It has been calculated from certain phenomena of refraction, that our atmosphere must extend to about forty miles from the surface of the earth. It may, in a state of extreme tenuity, extend still further; but it is probable that the intense cold produced by rarefaction sets limits to any extension much beyond this elevation. The uses of the atmosphere are many. It is the medium for regulating the dispersion of watery vapours over the earth. If there were no atmosphere, and that, as now, the equatorial climes were hot and the poles cold, evaporation would be continually going on at the equator, and condensation in the colder regions. The sky of the tropical climes would be perpetually cloudless, whilst in the temperate and arctic zones we should have constant rain and snow. By having a gaseous atmosphere, a more uniform state of things is produced; the vapours arising from the earth become intimately mixed with the air, and are borne by it over large tracts of country, and only precipitated when they enter some stratum much colder than that which involves them. There are opposite tendencies in an atmosphere of air and one of vapour. The air circulates from the colder to the warmer parts, and the vapour from the warmer to the colder regions; and as the currents of the air, from the distribution of land and sea--the land, from its low conducting power, being more quickly heated than the sea--are very complicated, and as some force is employed in keeping the vapour suspended in the air, water is less suddenly deposited on the earth than it would have been, had not these tendencies of the air and its hygrometric peculiarities been such as we find them. The blue colour of the sky, which is so much more agreeable to the eye than either red or yellow, is due to a tendency of the mixed gas and vapour to reflect the blue rays rather than red or yellow. The white light which falls upon the surface of the earth, without absorption or decomposition in its passage from the sun, is partially absorbed by, and in part reflected back from, the earth. The reflected rays pass with tolerable freedom through this transparent medium, but a portion of the blue rays are interrupted and rendered visible to us. That it is reflected light, is proved by the fact of its being in a polarized state.[226] Clouds of vapour reflect to us again, not isolated rays, but the undecomposed beam, and consequently they appear white as snow to our vision.