Page:Physical Geography of the Sea and its Meteorology.djvu/126

the sea, is (§ 232) no feeble agent^[1] in the production of wind, nor is it inconsiderable in its influence upon the trade-winds.

259. Vapour as one of the causes of the trade-winds.—Let us consider this influence. A cubic foot of water, being converted into vapour, occupies the space of 1800 cubic feet.^[2] This vapour is also lighter than the 1800 cubic feet of air which it displaces. Thus, if the displaced air weigh 1000 ounces, the vapour will weigh 623; consequently, when air is surcharged with vapour, the atmosphere is bulged out above, and the barometric pressure is diminished in proportion to the volume which flows off above in consequence of this bulging out. Thus, if we imagine the air over the Atlantic Ocean to be all in a state of rest, and that suddenly during this calm, columns of vapour were to commence rising from the middle of this ocean, we can understand how the wind would commence to flow into this central space from all around. Now, if we imagine no other disturbing cause to arise, but suppose the evaporation from this central area to go on with ceaseless activity, we can see that there would be a system of winds in the Atlantic as steady, but perhaps not so strong as the trades, yet owing their existence, nevertheless, merely to the formation of aqueous vapour. But this is not all.

260. Black's law.—"During the conversion of solids into liquids, or of liquids into vapours, heat is absorbed, which is again given out on their recondensation." ^[3] In the process of converting one measure of water into vapour, heat enough is absorbed—i. e., rendered latent, without raising the temperature of the vapour in the least—to raise the temperature of 1000 *such measures of water 1°; when this vapour is condensed again into water, wherever the place of recondensation may be, this heat is set free again. If it be still further condensed, as into hail or snow, the latent heat rendered sensible during the process of congelation would be sufficient to raise the temperature of 140 additional measures of water 1°.

261. The latent heat transported in vapour.—In this heat rendered latent by the processes of evaporation, and transported hither and

1. I am sustained in this view by a recent paper on "the forces that produce the great currents of the air, and of the ocean," recently read before the Royal Society by Thomas Hopkins.
2. Black and Watt's Experiments on Heat.
3. Black's law. It is an important one, and should be remembered.