468 Transactions. — Geology. 



temperature of the earth itself, irrespective of the heat 

 received from the sun, was greater than it now is, and with 

 the additional solar heat it is certain that greater contrasts of 

 heat and cold must have prevailed on the earth's surface and 

 its attendant atmospheric envelope. 



But if we assume that during the Pleistocene period the 

 earth was warmer, that evaporation was possibly greater, and 

 the movements of the atmosphere and the waters of the 

 ocean were more pronounced than they now are, it is of 

 the highest importance to understand the effect of increased 

 evaporation as a diffuser of heat over the earth's surface. 

 For example, a pound of water at boiling-point absorbs 

 during its evaporation as much heat as would raise 5-|lb. 

 of water from freezing-point to boiling-point, and this 

 without increasing the temperature of the vapour. If we 

 consider the vast quantity of evaporation that takes place in 

 the warmer zonal regions of the earth, it must be evident that 

 an enormous amount of energy is expended in this constant 

 conversion of water into vapour, and that large quantities of 

 heat are transferred in this way from the warm to the colder 

 zonal regions. As pointed out by Balfour Stewart, "Our own 

 earth is . . . an engine having the equatorial regions as 

 its boiler and the polar regions as its condensers, for at the 

 equator the air is heated by the direct rays of the sun, and 

 we have there an ascending current of air, up a chimney as it 

 were, the place of which is supplied by an indraught of colder 

 air along the ground or floor of the world from the poles on 

 both sides. . . . Very often, too, aqueous vapour as well 

 as air is carried up by means of the sun's heat to the upper 

 and colder regions, and there deposited, in the shape of rain or 

 hail or snow, which ultimately finds its way back again to 

 the earth, often displaying in its passage immense mechanical 

 energy." But what becomes of the heat — the latent heat 

 which was absorbed when the water was passing into vapour? 

 That it does not return to the earth is evident, for the rain 

 or snow or hail, as the case may be, is much below the tem- 

 perature of the vapour from which it was derived by con- 

 densation. 



In this process of carriage and condensation a large 

 quantity of heat becomes dissipated, and is finally lost by 

 passing away into outer space. Thus, although great stores 

 of heat are carried from the warmer to colder regions of the 

 earth by atmospheric agencies, in the performance of this 

 work much extra heat is demanded, and it is by the expendi- 

 ture or partial loss of this heat that the work of condensation 

 is carried on. The nearer the product of condensation is to 

 the freezing-point the greater the quantity of heat left behind, 

 and subsequently diffused in space. Thence it follows that the 



