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XXX. Appendix to the Account of the Earthquake-Wave Experiments made at Holyhead. 
By Robeet Mallet, C.E., F.R.S. 
Eeceived March 27, — Bead May 8, 1862. 
I AM now enabled to fulfill the intention expressed (vol. cli. p. 678) in concluding the 
above ‘ Account,’ as read to the Eoyal Society, having since then completed a series of 
experiments upon the compression of specimens of the Holyhead Rocks, and deter- 
mined their moduli of elasticity. These 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 
being taken to preserve perfect parallelism between the opposite boundary planes, so 
that, when compressed between hardened steel surfaces, fracture should not result by 
inequality of pressure. 
The compressions were made at the Royal Arsenal, Woolwich, ufith the very accurate 
and excellent machine used for testing compression and extension of metals in the 
gun-factory; and I have to express my thanks to Lieut.-Col. Andeeson, C.E., the 
Superintendent of that department, for the valuable assistance afibrded me through his 
attention. 
The specimens operated on consisted of two each from the following four classes, 
namely, the hardest and the softest slate-rock, 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, quam prox., 
parallel to the plane of natural lamination or jointing. The load (50 lbs.) first applied 
was considered zero, being only sufficient to ensure a complete bearing in all parts of 
the instrument. The subsequent loads advanced by 1000 lbs. per square inch of surface 
at a time, up to the crushing of the specimen ; and at each fresh load the amount of 
compression was measured by beam-calhpers, with instrumental arrangements that 
admitted of reading space to -0005 of an inch. 
The experimental results, as obtained, are recorded in the following Tables, from 
No. I. to No. VHI. inclusive ; and in the succeeding Tables IX. and X., 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. 
MDCCCLXII. 4 X 
