36 REPORT—1850. 
cause here the direction of shock is more vertical, and therefore less caleu- 
lated to overturn buildings, &c.; and without, because the power of the 
shock, though there more horizontal, has become weakened by distance of 
transmission. Thus let a be the centre of effort, ὦ, δ' the extreme limits of 
shock, α, 6 the vertical passing through the centre of effort, then some points, 
e,e', on the earth’s surface, 8, 6, 6, b, will lie in a circle, where the shock will be 
more potent in overthrowing buildings than in any other within or without. 
But when the surface of observation lies much further away from the 
centre of impulse, so that the shock advances along the surface, apparently 
horizontally, then there is more distinct evidence that its advancing crest is 
linear. Thus in the Calabrian earthquake, observers remarked many build- 
ings, or even whole villages, overthrown at the same moment along a distant 
line of country ; and this demolition appeared to progress in a similar way 
over the country. Similar facts are recorded by Prof. Rogers of American 
earthquakes. (Trans. Brit. Ass. for 1843-44.) 
There is every @ priori reason to suppose, that the crest of such a wave, 
being the intersection at the surface of the spherical shell of elastic com- 
pression produced by the original impulse of whatever sort beneath, moves 
upon the earth’s surface outwards, from the point immediately above its deep. 
centre of impulse, in lines parallel to themselves, and which are large circles, 
or several intersecting large circles, or possibly occasionally ellipses, and with 
the dimensions of the wave itself continually decreasing, but with unaltered. 
velocity of transit, save in so far as this is effected by changes in the character. 
of the formations through which it passes. This has not yet been proved by 
any direct observation, and it remains still to be found what is the curve or 
form assumed by the crest of the nearly horizontal travelling earth-wave or 
shock. But to this we shall more particularly allude when referring to the 
desiderata of our knowledge of earthquakes. To those lines along which 
the shock is simultaneously felt in passing outwards from the origin, I have 
proposed to give the name of coseismal lines. 
20th. The earth-wave or shock has determined dimensions in 
height and breadth, or in altitude and in amplitude, and these 
are dependent upon the force of the original impulse, the 
nature of the materials through which it passes, and the. 
distance it has travelled. 
Here also much evidence remains to be collected. Thus much, however, 
we know, that in some ratio the shock is greater, in other words, the wave is’ 
larger, as the originating impulse is more powerful. The absolute dimensions 
of the wave have never yet been correctiy ascertained, nor is this possible 
without the aid of well-constructed instruments. All that we know is, that’ 
these dimensions vary from waves whose altitude and amplitude are but a 
small fraction of an inch, to those whose motions were such and so great, as 
te throw down the heaviest buildings; to detach vast landslips and whole 
mountain-sides of rock, or even, as affirmed of the great stroke at Riobamba, 
to throw the bodies of human beings many feet into the air. [This latter’ 
case, however, though recorded on the authority of Humboldt, not from his 
own observation, however, but from testimony given to him, seems much to 
need confirmation. ] 
From all these, however, and generally from the narratives of the effects 
of all great earthquakes, there is good ground for believing that the altitude 
and amplitude of the great wave of shock may amount to many feet in either — 
dimension. [I have in a former publication (Admiralty Manual, ‘ Earth- 
rpms 
