OF GEOLOGICAL DYNAMICS. 117 



tons per million seconds, or 220 metre-tons per 

 annum. The whole area of the earth is 5 10,000,000 

 square kilometres ; and therefore the loss from the 

 whole earth is 3600 millions of metre-tons per 

 second, or 112 x io 15 metre-tons per annum. This 

 statement is not hypothetical in any respect. But 

 the numerical data assumed in it, being '005 

 gramme-water-units per centimetre per second for 

 conductivity, and i cent, per 30 metres for the 

 rate of increase of underground temperature down- 

 wards, are what Professor Huxley would justly call 

 loose, because we do not know the true average 

 conductivity of the upper strata for the whole 

 earth, nor the true average value of the rate of 

 augmentation of temperature per metre down- 

 wards ; and a considerable margin of probable error 

 must be allowed for any estimate that can yet be 

 made of the true rate at which energy is being lost 

 from the earth. This, however, does not at all 

 affect the principles in illustration of which I 

 adduce the numbers, or the importance of these 

 principles for the success of geology as a science. 

 33. The store of energy, transformations of 



