36 Transactions. 



respects for purposes of exact scientific inquiry, I trust that their discussion 

 may lead to the adoption of instrumental means for recording such pheno- 

 mena in future, as it is in this manner alone that sufficient accuracy can be 

 obtained. 



In the first place I will explain, in a few words, the exact nature of the 

 recently-experienced phenomena, concerning which there is a good deal of 

 misconception. 



NotAvithstanding the apparent rigidity of the rocks Y/hich form the crust 

 of the globe, they are nevertheless truly flexible and elastic ; and, for the 

 propagation of earthquake shocks, those Avhich appear to us to be most 

 compact and stubborn, are really the most elastic and susceptible of rapid 

 vibratory motion. 



The manner in which earthquake shocks affect the surface of the earth, 

 and the secondary phenomena by which they are accompanied, is now well 

 understood ; but only a few of the many causes which may lead to their 

 production are yet ascertained. 



Earthquakes occur, and perhaps originate, in every part of the earth's 

 crust ; and, from the researches of Mallet, there is good reason to believe 

 that the surface is, in some part or other, continually being subject to the 

 jarring motion which they produce. Volcanic regions are particularly liable 

 to them ; but there they seem to be only local phenomena, that fail to pro- 

 duce very distant effects. Yolcanic energy has indeed been generally 

 adopted as the cause of earthquakes ; but applying the term to those forces 

 by which masses of molten and chemically altered materials are heaped up 

 on the surface of the land or poured out beneath the ocean, it is more 

 probable that such convulsions are not originated, but only set loose, by the 

 passage of waves of motion through the crust of the earth, which in their 

 origin are quite independent of the local tension or constrained force which 

 gives rise to the volcanic eruption. 



"We are rather led to look on the passage of earth-waves as the normal 

 state of things, depending, like the ocean tides, on cosmical causes exterior 

 to our planet. "When their passage is interfered with they become percep- 

 tible to our senses : when they interfere with and let loose pent up forces or 

 tensions in some portions of the earth they lead to sudden convulsions, 

 which in their turn give rise to secondary phenomena that produce the most 

 terrific and appalling catastrophes. These great convulsions appear to 

 occur, nearly in every case, at the bottom of the ocean, and where it has a 

 profound depth at no great distance from land. 



The phenomena w^hich attend such convulsions in the order they woxild 

 appear to an observer on the shore of the neighbouring land, have been 

 described by Mallet as follows : — 



