220 REPORT — 1861. 



Mincow in Hungary, and by myself in the (late) Neapolitan kingdom, after 

 the great shock of 1857, Mhere I found that the velocity of propagation iu 

 the shattered limestone and argillaceous rocks of the shaken region was even 

 below what has been here determined for the harder and more compact rocks 

 of Wales, also of stratified structure. Experiment and observation have 

 thus alike sustained the three provisional conclusions anticipated by me as 

 to the transit-velocities of earthquake-waves in nature (at the conclusion of 

 "Second Report," &c., Report of Brit. Assoc. 1851, p. 316), in passing 

 through formations different in character. 



In experimenting with these great explosions at Holyhead, I have been 

 enabled to see that such great impulses, though offering the advantages of a 

 greatly extended range, and hence larger total time-period for measurement, 

 do not in reality admit, from various contingent circumstances, of greater, or 

 perhaps of as great accuracy of transit determinations, as do much smaller 

 explosions, such as those specially madeatKilliney Bay. These great explo- 

 sions, however, elicit phenomena visible in the seismoscope, which are too 

 faint to be distinct when due to smaller charges, and which analogize closely 

 with the succession of vibratory and wave movements observed in natural 

 earthquakes. In the larger of these great explosions, as the impulsive wave 

 approached the instrument, the previously steady reflected image of the cross 

 wires did not at once disappear ; the definition of the wires rapidly became 

 obscured, the obscuration increasing fur an instant to a flickering of the 

 image, preceding its obliteration, at the same moment that the oscillation 

 then communicated to the trough caused the mercury to sway from end to 

 end, in a liquid wave, whose amplitude was sufficient to cause variable flashes 

 of light to be transmitted to the eye, with the changing inclination of the 

 reflecting-surface of the undulating mirror, — the image of the cross Avires 

 reappearing (but now oscillating with the movement impressed upon the 

 mercury in the direction of the wave-transit) by passing through a second 

 phase of flickering and vibration, but in the reverse order, before becoming 

 perfect in definition as at the commencement. 



I had thus presented visibly before me the " tremors " that nearly invari- 

 ably are described as preceding and following the main shock and destructive 

 surface movement in every great earthquake. The phenomena appear to be 

 identical, however premature it may be to propose a precise and adequate 

 explanation of their production. 



There appear to be three elements upon which the wave-transmissive 

 power of a rock-formation mainly depends, viz. the modulus of elasticity of 

 its material, the absolute range of its compression by a given impulse or im- 

 pact, and the degree of heterogeneity and discontinuity of its parts. As has 

 been already described, the range of wave-transit of these experiments 

 passed through two rock-formations, quartz and slate, differing in name 

 and in several respects in structure, yet very much alike, as has been re- 

 marked, in intimate composition. It remains to show experimentally that 

 they do not differ in these conditions of transmissive power to such an extent 

 as materially to affect the results. 



If a perfectly elastic ball be dropped upon a mass of perfectly elastic rock, 

 whose volume may be considered as infinite with respect to that of the ball, 

 the latter will rebound to the height from which it descended ; and if the 

 same ball, though not perfectly elastic, be dropped in succession upon like 

 masses of two different rocks, it will rebound from each to a height less than 

 that from which it fell, and the value of which will depend mainly upon the 

 elasticity, the depth of the impression, and tlie degree of discontinuity of the 



