The great preponderance of water in the southern hemisphere is,-as Dove has pointed out, the prime cause of the southern hemisphere being a region of warm winters and cool summers, while the northern is characterised by cool winters and hot summers; an increasing quan tity of the sun's heat becoming latent, as, in passing from north decli nation and entering the southern signs, it is employed in evaporating an increasing quantity of aqueous vapour. And, as he observes, " the mild winter of the southern hemisphere, plus the contemporaneous hot summer of the northern, necessarily gives a higher sum of tempe rature than the cool summer of the southern plus the cold winter of the northern hemisphere." These relations, he suggests, appear to furnish, in the periodical conversion of the aqueous vapour into a liquid form, the motive power in the machinery of the general atmos phere of the earth. "The unequal distribution of land and sea in the northern and southern hemispheres appears to supply an effectual provision, from whence it necessarily follows that the aqueous vapour, which from the autumnal to the vernal equinox is developed to an immense extent over the southern hemisphere, returns to the earth in the other half of the year in the form of rain or snow. And thus the
wonderful march of the most powerful steam-engine with which we are acquainted, the atmosphere, appears to be permanently regulated. . . . It is probable that the northern hemisphere may be regarded, com paratively speaking and to a considerable degree, as the condenser in this great steam-engine, and the southern hemisphere as its water reservoir ; that the quantity of rain which falls in the northern hemisphere is therefore considerably greater than that which falls iu the southern hemisphere; and that one of the higher tempe rature of the northern hemisphere is, that tho large quantity of heat which becomes latent in the southern hemisphere in the formation of aqueous vapour is set free in the northern in great falls of rain and anew." (` Essay on Distribution of Beat,' p. 26, Eng. translation.)