and especially in Scotland. » 307 



strength as that they are nearly able to produce a disrup- 

 tion of the strata which confines them, a considerable depres- 

 sion of the barometer might enable them to accomplish that 

 result. It is thus easy to understand how shocks should be 

 more frequent in winter than in summer, and how at any season 

 shocks should generally be preceded by barometrical depres- 

 sion. 



It is thus shewn that the ordinary changes of atmospheri- 

 cal pressure may have an important effect on the elastic forces, 

 whether gaseous or liquid, which are supposed to exist beneath 

 the solid crust of the earth, and cause them to produce disrup- 

 tions, that would propagate vibrations to the earth's surface. 



(2.) But can all the electrical phenomena of earthquakes be 

 accounted for on the foregoing theory"? Would a mere change 

 of atmospherical pressure be attended by these phenomena, 

 without supposing the production and development of subter- 

 ranean electricity 1 Looking to the facts before described, it 

 seems difficult to deny, that, during earthquake-shocks, there 

 is an excess of electricity ; and from the way in which, on se- 

 veral occasions, it was indicated, it is reasonable to suppose 

 that it issued from the earth. The tvhizzing sound, — the cracks 

 or stiaps in the air, sometimes as loud as the report of a can- 

 non, — and the electrical shocks experienced by individuals 

 during earthquakes, seem almost to establish this proposition. 

 Electricity, as is well known, is produced by the conversion of 

 water into steam, so that if rain were to percolate from the 

 earth's surface far enough down into the interior of the globe, 

 so as to be converted into steam, electricity would be evolved 

 in considerable quantity. Now it is, in this point of view, im- 

 portant to observe, that earthquake-shocks are most frequent 

 in those places where there are deep and extensive fissures, 

 and at those seasons where rain falls in most abundance on 

 the earth's surface. From the table given on page 289 hereof, 

 it will be seen that the shocks are more frequent during the 

 first three months of winter than during the remaining three 

 months, a circumstance which is confirmatory of the suppo- 

 sition that electricity is evolved by the autumn rains. There 

 may be some hesitation in admitting that water could descend 

 in the fissures wliich intersect the earth's crust to the requi- 

 site depth. But that water docs descend very far, by such fi»- 



