Page:Physical Geography of the Sea and its Meteorology.djvu/470

444 melt; but it and its water, the heat being continued, will remain at 32° for 140 minutes, when all the ice will have become water at 32°. This 140° of heat, which is enough to raise the temperature of 140 cubic feet of ice one degree from any point below 32°, has been rendered latent in the process of liquefaction. Freeze this water again, and this latent heat will become sensible heat, for heat no more than ponderable matter can be annihilated. But if, after the cubic foot of ice has been converted into water at 32°, we continue the uniform supply of heat as before and at the same rate, the water will, at the expiration of 180 minutes more, reach the temperature of 212°—the boiling-point—and at this temperature it will remain for 1030 minutes, notwithstanding the continuous supply of heat during the interval. At the expiration of this 1030 minutes of boiling heat, the last drop of water will have been converted into steam; but the temperature of the steam will be, that only of the boiling water; thus, in the evaporation of every measure of water, heat enough is rendered latent during the process to raise the temperature of 1030 such measures one degree. If this vapour be now condensed, this latent heat will be set free and become sensible heat again. Hence we perceive that every rain-drop that falls from the sky has, in its process of condensation, evolving heat enough to raise one degree the temperature of 1030 rain-drops. But if instead of the liquid state, as rain, it come down in the solid state, as hail or snow, then the heat of fluidity, amounting to enough to raise the temperature of 140 additional drops one degree, is also set free.

830. The cause of the boisterous weather off Cape Horn.—We have in this fact a clew to the violent wind which usually accompanies hail-storms. In the hail-storm congelation takes place immediately after condensation, and so quickly that the heat evolved during the two processes may be considered as of one evolution. Consequently, the upper air has its temperature raised much higher than could be done by the condensing only. So also the storms which have made Cape Horn famous are no doubt owing, in a great measure, to this heavy Patagonian rainfall. The latent heat which is liberated by the vapour as it is condensed into rain there, has the effect of producing a great intumescence in the air of the upper regions round about them,