610 HISTORY OF GEOLOGY. 



earth might contract, as most bodies do when they 

 cool; and this contraction might lead to mechanical 

 results, as the shortening of the day. Laplace satis- 

 fied himself, by reference to ancient astronomical 

 records, that no sucfi alteration in the length of the 

 day had taken place, even to the amount of one 

 two-hundredth of a second; and thus, there was 

 here no confirmation of the hypothesis of a primi- 

 tive heat of the earth. 



Though we find no evidence of the secular con- 

 traction of the earth in the observations with which 

 astronomy deals, there are some geological facts 

 which at first appear to point to the reality of a 

 refrigeration within geological periods ; as the ex- 

 istence of the remains of plants and shells of tropi- 

 cal climates, in the strata of countries which are 

 now near to or within the frigid zones. These facts, 

 however, have given rise to theories of the changes 

 of climate, which we must consider separately. 



But we may notice, as connected with the doc- 

 trine of central heat, the manner in which this 

 hypothesis has been applied to explain volcanic and 

 geological phenomena. It does not enter into my 

 plan, to consider explanations in which this central 

 heat is supposed to give rise to an expansive force 5 , 

 without any distinct reference to known physical 

 laws. But we may notice, as more likely to become 

 useful materials of the science now before us, such 

 speculations as those of Mr. Babbage ; in which he 



6 Scrope On Volcanoes, p. 192. 



