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equal to 570. I repeated the experiment, at the temperature of 61°, and the following are the results:

Barom. Temp. of Water and Air. Dew Point

Under these circumstances, the amount of saturation was 651; an increase evidently dependent upon the force of the vapour, but not in exact proportion to its augmentation.

EXPERIMENT 5.-Being now desirous of ascertaining in what degree the temperature of an evaporating surface would be influenced by differences in the density of the air, I made the following disposition of the apparatus: To a brass wire, sliding through a collar of leathers, in a ground brass plate, I attached a very delicate mercurial thermometer; this was fixed air-tight, upon the top of a large glass receiver, which covered a surface of sulphuric acid of nearly equal dimensions with its base. Upon a tripod of glass, standing in the acid, was placed a vessel containing a little water, into which the thermometer could be dipped and withdrawn by means of the sliding wire. The bulb of the thermometer was covered with filtering-paper. At the commencement of the experiment, the barometer was at 30.2 inches, and the temperature of the air 50°. Upon withdrawing the thermometer from the water, it began to fall very rapidly, and in a few minutes reached its maximum of depression. The following table presents the results of the experiment, for different degrees of the air's density; the intervals were each of twenty minutes :

Barom. Temp. of Air. Temp. of wet Ther. Difference.

30.2

Here, in an atmosphere which a former experiment has proved to be in a state of almost perfect dryness, we find

that, at the full atmospheric pressure, the wet surface of the thermometer was reduced 9°. It is worthy of remark, also, how small a quantity of water is required to produce this effect. It has been previously shewn, that a surface of 2.7 inches diameter, only lost 1.24 grain in half an hour. This would have been 1.41 grain at the temperature of 49. The surface of the wet thermometer could not have exceeded th of that of the evaporating vessel, and the maximum effect was produced in ten minutes, or of the time, so that the weight of water evaporated in this case, was not more than (.0094 grains) one-hundredth of a grain.

The aqueous fluid is so abundantly spread over the face of the earth, that there can be no doubt that the permanently elastic atmosphere which surrounds it, would very speedily be saturated with its steam, did not some cause, analogous to the sulphuric acid in the receiver, prevent its universal diffusion. This never-failing cause is inequality of temperature. As in the small experiment we found that the degree of dryness was proportioned to the energy of the absorbent mass, and that the existing vapour was equally dif fused between it and the exhaling surface; so, in the larger operations of nature, we shall find that the state of saturation is dependent upon the point of precipitation, and that the aqueous atmosphere is nearly uniform between it and the source of steam.

Now, it is well understood, that the temperature of the gaseous atmosphere in its natural state must decrease with its density as we ascend to its upper parts; so that a great degree of cold is at all times to be found within a very moderate distance from the surface of the waters. It is this

temperature which determines the tension of the aqueous atmosphere; and it is evident that the evaporation which is thus caused at the base of the aerial fluid, must be accompanied by simultaneous and equal precipitation above. What then becomes of the precipitated moisture? Let us endeavour to trace the order of this phenomena. We will first suppose a calm state of the atmosphere, a temperature of 80°, and the barometer at 30 at the surface of the earth. By a calm state of the atmosphere is here meant, one that is free from any lateral wind, and in which the only currents being in an ascending and descending direction, evaporation would proceed at the rate exhibited in the first column of Mr Dalton's table. The dew-point at the surface of the earth is 64°, and this is determined by the temperature at the height of about 5000 feet, where the barometric column would maintain itself

at 24 inches. The degree of saturation below would therefore be 600, and the amount of evaporation 1.74 grain per minute from a surface of six inches diameter. This quantity we therefore suppose condensed at the height before named. But the state of saturation in the atmosphere above this point of precipitation, is again diminished; for we may suppose the force of the vapour to be determined by a temperature of 31° at a height of 15000 feet, where the barometer would stand about 16 inches. The force of evaporation would therefore be 1.71 grain per minute, at the full atmospheric pressure; and this amount increasing as the pressure diminishes, would give 2.13 grains per minute; so that the power of evaporation at this stage exceeds the supply of moisture, and no cloud could possibly be formed. Above the second point of condensation, let us now suppose the force of the vapour to be determined, in still loftier regions, by a temperature of 12°. The force of evaporation would then be 0.44 grain, increased in the proportion of 16 inches to 30, or 0.82 grain. Here, then, the power of evaporation would be insufficient to diffuse in the upper regions the whole of the moisture supplied from the surface of the earth, and a cloud, it might be supposed, must consequently result. But another modification of the process now ensues; the precipitated moisture has a tendency to fall back into the warm air below it, and consequently would again assume the elastic form with a rapidity proportioned to the rarefaction of the stratum in which it is diffused. There is, I think, no difficulty in supposing that no visible cloud, or one of extreme tenuity, would be formed during this double process of evaporation. A very important reaction, however, must take place upon the strata of vapour beneath; the elastic force being increased above, enables the water below to maintain an atmosphere of a higher degree, and the quantity of evaporation must decrease as the point of saturation rises. A different arrangement of the points of precipitation would ensue in the progress of these effects.

