppofed that any thing like a certain and established theory can be laid down: our utmost knowledge in this refpect goes no further as yet than the establishment of a few facts; and in reasoning upon thefe, we are involveď every moment in queftions which feem fcarcely within the compass of human wisdom to refolve.

To treat it in a fatisfactory manner, we ought to have an intimate acquaintance with the conftitution of the atmosphere, and the nature of those powerful agents, fire, light, and electricity, by which it seems to be principally influenced; with their peculiar influences upon one another, and upon the atmosphere, and this in every poffible variety of circumftances. Many of the qualities of air, earth, water, and fire, have been indeed difcovered and eftimated; but when these come to be united by nature, they often produce a refult which no artificial combinations can imitate. Every cloud that moves, and every fhower that falls, ferves to mortify the philofopher, and to fhew him hidden qualities in air and water that he is unable to explain.

The greater part of the received notions on meteorology are vague and incorrect, not only thofe which relate to the nature of the caufes, but thofe alfo which concern the laws of their effects. The fame may be faid of our notions of the elafticity of the air, of beat when applied to this fluid, of both igneous and aqueous meteors, of fudden and

partial

partial winds; they are all fo many enigmas to the philofopher.

Indeed, till we were in poffeffion of a good hygrometer, it was impoffible to form any certain conclufions concerning the moisture of the air: this difficulty is removed; Mr. de Luc has by numerous experiments and obfervations furnished us with a comparative hygrometer, by which together with a thermometer the air can neither lofe nor acquire moisture without our being advertised

thereof.

By the use of this hygrometer we have obtained clear and certain ideas of the causes, by which water, fimply evaporated in air, may be precipitated therefrom. These causes are the fame with thofe, which, in air where the quantity of evaporated water remains the fame, always produce an increase of moisture, the neceffary forerunner of the precipitation of water; and thefe are two; the compreffion of the air, or it's being cooled: no other caufes are indicated by experiment. Some philofophers have thought that the air, when rarified, quitted a portion of the water which, according to them, it held in folution; but I have fhewn you, that this idea is erroneous, and that rarifaction occafioned drynefs inftead of moisture.

The great question therefore in the inquiry concerning the immediate caufe of clouds, &c. is, What becomes of the water that rifes as vapour into the atmosphere? What is the ftate in which it fubfifts there between the time of it's evaporation and the time of it's falling down again in rain?

If it continues in a state of watery vapour, or fuch as is the immediate product of evaporation, it muft poffefs the diftinctive characters effential to that fluid. It must make the hygrometer move towards humidity in proportion as the vapour is VOL. IV.

LI

more

more or lefs abundant in the air. On a diminution of heat, the moisture, as fhewn by the hygrometer, would increafe; but on an increase of heat, the humidity would decrease. Again, on this fuppofition, if hygrofcopic fubftances dryer than the air be introduced therein, they must have the fame effect as an augmentation of heat: for fuch are always the properties of aqueous vapour on every hypothefis of evaporation. If therefore water exifts in the atmosphere, without thefe properties, it is no longer vapour, it must have changed it's nature. Mr. de Luc has fhewn, that the water which forms rain, does not poffefs these propertics; it must therefore have paffed into another fate.

Repeated obfervations have fhewn, that the upper regions of the atmosphere, notwithstanding the continual afcent of vapours there, are dryer than the inferior regions; on the fummits of high mountains a degree of drynefs prevails, unknown on the plains.

If rain be the immediate product of evaporation, it ought always to be preceded and accompanied by a diminution of heat, in that ftratum of air where it originated; and this diminution, to produce it's effect, should be greater in proportion as the moisture was further removed from it's extreme term in this ftratum; but in a great ftorm on the mountain of Sixt, Mr. de Luc found that the heat had increased instead of diminished; this caufe could not operate here, and it was therefore impoffible that the quantity of water which was then precipitated from the air could have been contained there in the form of the immediate produc of evaporation.

On every hypothefis of the formation of rain from vapour, it is heat that produces the evaporation, and a diminution of heat that occafions the

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