ON THE PACTS OF EARTHQUAKE PHiENOMENA. 273 



the transit through the earth's crust of a wave of elastic compression," though 

 received at first with some hesitation, perhaps with doubt, appear rapidly to 

 have claimed tlie general assent that verisimilitude warranted, and indeed now 

 have been admitted as a portion of our systematic knowledge in the works 

 of Somerville, De la Beche, Lyell, and others. It always however appeared 

 desirable to me that the truth should be submitted to an " experimentum 

 crucis," and in the original paper above noticed, as also in the ' Admiralty 

 Manual,' I proposed, as an important object to physical experimentalists, to 

 determine the actual rate and other conditions of propagation of elastic waves 

 or pulses through the various formations of the earth's crust when artificially 

 produced by the explosion of gunpowder, to be fired at known distances from 

 the points of observation by means of the galvanic battery, the time of wave 

 transit to be noted. 



The subject not having been taken up by others, doubtless, amongst other 

 reasons, because few competent experimenters are found in circumstances of 

 locality suited for such experiments, or can command the time and personal 

 labour demanded for conducting them, not to speak of the large expenditure 

 in money which such operations require, I undertook the task, and with the 

 able and zealous assistance of my eldest son, William Mallet, whose help, 

 efficient beyond the promise of his years, I acknowledge, with I trust a par- 

 donable pride, have been enabled to complete it during the summers and 

 autumns of the years 1849-50. 



The precise object, then, that I set before me was to measure the transit 

 rate of pulses in two different media, such as might be considered to give 

 the extreme limits of speed, viz. the fastest and the slowest, likely to be found 

 in any of the formations occupying considerable tracts of the earth's crust. 



It would doubtless be found that the limits of wave slowness should occur 

 in some discontinuous medium, and in this the most rapid extinction of impulse 

 would also occur. Preliminary knowledge already pointed out solid granite 

 or other crystalline rock as the medium giving the limits of loave fastness, 

 and that in what the impulse would be transferred furthest. 



It fortunately happened that both these media could be commanded at the 

 distance of a few miles from Dublin, at a point of the coast easily accessible 

 by railway, and offering the requisite conditions of uniformity of medium 

 over a sufficient range and depth for experiment, — nearly level surface for 

 admeasurement of the ranges, and a locality such that the required explo- 

 sions could be made free from danger to other persons and without inter- 

 ruption. 



Killihey Bay, on the coast of the County of Dublin, offered, along its 

 widely extended beach, a mass of wet sand admirably circumstanced and 

 suited for one limit, while the granite of Dalkey Island, closely adjacent and 

 not half a mile from the shore, about a mile northward, gave as a medium 

 the other limit of wave transit which is not likely to be exceeded by at least 

 any European formation. 



The map (Plate XII.) shows the positions finally chosen for both sets of 

 experiments, the directions and lengths of the two measured ranges being in- 

 dicated by dark lines : AB, the Killiney Bay range in sand ; CD, the Dalkey 

 Island range in granite. 



The sites being determined, I made application to the commanding officers 

 of Engineers, and to those of the Coast Guard, to obtain the assistance of 

 the non-commissioned officers and men of both services, stationed in the 

 several batteries along the line of adjacent coast, and for permission to use 

 one or more of the batteries as places of deposit for our instruments ; and I 

 have to acknowledge the prompt and cordial manner in which my requests 



1851. T 



