^676 ^IR. MALLET ON THE TEANSIT-VELOCITY OE EAETHQTT ANE WAYE&. 
by Noggerath*, who found it 1376 Paris feet per second; by ScHMiDif, of the shock 
about Mincow in Hungary, and by myself in the (late) Neapolitan kingdom, after the 
great shock of 1857, where I found that the velocity of propagation, in 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 natm'e 
(at the conclusion of “ Second Report,” &c.. Report of Brit. Assoc. 1851, p. 316), in 
passing through formations difierent 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 determi- 
nations, as do much smaller explosions, such as those specially made at Killiney Bay. 
These great explosions, however, elicit phenomena visible in the seismoscope, which are 
too faint to be distinct when due to smaller charges, and which analogize closely vdth 
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 defi- 
nition of the wires rapidly became obscured, the obscuration increasing for an instant to 
a flickering of the image, preceding its obliteration, at the same moment that the oscil- 
lation 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-sm’face of the 
imdulating mirror, — the image of the cross wires reappearing (but now oscillating until 
the movement impressed upon the mercury in the direction of the wave-transit) by pass- 
ing 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 invariably are de- 
scribed as preceding and following the main shock and destructive surface movement in 
eveiy 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 impact, 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, difiering in 
name, and in several respects in structure, yet very much alike, as has been remarked, m 
* Das Erdbeben vom 29 Juli, 1846, im Ebeingebiet, &c. V. Dr. Jakob Nogoekath. 4to. Bonn, 1847. 
t TTntersucbungen iiber das Erdbeben am 15 Jan. 1858. J. E. Schmidt, Astronom, Mittbeilungen der 
Kais.-Konigl. Geog. Gesellscbaft, 11. Jabrgang, 1858. 
