386 PLEASANT WA YS IN SCIENCE. 



levelling power of rain is so effective. The sun's heat is 

 the true agent in thus levelling the earth, and if we regard, 

 as \ve justly may, the action of water, whether in the form 

 of rain or river, or of sea-wave raised by wind or tide, as 

 the chief levelling and therefore destructive force at work 

 upon the earth, and the action of the earth's vulcanian 

 energies as the chief restorative agent, then we may fairly 

 consider the contest as lying between the sun's heat and the 

 earth's internal heat. There can be little question as to 

 what would be the ultimate issue of the contest if land and 

 sea and air all endured or were only so far modified as they 

 were affected by these causes. Sun-heat would inevitably 

 prevail in the long run over earth-heat. But we see from 

 the condition of our moon how the withdrawal of water 

 and air from the scene must diminish the sun's power of 

 levelling the irregularities of the earth's surface. We say 

 advisedly diminish, not destroy ; for there can be no question 

 that the solar heat alternating with the cold of the long 

 lunar night is still at work levelling, however slowly, the 

 moon's surface ; and the same will be the case with our 

 earth when her oceans and atmosphere have disappeared by 

 slow processes of absorption. 



The power actually at work at present in producing rain, 

 and so, indirectly, in levelling the earth's surface, is enor- 

 mous. I have shown that the amount of heat required 

 to evaporate a quantity of water which would cover an area 

 of 100 square miles to a depth of one inch would be equal 

 to the heat which would be produced by the combustion 

 of half a million tons of coals, and that the amount of force 

 of which this consumption of heat would be the equivalent 

 corresponds to that which would be required to raise a 

 weight of upwards of one thousand millions of tons to a 

 height of one mile.* When we remember that the land 

 surface of the earth amounts to about fifty millions of square 

 miles, we perceive how enormous must be the force-equiva- 

 lent of the annual rainfall of our earth. We are apt to 

 * See my " Science Byways," pp. 244, 245. 



