28 
follow a force of paroxysmal character. Periods of large and long continued 
deposition of sedimentary strata would usually be periods of subsidence. 
The consequent rise of the isothermal line is thought by Babbage to cause 
expansion and therefore elevation, but Hopkins objects to this as usually 
contrary to facts. 
Mr. Hopkins proceeds to consider the manner in which vibrations may be 
generated and propagated through the crust of the globe, thus causing 
Earthquakes. It is shown what would happen if an impulse were impressed 
along a cylindrical tube, and how waves would be propagated along the 
surface of water in a uniform canal ; and how they would move in fluids in 
all directions from a centre. Vibrations along a solid bar are of two kinds— 
the first longitudinal, and the second transverse, as those of a musical chord. 
The same would happen with vibrations through a solid mass. The normal 
wave has its direction from the centre to the moving particle, and its velocity 
is greater than that of the tangential wave, the direction of which is per- 
pendicular to that of the normal wave. If a wave passes from one medium 
into another it will be refracted, as shewn by a diagram. It is likewise 
reflected, and if the angle of incidence upon the separating plane be very 
small, it will be wholly reflected, as in the case of a ray of light. 
Mr. Hopkins proceeds to apply his investigations to earthquake phenomena. 
The simplest case is when the shock is violent and limited to a small space, 
and likewise the strata homogeneous. Then the direction of the initial wave 
must be observed, that being the normal direction. And if the horizontal 
direction be known at two places, the intersection of these lines gives the 
point on the surface above the focus of disturbance. Also the difference of 
the times compared with the distances of the places will determine the 
normal velocity. 
The mathematical process was then explained, by which the position of the 
origin of impulse was ascertained when the conditions of direction of vibra- 
tion and the normal and horizontal velocities had been obtained by observation. 
Mr. Hopkins then discusses cases more complicated, in which the vibrations 
are supposed to pass through strata of varying density. If the sphere of 
disturbance is large, the shocks over a large area would be contemporaneous, 
and probably an elevation would take place. If that sphere is small, and the 
intensity great, then the results will not differ from those mentioned. But 
suppose that the impulse takes place at the bottom of the sea, a uniform 
velocity would be maintained, depending on the depth of the water. Should 
the water become shallower by approaching the shore, the velocity will 
diminish, but the front of the wave will be steep, like what is termed the 
