one of the most powerful modifiers of climates. It is to the winds precisely what coals are to the steam-ship at sea—the source of motive power. The condensation of vapour is for one what the consumption of fuel is for the other; only with the winds the same heat may be used over and over again, and for many purposes. By simply sending moist air to the top of snow-capped mountains, condensing its moisture, and bringing it down to the surface again, it is made hot. Though by going up the air be cooled, it is expanded, and receives as sensible heat the latent heat of its vapour; being brought down to the surface again, and compressed by the whole weight of the barometric column, it is hotter than it was before by the amount of heat received from its vapour. That we may form some idea as to the modifying influences upon climate which might arise from this source, let us imagine the air as it impinges upon the antarctic continent to be charged with vapour at the temperature of of 40°. In order to arrive at the place of polar calms, it has to cross a mountain range, we will suppose, the summits of which are pushed high up into the regions of perpetual snow. As this air, with its moisture, rises, it expands, cools, and liberates the latent heat of its vapour, which the air receives in the sensible form. Now suppose the expansion due the height of the mountain-top to be sufficient to lower the temperature of dry air to —50°, but, on account of the latent heat which is liberated from the vapour of the moist air, the temperature of the air that has ascended, instead of falling as it crosses the mountain to —50°, as dry air would do, falls, in consequence of the condensation of its vapour, no lower than —30°. Thus, in the case supposed, heat enough has been set free to raise the temperature of the newly-arrived air 20°. Consequently, when this air, which, at the temperature of 40°, came from the sea loaded with vapour, passes the mountain, it loses vapour, but receives heat; descending into the valleys beyond, it is again compressed by the weight of the barometric column, which, let us assume, is the same in the valley as at the sea level on the other side of the mountain. The temperature of this air now, instead of being 40°, will be 60°. A powerful modifier of climate is the latent heat of vapour in the air.
870. Local variations of climate, how caused.—At one time, as has been shown in Chap. IV., this heat is brought down from the cloud region to scorch the earth; at another time it causes
