146 Royal Society : — 



May 8.— Major- General Sabine, President, in the Chair. 



The following communication was read : — 



" Appendix to the Account of the Earthquake-wave Experiments 

 made at Holyhead." By Robert Mallet, Esq., C.E., F.R.S. 



This communication contributes the sequel of the author's " Re- 

 port on Earthquake-Wave Experiments " (made at Holyhead), 

 as published in part 3 of the ' Philosophical Transactions ' for 

 1861. At the conclusion of that paper the author expressed his 

 hope of being able soon to lay before the Royal Society some experi- 

 ments for the determination of the modulus of elasticity of perfectly 

 solid portions of both the slate and the quartz rock formation 

 through which his wave-transit experiments had been made at Holy- 

 head, with a view to throw light upon the relations between the 

 theoretic velocity of transmission (if the rocks were all solid and 

 homogeneous) and the actual velocity as determined by experiment. 

 He has now determined the elastic modulus for both rocks, and 

 for each rock in two directions, viz. parallel to and transverse to its 

 lamination ; and he has extended his determinations to specimens of 

 each rock of maximum and of minimum compactness and hardness, 

 so that the series of experiments upon the compressibility of these 

 rocks (from which the modulus is derived) assumes the following 

 divarication, viz. : — 



rTX , . f B. Parallel to laminae, Table 2. 

 f Hardest . < ^ Transverse to lararnge , Table 1. 

 Slate rock ,J ] F< Parallel to lamina, Table 6. 



I feoltest • • { E. Transverse to laminae, Table 5. 



TT , f D. Parallel to laminae, Table 4. 



f Hardest . j Q Transverse to laminae, Table 3. 

 Quartz rock .J r H Para i le l to laminae, Table 8. 



I feottest • • j G. Transverse to laminae, Table 7. 



Involving thus eight distinct series of experiments. 



The compressions were conducted at the Royal Arsenal, Woolwich, 

 by the aid of the excellent American machine belonging to the Royal 

 Gun-factories, permission to use which was accorded to the author. 



The specimens of rock submitted to pressure were all equal cubes 

 of 0*707 inch on the edge, presenting thus a surface on each side 

 of O'o square inch — a dimension presenting facilities for tabular 

 reduction, &c. 



The cubes were cut from the chosen rock specimens (selected with 

 care as fairly representative) by means of the lapidary's wheel, and 

 had opposite faces rigidly parallel and equal. 



The pressures advanced by 1000 lbs. per square inch of surface, 

 from zero up to the crushing point of the specimen ; and at each 

 advance the actual compression of the column of rock was measured 

 by instrumental arrangements that admitted of reading space to 

 •0005 of an inch. The results are given in Tables numbered 1 to 8, 

 referred to above, and these are compared in two Tables numbered 

 9 and 10. 



