Now the quantity of moisture in the atmosphere will at all times be nearly equal to the greatest quantity that can be maintained in It in a state of vapour at the existing temperature. Therefore if two volumes of air thus saturated with moisture, but of different temperatures, become by any means mixed together, a mean degree of heat results from the union ; hut the whole quantity of moisture In the sum of the volumes of air will, agreeably to the theory, be greater than that which is due to the mean temperature, and the excess will of course be condensed or precipitated. The vapour so condensed forms a cloud [Cumin] • and if this be specifically heavier than the air in which it is formed, it will begin to sink : the internal friction of the air, as Professor Stokes has recently shown, will for a time retard its descent, allowing it to remain suspended until the resistance from this cause is overcome by the increasing magnitude of the drops. Should the atmosphere near the earth be less dense than the cloud, the latter will continue to descend till it touches the ground, when the aqueous particles, if small, will form what is called a mist; or if large, and particularly if the condensation of the vapour has been rapid and copious, they will descend by their gravity in rain, snow, or hail, according to the temperature of the region in which they have been formed or through which they pass. It may happen, however, in the descent that a cloud arrives in a warmer region than that in which it was formed ; in this case the condensed moisture may again become vapour, which may re-ascend to a region at which a new condensa tion takes place. But though it be true that some precipitation must follow, whatever be the difference between the temperatures of the two volumes of air, yet unless the mean of the two quantities of vapour should be greater than the quantity necessary for coin pieta saturation at the mean of the two temperatures, the precipitation will not be perceptible in the form of rain.
In order to illustrate the general subject of clouds and rain, the late Professor Daniell, in his Essays on the Constitution of the Atmo sphere' and Meteorology,' supposes, first, that the earth is a sphere of uniform temperature, and surrounded by an atmosphere of dry and permanently elastic fluid ; and he shows that on this supposition the density of the air would diminish in a geometrical progression at eleva tions increasing by equal increments. He observes also that the tem perature would decrease with the densities, and that the atmosphere would be constantly in equilibrio. This would continue to be the case if the general temperature of the sphere were to be increased, provided that increase were uniform at all points on its surface. Now, if the temperature of the sphere, instead of being uniform, were supposed to increase from the poles towards the equator, the unequal densities pro duced in vertical columns of the air by the differences of temperature at equal heights above the surface of the sphere, would give rise to lateral pressures which, in the lower strata, would produce currents tending from the poles towards the equator ; but the elasticity of the air, which is constant near the surface of the sphere, varies with the height above that surface, according to such a law that, beyond a cer tain elevation, it would produce lateral pressures exceeding those which arise from the density in the neighbouring columns at equal altitudes, and thus there would arise a current in the upper regions flowing con tinually from the equator towards the poles. ,
He supposes next that the sphere is covered everywhere with water of equal temperature, and is surrounded by an atmosphere of pure aqueous vapour ; and he shows not only that the density of this vapour would diminish upwards, according to the law before men tioned, but that the atmosphere would in this case also be in equilibrio and transparent even when the general temperature of the sphere experiences a uniform increase. But if the temperature of the sphere were to increase as before, from the poles towards the equator, the density and elasticity of the vapour varying also with the temperature, there would arise by evaporation at the equator a current tending from thence to the poles, and this, being condensed in its course, would return from the poles towards the equator in the form of water. The condensation. thus going on would cause the atmosphere to be con stantly charged with clouds and rain. Unless, however, the excess of temperature at the equator were maintained by some foreign power, as solar radiation, the temperature over the whole sphere would by degrees become equalised; the equatorial parts becoming cooled by evaporation, while the polar regions would become warmed by the condensation.
Mr. Daniell afterwards contemplates an atmosphere consisting of a permanently elastic fluid mixed with aqueous vapour, and surrounding a sphere of water of uniform temperature ; and he observes that, since the evaporation would be slow, the small quantity of water precipitated would be almost immediately dissolved by the superior temperature of the stratum below, into which it would tend to fall; therefore this atmosphere would be free from clouds. But in the event of the tem perature of the sphere increasing from the poles to the equator, the evaporation in the latter region would destroy the regular gradation of temperature in the atmosphere from the surface of the sphere upwards; the evaporated water rising to the middle regions would there, in con sequence of the diminished temperature, give out its latent heat, and become condensed ; then descending, it would acquire from below a new portion of heat, with which it would rise till it was again forced to part with its caloric. This process may be supposed to continue till those regions of the atmosphere become saturated with vapour, and at the same time rarefied by the heat. The rarefaction of the air would diminish its resistance to the general movement of the vapour towards. the poles, and thus the vapour would rush with force in those directions; but on arriving in latitudes at which the temperature is too low to allow the air to bold it in solution, condensation would take place, and clouds would be formed.