THE SPECIFIC GRAVITY OF THE SEA, ETC. 231 



day in July, wlien it is in aphelion.* January is tlic midsummer 

 month of the southern hemisphere, consequently that half of tho 

 ^iobe 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 tho 

 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° Fah- 

 renheit, into steam it absorbs 1000° of heat, which becomes in- 

 sensible to the thermometer, or latent ; and conversely, w^hen 

 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* 



* Sir Johu Herscliel. 



