ON THE TRANS IT- VELOCITY OF EARTHQUAKE WAVES. 223 



No simple relation appears as yet between the transit-velocity and that of the 

 particles ; and however interes'ting and important both to general physics and 

 to seismology may be further determinations with exactness of the former, 

 it is to the observation and measurement of the latter, by the methods pointed 

 out in the Report upon the Neapolitan Earthquake*, and there employed, 

 that we must look as instruments of future seismological research. 



I proceed to lay before the Association the results of some experiments 

 upon the modulus of elasticity of perfectly solid portions of both these 

 rocks, with a view to the interesting question of the relation between the 

 theoretic velocity of transmission, if the rock were all solid and homo- 

 geneous (y= ^2q -, e being that modulusY and the actual velocity found 



by the preceding experiments. 



Subsequently to the conclusion of the experiments at Holyhead, referred 

 to above, I have been enabled to complete a series of experiments upon the 

 compressibility of the rocks which formed my range there, and have de- 

 termined their moduli of elasticity, &c. The inferences derivable from this 

 latter series form the proper sequel to what has preceded, and they throw 

 some new and not unimportant light upon several points of earthquake 

 dynamics. The experiments were made upon cubes cut from solid and 

 perfect pieces of the rocks by the lapidary's wheel, each 0-707 inch upon 

 the edge — each side, therefore, presenting a surface of 0-5 square inch; 

 and the utmost care was taken to preserve perfect parallelism between the 

 opposite boundary planes, so that, when compressed between hardened steel 

 surfaces, fracture should not result by mere inequality of pressure. 



The experiments were made at the Royal Arsenal, Woolwich, with the 

 very accurate and excellent machine used for testing compression and ex- : 

 tension of metals in the gun-factory ; and I have to express my thanks to 

 Lieut.-Col. Anderson, C.E., the Superintendent of that department, for the 

 valuable assistance afforded me through his attention. The specimens ope- 

 rated on consisted of two each from the following four classes, namely — 



The hardest and the softest slate-rock, and the hardest and the softest quartz- 

 rock, which occur within the range or neighbourhood of my experimental 

 explosions at Holyhead ; and from each of these classes or varieties of the 

 two rocks, cubic specimens were compressed, 1st, in a direction transverse 

 to the plane of lamination, 2nd, parallel to the same, all the cubes being so 

 cut out of the rock that two sides were, guam prox., parallel to the plane of 

 natural lamination or jointing. The load (50 lbs.) first applied was consi- 

 dered zero, being only sufficient to ensure a complete bearing in all parts 

 of the instrument. The subsequent loads advanced by 1000 lbs. at a time, 

 up to the crushing of the specimen ; and at each fresh load the amount of 

 compression was measured by beam-callipers, with instrumental arrange- 

 ments that admitted of reading space to -0005 of an inch. 



The experimental results, as obtained, are recorded in the following 

 Tables, from No. 1 to No. 8 inclusive ; and in the succeeding Tables 9 

 and 10, the results of the former are compared, and the mean compression 

 deduced for each 1000 lbs. of pressure applied upon a prism of each of the 

 four classes of rock (two of slate and two of quartz), of one inch square 

 surface, and one inch in height, and under both conditions as to the relative 

 direction of pressure and of lamination. 



* Now in the press. Chapman and Hall, London : 2 vols. 8vo. 



