Mr. R. Mallet on Earthquake-wave Experiments at Holyhead. 147 



The following are the mean compressions for each 1000 lbs. per 

 square inch : — 



Slates. 



Quartz. 



A. 



B. 



E. 



F. 



C. D. 



a. 



H. 



inches. 



•000627 



up to 



23,000 lbs. 



inches. 



•0025000 



up to 

 26,000 lbs. 



inches. 



•0039144 



up to 

 14,000 lbs. 



inches. 



•0037000 



up to 

 7000 lbs. 



inches. 



•0007085 



up to 

 35,000 lbs. 



inches. 



•0010947 



up to 

 19,000 lbs. 



inches. 



•0014666 



up to 

 12,000 lbs. 



inches. 



•0172666 



up to 

 6000 lbs. 



Crushing usually took place at 1000 to 2000 lbs. additional pres- 

 sures beyond the above limits, up to which the compressions were 

 tolerably uniform. 



The discussion of these Tables fully presents some interesting and 

 novel results. 



Generally the quartz rock is less compressible than the slate ; the 

 softest quartz, however, is much more compressible than the softest 

 slate in a direction parallel to the lamination of both. In this di- 

 rection also the hardest slate is more than double as compressible as 

 the hardest quartz. Transverse to the lamination, however, both the 

 hardest slate and quartz have nearly the same coefficient of compres- 

 sibility, which is very small for both. In the latter direction also 

 the softest slate and quartz have almost the same coefficient, but one 

 about four times as great as for the hardest like rocks. 



The author points out several conclusions of much interest dedu- 

 cible from these experiments as to the physical and geological con- 

 ditions under which these rocks were formed and consolidated. The 

 compression by natural forces has already been greatest in directions 

 transverse to the lamination. The great compressibility in the op- 

 posite directions, or parallel to the lamination, appears to arise chiefly 

 from the mass of the rock being made up of minute wedge-shaped 

 mineral particles, deposited all with their largest dimensions on the 

 plane of lamination, and so acting on each other like wedges. 



Some curious circumstances in the mode of giving way of the rocks 

 under pressure are shown by the author to be probably connected 

 with their mass being formed of an aggregate of several simple 

 minerals. 



He points out the great differences in wave-transmissive power in 

 directions transverse to and parallel to the lamination which these ex- 

 periments disclose. The specific gravities of the several specimens 

 of rock are then given, to enable the modulus of elasticity to be ob- 

 tained in feet, and the general results of the experiments are com- 

 prised in the following Table (p. 148) : — 



The author then proceeds to apply these results to the comparison 

 of the theoretic and actual transit-periods of the wave of impulse. 



The general expression for elastic wave-propagation in a homo- 

 geneous medium may be expressed by an equation of the form 



V=V^L= 8-024 VL, 



L2 



