58 Mr. Mautet on the Dynamics of Earthquakes. 
word “undulation,” and, in so far, have come nearer to the truth ; for it appears 
to me that the fact that displaced bodies are not occasionally replaced in earth- 
quakes, is conclusive evidence of either one of two things :—either the motion 
is limited to horizontal, direct movement in one or more directions, and, if so, the 
whole mass of the disturbed country must be pushed bodily forward, and remain 
so, of which there is no evidence ; and all bodies must, as the effect of one 
shock, fall in the one direction, and not in opposite directions, which is contrary 
to observed facts : or, on the other hand, if the movement be an alternate, horizon- 
tal motion, as all observations go to prove it is, then the motion in one direction 
must be slower than in the other; or attended with other differences of circum- 
stances; the backward motion must be different from the forward motion, or 
otherwise displaced bodies would be sometimes replaced by the recurrence, in the 
opposite sign, of forces similar and equal to those that first set them in motion : 
but they are not found so replaced. 
Now, of all conceivable alternate motions, the only one that will fulfil the 
requisite conditions observed, namely, that shall move with such an immense 
velocity as to displace bodies by their inertia, or even shear close off great but- 
tresses from the walls they sustained, or project stones out of their beds by iner- 
tia,—that shall have a horizontal, alternate motion, either much quicker in one 
direction than in the other, or different in its effects, and that shall be accompa- 
nied by an upward and downward motion at the same time (a circumstance 
universally described as attendant on earthquakes) ;—the only motion that will 
fulfil these conditions, is the transit of a wave of elastic compression, or of a 
succession of these, in parallel or in intersecting lines, through the solid substance 
and surface of the disturbed country. 
The idea that earthquake motion consists of a wave of some sort is not new, 
although so entirely neglected by the great mass of recent geological authors. 
To the Rev. John Mitchell, M. A., Fellow of Queen’s College, Cambridge, the 
merit of this idea appears to be due. Ina paper communicated to the Royal 
Society, read in 1760,* he treats at length of the origin and phenomena of 
earthquakes, and distinctly enunciates the following views : 
That the motion of the earth is due to a wave propagated along its surface 
* Phil. Trans. vol. li, part 2. 
