364 The Philosophy of Earthquakes. 



to fall in and stop their rolling, their overthrow would then 

 give us much information. 



First, we should get to know the force of the shock, because 

 it either knocked down all, and thus was competent, or more 

 than competent, to the hardest work we had set it, namely, to 

 overthrow the cylinder of the broadest base ; or it would 

 knock down some, and leave others, and then we could say 

 that it was able to upset cylinders not exceeding a known 

 power of resistance. In the next place, we could tell the 

 direction of the shock from that in which the cylinders fell. 

 Now, excepting that these cylinders could not, like walls, be 

 cracked by a concussion not sufficient to overthrow them, they 

 would resemble, more or less, the buildings in a town, of course 

 making allowance for difference of shape. If therefore we exa- 

 mine a town that has been damaged or destroyed by an earth- 

 quake, we ought to be able to learn from it what the cylinders 

 taught, together with some other facts they could not show. 



Hitherto, for the sake of simplification, we have omitted to 

 speak definitely of the opposite motions of a concussion wave, 

 which might without explanation be supposed capable of im- 

 parting movement in one direction only. The following ex- 

 tract from Mr. Mallet will place the composite action in a 

 clear light. He tells us that, " as the co-seismal curve (or 

 crest of a wave-shock) enlarges its area, travelling outwards in 

 all directions from the seismic vertical — that is, from the ver- 

 tical line passing through the earth's surface (and centre) and 

 the focus — every point in and upon the surface, in succession, 

 moves once forward and back in the direction of the wave-path, 

 and to the extent of its amplitude at that point, or in two 

 components, vertical and horizontal, that shall give such direc- 

 tion.-" An object may be overthrown by either the backward 

 or the forward motion, and when the concussion wave emerges 

 in a slant, the form of the body and its mode of standing, or 

 being supported, will determine whether the forward or the 

 backward motion is most dangerous to it. 



When an earthquake makes an onslaught upon a town, tall, 

 old, badly-built edifices may be reduced to a heap of ruins, 

 while stronger buildings escape with slighter injury. In nar- 

 row streets of weak houses, a smart shock throws some down 

 first, and they in falling tumble against neighbouring struc- 

 i u res, and reduco the abodes of thousands to a pell-mell heap 

 of rubbish, from which little more than the magnitude of the 

 calamity can be learnt. When the buildings are stronger, and 

 less affected by the disasters that happen to adjacent edifices, 

 the work of devastation is moro methodical, and frequently 

 goes no further than the production of cracks, or fissures, indi- 



