THE CAUSE OF EARTHQUAKES. 289 



elastic tides must be produced. A consequent pheno- 

 menon, dependent on the existence of these tides, would 

 be a marked regularity in the occurrence of earthquakes. 

 As this marked regularity does not exist, we must 

 conclude that earthquakes are not due to the attractive 

 influences of the sun and moon acting upon the thin 

 crust of the earth covering a fluid interior. The period- 

 icity of earthquakes corroborates the conclusions of Sir 

 William Thomson, who remarks that if the earth were 

 not extremely rigid the enormous elastic tides which 

 must result would be sufficient to lift the waters of the 

 ocean up and down so that the oceanic tide would be 

 obliterated. 



Assuming that the earth has the rigidity assigned to it 

 by mathematical and physical investigators, we neverthe- 

 less have travelling round our earth, following the attrac- 

 tions of the moon and sun, a tidal stress. This stress, im- 

 posed upon an area in a critical state, may cause it to give 

 way, and thus be the origin of an earthquake. Earth- 

 quakes ought therefore to be more numerous when these 

 stresses are the greatest. 



The periods of maximum stress or greatest pull ex- 

 erted by the moon and sun will occur when these bodies 

 are nearest to our planet — that is, in perigee and peri- 

 helion, and again when they are acting in conjunction 

 or at the syzygies. That earthquakes are slightly more 

 numerous at these particular periods than at others 

 is a strong reason for believing that the attractions of 

 the moon and sun enter into the list of causes producing 

 these phenomena. 



Had there been a strongly marked distinction in 

 the number of earthquakes occurring at these particular 

 seasons as compared with others, we might have attributed 

 earthquakes to the existence of elastic tides of a sensible 



