308 REPORT— 1851. 



For the seismoscope wave, the former correction is 



1220-44x0"-065 

 1" 

 and the latter 1559-96 xO"-065 



1 220-44 x0"-065 „ „^^ „ ^ 

 p7 =79*30 feet, 



1' 



= 101 -40 feet; 



and adding these to the uncorrected rates, we obtain the true transit rates :— 

 For jointed granite 1220-44 + 79-30=1299-74 feet per second, and for the 

 more solid granite 1559-96 + 101-40=sl661-36 feet per second. 



We have now experimentally ascertained, with considerable accuracy, the 

 rate at which an elastic wave of impulse is transmitted through two distinct 

 media, sand and granite, which, as being perhaps the worst and the best for 

 the rapid transmission of such movements to be met with in extensive 

 masses on the surface of the earth, may be viewed as those in which the limits 

 of wave motions, analogous to those of earthquake waves, are to be found. 



The results are as unexpected as remarkable. In the original memoir upon 

 this subject, published in the Transactions of the Royal Irish Academy, vol. 

 xxi. part 1, on the hypothesis that earthquake waves were true waves of 

 elastic compression, I ventured to conclude that their transit rates in various 

 solid and homogeneous media would be found to be proportionate to the 

 square roots of their moduli of elasticity. What the transit rate might be in 

 discontinuous or interrupted media, I could not venture even to guess. It 

 was not without surprise, however, that I found it in sand reduced so low 

 as under 966 feet per second; and this was again increased, when it proved 

 in granite not to exceed a maximum of 1662 feet per second ; the former 

 falling below the rate of ordinary sound in air about as much as the latter 

 rises above it. Had I been asked beforehand what velocity I expected to 

 find in solid granite, I should have replied, possibly 8000 feet per second, or 

 more ; but the extremely low velocity previously found in the sand had in 

 some degree prepared me to expect one of lower value in the granite, and 

 on careful review of the whole research I am disposed to consider the results 

 given as true answers to questions truly put to nature. Whence then this 

 apparent discrepancy between hypothesis and experiment? I believe the ex- 

 planation is twofold. 



No such thing as an absolutely solid and homogeneous rock formation exists 

 anywhere on the earth's surface. Whatever may be the rate of wave-transit 

 in any one solid block or mass of rock, the transit rate through the whole 

 depends not only upon the density and elasticity of the minerals, but upon 

 the extent and degree to which its mass is broken and fissured, and upon 

 the nature and direction of these fissures. If the fissures be but fractures 

 M'ith closely abutting surfaces, and these surfaces, chiefly either in planes 

 perpendicular to, or in the direction of the wave's transit, they will produce 

 the least degree of retardation in the velocity of the wave, and have the least 

 effect in the extinction of its volume ; but when such fissures are either alter- 

 nations of bedding filled in with thick or thin layers of material, heterogene- 

 ous to the mass of the formation of invaded rock, or shatterings in various 

 planes at many and uncertain angles of direction to that of the wave's mo- 

 tion, and with surfaces loosely approximating, or even altogether out of con- 

 tact, then the effect, of loss of vis viva, at every such alternation of medium, 

 in retarding the velocity of the wave, and by innumerable reflexions and 

 alterations in direction, dispersing its volume, will be enormous. 



In some instances, where the angle at which the new medium meets the 

 line of transit of the emergent wave is suitable, reflexion (as in the case of 



