66 Original Articles. [Jan. 
velocity of transit due to the elasticity of the rocks, which was also 
experimentally obtained, was extinguished thus by their want of con- 
tinuity, &e. 
Now, from these different rates of wave transit in diverse materials, 
it results that if an impulse be given at a single point, it may be per- 
ceptible several times in succession by a person so situated as to re- 
ceive it through different media. 
Let, for example, one stand near a line of railway, and a heavy blow 
be delivered upon the iron rail; it will be heard first, through the iron 
rail; almost directly afterwards a second sound will be heard through 
the air; and almost at the same time the person will feel the pulse of 
the blow reach his feet through the ground. While, if another person 
had his head immersed in the water filling a side drain along the line, 
he would have heard the sound through the liquid at a moment dif- 
ferent from the arrival of any of the other waves. 
Such waves, only on a larger scale, constitute an Earthquake shock. 
An originating impulse (something of the nature of a blow, or hav- 
ing the effects of one) there must be for every shock, but we are not here 
concerned with the source from which that impulse may be produced. 
It may be an explosive production or condensation of high-pressure 
steam in heated cavities, deep beneath the surface, or sudden increase 
or decrease of its tension, or sudden fracture or fall, or forcing up or 
down or against each other of great rocky masses, or if (in near pro- 
pinquity to active voleanoes), it may be any of their throbs or throes, 
or explosive ejections, or the recoil from these; it matters not as 
respects the physical theory of Harthquake-motion, and the expla- 
nation this renders of Earthquake-phenomena, what or which or 
whether any of these be the cause of the blow, so long as some sort of 
impulse be given, and the seat of this be more or less deep beneath 
the earth. 
Then in all directions outwards from this centre of impulse, there 
will be transmitted an elastic wave. ‘The form of the wave, if origi- 
nated at one point, would be that cf a spherical shell concentric with 
the centre of impulse, 1f the medium were quite homogeneous; but 
in nature, the wave assumes ellipsoidal and various other more com- 
plex forms, and rapidly gets broken up into smaller and still more 
complex waves, by dispersion, by interference, refraction and reflec- 
tion, in consequence of the shattered and varying nature of all the 
uperficial formations through which it is transmitted. 
The wave starts from the origin with one normal and two trans- 
versal vibrations, 7. e. every particle vibrates not only to and fro, 
in the radial direction from the centre, but also at right angles to this, 
in two directions at once. The former is the larger vibration and the 
more important to attend to, so that we may often, in investigating 
Harthquake-phenomena, altogether pass over the transversals. These 
vibrations constitute the proper motion of the wave as contradistin- 
euished from its motion in transit. 
A plumb line passing from above the surface of the earth and 
through the centre of impulse is called The Seismic Veriical. The 
wave or Shock passing outwards from this centre, reaches the earth’s 
