assumed to accompany Earthquakes. 543 



is no evidence ; and all bodies must, as the effect of one shock, 

 fall in the one direction, and not in opposite directions, which 

 is contrary to observed facts: or, on the other hand, if the 

 movement be an alternate horizontal motion, as all observa- 

 tions go to prove it is, then the motion in one direction must 

 be slower than in the other, or attended with other differences 

 of circumstances. The backward motion must be different 

 from the forward motion, or otherwise displaced bodies would 

 be replaced by the recurrence, in the opposite sign of forces 

 similar and equal to those that first set them in motion ; but 

 they are not found so replaced. 



Now, of all conceivable alternate motions, the only one that 

 will fulfill the requisite conditions observed, namely, that shall 

 move with such an immense velocity as to displace bodies by 

 their inertia, or even shear close off great buttresses from the 

 wall, they sustained, (Darwin) or project stones out of their 

 beds, by inertia; that shall have a horizontal alternate motion, 

 either much quicker in one direction than in the other, or dif- 

 ferent in its effects ; and that shall be accompanied by an up- 

 ward and downward motion at the same time — a circumstance 

 universally described as attendant on earthquakes — the only 

 motion, I say, that will fulfill these conditions, is the transit of 

 a great solitary wave of elastic compression, or of a succession 

 of these, in parallel or in intersecting lines through the solid 

 substance and surface of the disturbed country. 



The general idea of the nature of earthquake motion, viz. 

 that it consists of a wave of some sort, is not however new, 

 although so entirely neglected by the mass of recent geologi- 

 cal authors. To the Rev. John Mitchell, M.A., Fellow of 

 Queen's College, Cambridge, the merit of this idea appears 

 originally due. In a paper communicated to the Royal So- 

 ciety, read in 1760, and published in the 51st vol. of the Philo- 

 sophical Transactions, part 2nd, he treats at length of the 

 origin and pheenomena of earthquakes, and distinctly enun- 

 ciates the following view: — 



That the motion of the earth is clue to a wave, propagated 

 along its surface, from a point where it has been produced by 

 an original impulse. This impulse, he conceives, to arise from 

 the sudden production or condensation of aqueous vapour, 

 under the bed of the ocean, by the agency of volcanic heat, 

 the supposed mechanism of which he minutely describes; but 

 while he was so far right in his conception of an elastic wave 

 of some sort, 1 expect to be able shortly to prove that he has 

 wholly mistaken the nature of the wave that actually occurs, 

 and that a wave, such as he assumes, can have no existence 

 consistently with the physical structure of our globe, with the 



