260 C. Davison — Vorticose Earthquake Shocks. 



Dynamics of Earthquakes," and therefore before the mechanism of 

 earthquakes was clearly understood. 



M. Eamond, who had for many years explored the Pyrenean range 

 and experienced numerous earthquakes, says, " I have felt as many 

 as ten or twelve shocks in the course of one night, and I am quite 

 sure that, although the oscillations usually took place in the direc- 

 tion from north to south, the propagation was made in the direction 

 of the chain, in such manner that all the rocks successively dis- 

 turbed were found on the same parallel. This apparent contradiction 

 is easy to explain : an explosion which traverses a series of distant 

 cavities, must exert all its energy on their lateral walls, and impress 

 on them movements of vibration, which open them and draw them 

 together in a direction that crosses at right angles the line of pro- 

 pagation." ^ 



Prof. Perrey suggests two theories, but does not state which he 

 considers the more probable. One is similar to that of Baron 

 Eamond ; the other involves the hypothesis of a thin solid crust, and 

 is as follows: "Let us admit at the internal surface of the solid 

 crust of the globe (supposed fluid in the interior), an immense pro- 

 tuberance, a kind of mountain, which, to fix our ideas, we shall sup- 

 pose directed parallel to the axis of an external valley, a great river- 

 basin for example. Let us imagine the central nucleus, or a part of 

 the materials which compose it, to strike this mountain at one of its 

 extremities and in the direction of the axis of the chain ; the move- 

 ment will certainly propagate itself, according to the above hypo- 

 thesis, along the axis of the external basin ; but, on the other hand, 

 will there not be produced derived movements which will manifest 

 themselves outside on the flanks or slopes of the basin, by vibra- 

 tions, diverging oscillations, or oscillations which will even be 

 perpendicular to the axis of the basin." ^ 



Mr. Mallet supposes these shocks to be caused by the transversely 

 vibrating particles^ describing curves similar to 

 those in the annexed figure (Fig. 1). He con- 

 siders that for small angles of emergence the 

 transverse vibrations in nearly vertical planes are 

 of very little importance, and may be neglected in 

 comparison with the direct vibrations ; but that 

 for vertically arriving shocks the horizontal trans- 

 verse movements would be powerful enough to 

 cause the " twisting and wriggling motion, violent 

 in its changes of direction," whilst the direct 



1 Journal des Mines, vol. xii. pp. 95-96. Quoted by M. Perrey. " Memoire sur 

 les ti'emblements de terre ressentis en France, en Belgique et en HoUande, etc." : 

 Mem. Cour. de I'Acad. Eoy. de Bruxelles, vol. xviii. p. 103. 



- M. Perrey. " Sur les tremblements de terre ressentis dans le bassin du Ehone " : 

 Ann. de la Soc. Eoy. d'Agric. de Lyon (1845), vol. viii. p. 341. 



^ If a disturbance take place at any point of a homogeneous and isotropic solid, 

 two spherical waves will diverge from that point with unequal velocities. In the first 

 every particle moves approximately along a line joining it to the centre of disturb- 

 ance, and the corresponding wave is said to consist of direct vibrations. In the 

 second the particles vibrate in planes perpendicular to the lines joining them to the 

 centre of disturbance, and are said to execute transverse vibrations. 



