ON THK FACTS OF EARTHQUAKE PHiENOMENA. 



309 



light) will no doubt be total, and thus an earthquake shock may emerge from 

 the surface at a point totally different from that indicated by its original path. 

 An analogous phaenomenon of continued reflexion seems to be the pro- 

 bable solution of the frequently-observed case of shocks felt in mines at cer- 

 tain depths, and yet not at the surface or still deeper in the mine. Thus, if 

 o, b constitute a couche of formation of a certain elasticity and density, lying 



Diag. 11. 



— h 



upon another of very different and perhaps much lower elasticity, and with 

 a third different formation superimposed, a wave transmitted at a certain 

 angle in the direction of the arrow from a, may be reflected downwards and 

 upwards alternately until extinguished, and while scarcely or not at all 

 perceptible in the beds above or below, may produce a powerful shock 

 throughout the intermediate beds «, h. The Saxon and South American 

 mines afford frequent record of this phaenomenon. 



But to return. To these causes I ascribe the great deficiency in velocity 

 which we have ascertained, below that due to the probable moduli of the 

 materials. In the lower limit, the sand, which may be viewed as only the 

 extreme case of a rock formation shattered and broken up indefinitely, the 

 velocity due to its material would theoretically be the high one due to the 

 highly elastic quartz of which its particles mainly consist ; but the extreme 

 want of continuity occurring at every fraction of an inch through which the 

 path of the wave lies and the imperfect juxtaposition of the particles, neu- 

 tralize all this ; and the low conducting power of sand for impulse (elastic 

 waves) is matter of even popular knowledge : thus we find sand-bags for this 

 reason employed to form the breastworks of batteries, as the best material to 

 absorb and neutralize the stroke of cannon-shot. 



The retarding effects of heterogeneity of medium have been well shown by 

 the results of Mr. Goldingham's elaborate experiments at Madras upon the 

 rate of sound in dry and in damp air, from which he concluded that sound 

 in common air suffered a retardation of I'^Ofoot per second for each degree 

 of his hygrometer, due to the admixture of moisture. If water in the state of 

 intimate suspension, in which it exists invisibly dissolved in air, has such an 

 effect, how much more might we look for it in sand mixed with air and water 1 



It would be premature and useless in the as yet imperfect state, both ex- 

 perimental and theoretic, of what we may hereafter call Stereo-acoustics, 

 or knowledge of the nature of sound in solids, to affirm that these are all 

 the conditions that may modify the transit rate of waves in such media, or 

 that if we had presented to our experiments extended formations of mineral 

 matter, absolutely homogeneous and unbroken, we then should find the 

 transit rate of longitudinal propagation of the wave such as theory has 

 hitherto predicted it, or that it would be the same as the transit rate of/ree 

 vibration in the same medium. On the contrary, there is reason already to 

 believe that hereafter it may prove to be otherwise. 



We may recall the experiments made long since by M. Biot, upon the 

 transit rate of sound in the cast iron water-pipes of Paris. He announced as 

 his result that sound in cast iron travels longitudinally with a velocity 10-5 