An important distinction must here be drwan between the ultimate effects of the superior and inferior evaporation denoted above. In the first, the whole weight of water is condensed and simultaneously exhaled; and although it constitutes steam of an inferior degree of force, there is little or no difference in the quantity of its latent heat, and no effect is therefore produced upon the temperature of that portion of the atmosphere in which the change takes place. But in the second, the condensation happens at one spot, and the

vaporization at another inferior to it; the latent heat is therefore evolved at the former, and communicated to the air, while at the latter the process is reversed, and the air is cooled. The process of this operation would, therefore, tend to equalize the temperature of the atmosphere.

We will next imagine that the surface of the earth is swept by a high wind, and that the atmosphere, instead of resting calmly upon its base, moves laterally with great volocity. Under these circumstances experience has shewn that the amount of evaporation will be nearly doubled; but the force of evaporation is not altered in the upper regions. The inferior exhaling surface being immoveable, the motion of the air perpetually changes, and renews the points of contact, and prevents accumulation at any one place; but in the heights of the atmosphere the exhaling surface of the cloud is borne upon the wind, and their relative situations never change.

The progress of precipitation must, therefore, necessarily, under these circumstances, outstrip that of evaporation, and the disturbance of the atmospheric temperature will be greatly accelerated.

There is another cause which would also quicken evaporation below, without equally increasing its power of diffusion at any given height above; and that is a decrease in the density of the air at the surface of the earth. Under the circumstances of our first supposition, imagine the barometer to fall 28 inches, the evaporation would be increased from 1.74 grain per minute, to 1.86 grain; but this decline of two inches at the surface would indicate a contemporaneous fall of little more than one inch at the height of 15,000 feet, and the rate of diffusion would vary accordingly. When it is considered that great falls of the barometer are generally accompanied by high winds, and that this disparity is multiplied by the force of the current, it is easy to appreciate the influence of this local increase of the power of evaporation.

The facility of evaporation in the rarer regions of the atmosphere will also go far to account for the state of saturation in which the air of mountainous countries is generally found, and many minor meteorological phenomena might probably meet with their explanation from variations of the same cause; such as the fogs which frequently accompany a very high degree of atmospheric pressure, and that peculiar transparency of the air which often precedes rain, and is accompanied by a falling barometer. But to return again

to the more general and extended influence of the vapour upon the boundless strata of the atmosphere :—that the phenomena of evaporation and condensation, as we have been contemplating their progress, have not been described with any bias to theoretical considerations, but are in strict accordance with facts and observations, any one might easily convince himself with less difficulty than would at first be supposed. To prove the assertion I shall extract the following passages from the works of De Luc, who was probably one of the most accurate observers of nature that ever existed, and who seldom, indeed, allowed any hypothetical considerations to warp his description of what he had observed. They will afford a complete illustration of the preceding remarks, although they were penned by him to support a very different hypothesis.

It

"If we give but a slight attention to the surface of the mists seen from the mountains, merely enjoying them as a fine spectacle, we may think that they are permanent; that the evaporation has arrived to its maximum at the surface of the water, because the air has attained an extreme humidity, and because the vapours which cloud the transparency of this air remain for weeks and even months; that is to say, as long as the mists preserve themselves at the same elevation. But the phenomenon is very different from this first appearance; the evaporation continues at the surface of the water, the vapours thus formed ascend continually, and a new evaporation takes place at the surface of the mists. is a spectacle equally amusing and instructive, when that which forms this surface is seen from a place a little elevated above it, and in a great valley, where there is at some distance mountains darkened by forests of fir-trees. Such a valley lighted by the rays of the sun, seems to be heaped with cotton, drawn through all its surface by invisible beings into invisible threads; there are every where made bunches, resembling those made by a spinster over her distaff, in drawing the cotton to form her thread, and these disappear successively and waste themselves in the air. Sometimes these bunches elongate, and separate themselves from the mass; by their tendency to rise we see them again stretch out as a parcel of gauze, unfolding itself, and in a little time disappearing. In this way the mists are constantly forming at the surface of the water, and the earth; but they are also constantly wasting themselves in the upper air, and at the same time we do not perceive that the humidity is thereby augmented." Ideés sur la Météorologie, tom. 2, p. 78.