308 Mr D. Milne on Earthquake-Shocks in Great Britain, 



sures, is known to miners, as the deepest mines are never free 

 from water, which, the greater the depth, exerts the greater 

 hydrostatic pressure. Besides, it will be remembered that 

 earthquake-shocks are most frequent at those seasons when 

 the weight of the atmosphere is least, and when, therefore, 

 the subterranean elastic forces may, to some extent, exert a 

 lifting or heaving power on the superincumbent strata, and 

 thus facilitate the progress of the water along the cracks and 

 fissures. 



(3.) The suggestions now thrown out to explain the cause of 

 earthquake-shocks, and the phenomena accompanying them, 

 are founded on the assumption, which most geologists seem to 

 have adopted, that these shocks arise either from disturbances 

 in the molten lava on which the earth's crust is floating, or 

 from disruptions caused by an explosion or sudden expansion 

 of elastic vapours. On that view, it is not difficult to see that 

 a considerable variation, and especially a diminution, of atmo- 

 spheric pressure over any portion of the earth's surface, may 

 facilitate there the production of such subterranean commo- 

 tions ; and in this case the electricity evolved would be classed 

 among the phenomena attending, and not among the causes 

 of, earthquake-shocks. 



But, on the other hand, it seems to deserve consideration 

 whether electricity may not itself be, occasionally at least, in* 

 strumental in the production of shocks. If (as cannot be 

 doubted) electricity be generated to a great extent in the in- 

 terior of the earth, and is, at the very time that the shocks 

 occur, transmitted'upwards to the surface, is it not reasonable 

 to suppose that vibrations would thereby be caused in the 

 earth's crust as effectually as by the alleged disruption of 

 strata ? And is it not more philosophical to attribute such vi- 

 brations to a known and sufficient cause, than to one entirely 

 hypothetical ? 



It may be asked how, in this view, the diminution or the 

 variation of atmospherical pressure is connected with the pro- 

 duction of earthquake-shocks, if they are caused solely or 

 chiefly by electricity 1 In answer to this objection, it may be 

 remarked, that those circumstances which aid in the produc- 

 tion of subterranean electricity, are generally indicated by a 

 *low barometer, and at all events occur chiefly at those seasons 



