362 The Philosophy of Earthquakes. 



if the sliock communicated is not very violent, and the object 

 not top-heavy, it will not be overthrown, but simply raised, to 

 fall back where it was before. But any hammer stroke will 

 have given a concussion to the whole table — one concussion 

 wave after another mil spread, like those produced by throw- 

 ing a stone in a pond — and they will reach the surface and 

 emerge further and further off the centre of the shock and the 

 seismic vertical, each one coming out more slantingly than 

 that which preceded it. Thus we might draw on the surface 

 of the table concentric circles, each representing the crest of a 

 concussion wave, the waves growing weaker and weaker as 

 they extend further and further from the original point of 

 shock. Every circle thus traced would represent a Hue of 

 equal shock, or, in technical terms, a coseismal line ; that is to 

 say, a line on which objects would be equally shaken on what- 

 ever part of it they might happen to be placed. 



Having thus formed an idea of lines of equal shoclc, we must 

 consider the line of maximum shock, or, rather, maximum, power 

 of overthrowing objects. A body like a sugar-loaf, or a skittle, 

 would not be readily overthrown by striking the table imme- 

 diately under it. The blow might be severe, but its direction 

 would make it feeble in overthrowing 'power, while a concussion 

 that reached the skittle more obliquely, though of less violence, 

 might be quite sufficient to throw it down. We have seen that 

 the greater the horizontal distance from the seismic vertical at 

 which a concussion wave emerges, the greater its slant. Now, 

 the most advantageous mode of exerting force to upset a skittle 

 is to give it a shove horizontally, and above its centre of gravity; 

 and thus the concussion wave improves the direction of its force 

 as an upsetting power by slanting, from the seismic vertical; 

 but as it slants it gets further off its origin or source — its 

 energy is spread over a greater area, and is weakened in con- 

 sequence. There will, however, be a certain distance from the 

 sciM.iio vertical at which loss of power will be compensated by 

 gain in direction, and a line connecting all such points will be 

 the line of maximum overthrow, or meizoseismic line. In our il- 

 lustrative diagram this may be a circle, but in nature some of 

 our supposed conditions never occur. In real earthquakes the 

 focus from which the disturbance proceeds will not be a point, 

 but as irregular, and often very large space, and the surround- 

 ing earth stfata will be of different structure and density, better 

 able to transmit a sliock in one direction than another. Thus 

 we shall not, find b] eismal lines (in nature) making circular 



curves; but, although less regular and more dillier.lt to trace, 

 tliey will still be distinguishable when the circumstances are 

 favourable. 



To prevent a misconception, we must now advert to a fact 



