Page:Physical Geography of the Sea and its Meteorology.djvu/257

Rh day in July, when it is in aphelion. January is the midsummer month of the southern hemisphere, consequently that half of the globe receives more heat in a day of its summer than tho other half receives in a day of the northern summer. But the northern summer is a week the longer, by the reason of the ellipticity of the earth's orbit. What becomes of this diurnal excess of southern summer heat, be it in its aggregate never so small, and why does it not accumulate in trans-equatorial climes? So far from it the southern hemisphere is the cooler.

454. The latent heat of vapour.—In the southern hemisphere there is more sea and less land than in the northern. But the hydrometer indicates that the water in the seas of the former are Salter and heavier than the waters of seas cis-equatorial; and man's reasoning faculties suggest, in explanation of this, that this difference of saltness or specific gravity is owing to the excess of evaporation in the southern half, excess of precipitation in the northern half of our planet. "When water passes, at 212° Fahrenheit, into steam it absorbs 1000° of heat, which becomes insensible to the thermometer, or latent; and conversely, when steam is condensed into water, it gives out 1000° of latent heat, which thus becomes free, and affects both the thermometer and the senses. Hence steam of 212° Fahrenheit will, in condensing, heat five and a half times its own weight of water from the freezing to the boiling point."—M'Culloch. Now there is in the southern a very much larger water surface exposed to the sun than there is in the northern hemisphere, and this excess of heat is employed in lifting up vapour from that broad surface, in transporting it across the torrid zone and conveying it to extra-tropical northern latitudes, where the vapour is condensed to replenish our fountains, and where this southern heat is set free to mitigate the severity of northern climates.

455. Its influence upon climates.—In order to trace a little farther, in our blind way, the evidences of wisdom and design, which we imagine we can detect in the terrestrial arrangement of land and water, let us fancy the southern hemisphere to have the land of the northern, and the northern to have the water of the southern, the earth's orbit remaining tho same. Is it not obvious to our reason that by this change the whole system of climatology in both hemispheres would be changed? The climates of our planet are as obedient to law as the hosts of heaven.