DISTRIBUTION OF EARTHQUAKES IN TIME. 235 



as deduced from many observations appears to be about 

 1° F. for every fifty or sixty feet of descent. 



To explain this and other kindred phenomena it is 

 assumed that the earth was once very much hotter than 

 it is at present, and to reach its present stage it has been 

 gradually cooling. As the laws of cooling are perfectly 

 known, to calculate how many years it must have taken a 

 body like our earth to cool down to its present tempera- 

 ture is a definite problem. Sir William Thomson, start- 

 ing with the temperature of 7,000° F., when all the rocks 

 of the earth must have been molten and a skin or crust 

 upon the surface, such as is so quickly produced upon the 

 surface of molten lava, finds by calculation that the time 

 taken to reach the present temperature must have been 

 about one hundred million years. Into this period he 

 and other physicists desire to compress the history of all 

 the stratified deposits. Geologists find this period too 

 short. Others seeking to reconcile the views of physicists 

 and geologists endeavour to show that the various agencies 

 engaged in degrading rocks and accumulating sediments 

 in former ages are not to be judged of by the agencies 

 we now see around us ; in former times they were more 

 active. At one period the elastic tides in the earth may 

 have been so great that they resulted in the fracturing 

 off from our planet its satellite the moon, and subse- 

 quently the moon, acting on the waters of the earth, may, 

 even as late as 150,000 years ago, have produced every 

 three hours tides 150 feet in height. 



Whatever may be the value of the figures here quoted, 

 reasonings like these bring us to the conclusions that the 

 various agencies which we now know to be acting upon 

 our earth were once far more potent than they are at 

 present, and if the moon, as a producer of elastic tides, 

 has any influence upon the occurrence of earthquakes. 



