64 Original Articles. [Jan.. 
replacement of those particles within a determinate volume of the 
material, transferred through and affecting in succession, the whole 
mass. 
The shaking of the ground by the rolling of carriages, beating 
their wheels upon the paving-stones in the streets of cities, and the still 
more perceptible rocking of the ground beneath our feet as we stand 
near a heavy railway train at speed, are examples of such waves 
in solids. 
The ordinary sounds we hear, are examples of like waves in air; 
and the noise of the grating and rolling pebbles moved by the waves 
as they approach the shore on which we stand, is an instance of such 
waves, transmitted from the mutually struck pebbles to the water, and 
through the water to the air, by which it reaches our hearing organs. 
While the shock or jar felt in a boat floating at some distance from a 
blast exploded at the bottom of the sea, is a case of such an elastic 
wave, originated by the blow of the powder, and transmitted directly 
to and through the water and the boat, to our bodies. 
Now the velocity with which such a wave-form travels, varies in 
different materials, and if these be homogeneous, depends for any given 
substance, principally upon its specific degree of elasticity —technically 
called its elastic modulus, and upon its density, upon which its mass 
and inertia are dependent in a given volume. The rate at which the 
wave-form, 1.e. the whole group of displacing and replacing particles 
in simultaneous movement, is transmitted in any particular substance, 
is called its transit period. 
This period is constant (always the same) for the same material, 
under the same conditions as to temperature, molecular state, &c., 
and for small originating impulses is irrespective of the amount or 
kind of the original impulse which produced the wave. Experiments 
conducted within a few years past at Holyhead, as to the time that 
the wave or shock, transmitted through the Quartz and Slate Rocks 
there, to traverse a measured mile of rock, from the moment of 
production by certain of the explosions of the great mines, employed 
in the adjacent Government Quarries, which vary from less than a ton 
up to six or seven tons of powder fired at once, appear to indicate that 
in elastic waves of this great magnitude and transmitted through hete- 
rogeneous material, 7. e. laminated, contorted, and shattered rocks of 
various degrees of hardness, density, and elasticity ; the transit period 
is not independent of the amount of the original impulse, but that the 
larger this is—and the greater consequently the original magnitude of 
the wave—the less (in some ratio) is the time of the transit period ; 
in other words, the faster the wave travels. 
In air, the transfer of this elastic wave, which is identical with 
that of sound, has a velocity of about 1,140 feet per second. In water, 
the transit period is about 4,700 feet per second; and in hard erys- 
tallized rocks, such as porphyry or granite, if they were perfectly 
solid and homogeneous, it would be from 5,000 to 10,000, while in iron 
and steel it reaches 11,000 or 12,000 feet per second. An enormous 
retardation of this transit velocity occurs however when the material 
through which the wave passes is heterogeneous, broken up and 
