274 ANNUAL OF SCIENTIFIC DISCOVERY. 



Mr. Mallet's inquiry commences with a demonstration of the cause 

 of the rotary or vortical motion supposed to be peculiar to earth- 

 quakes ; one of the most remarkable instances of this movement is 

 that mentioned by Sir C. Lyell, where the upper stones of a square 

 pedestal are described as having- been turned round several inches from 

 their place without falling, while the lower blocks retained their origi- 

 nal position. An analogous case is cited by Darwin ; the buttresses 

 of the Cathedral at Concepcion were twisted, so to speak, clean olF 

 from the walls, while the walls themselves remained standing 1 , and 

 comparatively uninjured. Assuming the vortical theory to be the 

 true one, the rotary force sufficient at its centre to displace such vast 

 masses of masonry would be inconceivably great at a few hundred 

 feet distant ; so great, indeed, that nothing could resist it, and the 

 tremendous phenomena of earthquakes would be a thousand times 

 more terrible than they already are. Mr. Mallet in brief terms shows 

 that the effects described can be satisfactorily accounted for on other 

 grounds. " I assume," he observes, " nothing more than what is 

 universally admitted, that during earthquakes a motion of some sort 

 takes place, by which the ground itself, and all objects resting upon it, 

 are shaken, or moved backwards and forwards by an alternate horizon- 

 tal motion, within certain limits, which, for all present evidence to the 

 contrary, may be a straight-line motion, though possibly variable in 

 direction at different and sometimes closely successive times, and the 

 velocity of which is sufficient to throw down or disturb the position of 

 bodies supported by the earth, through their own inertia." Whether 

 a building shall be twisted round or completely overturned by the pro- 

 gressive motion in a straight line depends on the centre of gravity of 

 the edifice. " The effect of the rectilinear motion in the plane of the 

 base," we read, " will be to twist the body round upon its bed, or to 

 move it laterally, and twist it at the same time, thus converting the 

 rectilinear into a curvilinear motion in space." Next we have the 

 question as to whether the motion is alternate backward and forward, 

 by which, as some have assumed, displaced objects should be restored 

 to their former position. But as the back stroke cannot be so power- 

 ful as the forward one, it necessarily fails to move the disturbed bodies 

 from the situation in which the forward stroke left them. This gen- 

 eral view is not affected by the fact of bodies being occasionally 

 thrown down in opposite directions, as the east and west walls of a 

 building ; such anomalies are to be explained by differences of resist- 

 ance, weight, or connection, or simply by the return of the secondary 

 or reflected wave. 



The only conceivable alternate motion that answers to all the cir- 

 cumstances observed in earthquakes, is that of an elastic compression ; 

 in other words, a wave of elastic compression, passing along through 

 the crust of the earth, in parallel or intersecting lines. Earthquake 

 shocks originate either on the land, or under the ocean, the latter be- 

 ing the most disastrous in their consequences. " When the original 

 impulse comes from the land, an elastic wave is propagated through 

 the solid crust of the earth, and through the air, and transmitted from 

 the former to the ocean water, where the wave is finally spent and 



