LIFE ON THE EARTH. 135 



supposed to have declined to the actual condition, 

 and with the depression of temperature the atmo- 

 spheric wasting power to have declined also, and in 

 a more rapid progression. For the quantity of mois- 

 ture sustained in air of different temperatures freely 

 in contact with water diminishes more rapidly than 

 the temperatures 1 ; so that if we take 56 for the 

 present mean temperature of the surface of the 

 earth, and suppose it to have been formerly 76, the 

 quantity of moisture held up by the atmosphere 

 formerly may be taken at nearly double that now 

 supported in it. 



This being the case, and the causes of rain and 

 vicissitudes of seasons the same in kind, we may 

 admit the atmospheric power in wasting the earth's 

 surface to have been double what it is now, and 

 that from that time to the present it has sunk in 

 geometrical progression. Under these conditions the 

 time consumed in the production of the same total 

 effect as that already stated, under the actual 

 powers of the atmosphere, could not be so little as 

 two-thirds of that computed on the hypothesis of 

 uniform action, viz. 63,936,000 years. 



But according to the same hypothesis the action 

 of the sea and atmosphere in early times must have 

 been more effective than at present, because of the 



1 It varies in geometrical proportion to the temperature. 



