FORMATION CF CLOUDS AJ>ID RAIN. 41 



ually dissipated in the coldest weather, and sometimes with a degree 

 of velocity which at first sight seems truly surprising.* 



It thus liappens that the atmosphere is constantly impregnated with 

 watery vapour, which in this gaseous state accompanies the air where- 

 ever it penetrates, permeates the soil, pervades the leaves and pores of 

 plants, and gains admission to the lungs and general vascular system of 

 animals. We cannot appreciate the influence which, in this highly 

 comminuted form, water exercises over the general economy of organic 

 nature. 



But it is chiefly when it assumes the form of rain and dew, and re- 

 descends to the earth, that the benefits arising from a previous conversion 

 of the water into vapour become distinctly appreciable. The quantity 

 of vapour which the air is capable of holding in suspension is depend- 

 ent upon its temperature. At high temperatures, in warm climates, or 

 in warm weather, it can sustain more— at low temperatures less. 

 Hence when a current of comparatively warm air loaded with moisture 

 ascends to or comes in contact with a cold mountain top, i is cooled 

 down, is rendered incapable of holding the whole of the vapour in sus- 

 pension, and therefore leaves behind in the form of a mist or cloud, a 

 portion of its watery burden. In rills subsequently, or springs, the 

 aqueous particles which float in the midst, re-appear on the plains be- 

 neath, bringing nourishmentf at once, and agreateful relief to the thirsty 

 soil. 



So when two currents of air charged with moisture, but of unequal 

 temperature, meet in the atmosphere, they mix, and the mixture has 

 the mean temperature of the two currents. But air of this mean tem- 

 perature is incapable of holding in suspension the mean quantity of wa- 

 tery vapour ; hence, as before, a cloud is formed, and the excess of 

 moisture falls to the earth in the form of rain. In descending to refresh 

 the earth, this rain discharges in its progress another ofUce. It washes 

 the air as it passes through it, dissolving and carrying those accidental 

 vapours which, though unwholesome to man, are yet fitted to minister 

 to the growth of plants. 



The dew, celebrated through all times and in every tongue for its sweet 

 influence, presents the most beautiful and striking illustration of the agen- 

 cy of water in the economy of nature, and exhibits one of those wise and 

 bountiful adaptations, by which the whole system of things, animate and 

 inanimate, is fitted and bound together. 



All bodies on the surface of the earth radiate, or throw out rays 

 of heat, in straight lines — every warmer body to every colder ; and the 

 entire surface is itself continually sending rays upwards through the 

 clear air into free space. Thus on the earth's surface all bodies strive, 

 as it were, after an equal temperature (an equilibrium of heat), while 



* Mr. Howard states that a circular patch of snow 5 inches in diameter lost in the month 

 of January 150 grains of vapour between sunset and sunrise, and 56 grains more before tho 

 close of the day, when exposed to a smart breeze on a .house-top. From an acre of snow 

 this would be equal to 1000 gallons of water during the night only.— Prout' 8 Bridgewater 

 Treatise, p. 302; Encyclopcsd. Metropolian. Meteorology. 



In Von Wrangell's account of his visit to Siberia and the Polar sea, translated by Major 

 Sabine (p. 390), it is stated that, in the intense cold, not only living bodies— but the very 

 gnaw — smokes and fills the air with vapour. 



t For the nature of this nourishment see the subsequent Lectures, " On the inorganic con 

 ttitutnta of plants." 



