EFFECTS PHODUCED UPON BUILDINGS. 137 



the effect is to transmit a push to the first boy, who, 

 being unsupported, flies forward. 



In the case of some earthquakes, most disastrous 

 results have occurred which seem only to admit of an 

 explanation such as this. A remarkable instance of this 

 kind occurred when the great earthquake of 1857 'swept 

 along the Alps from Geneva to the east-north-east, and 

 its crest reached the edge of the deep glen between 

 Zermatt and Visp. Then the upper part of the wave- 

 movement, a thousand or two thousand feet in depth 

 from the surface, came to an end ; the forward pulsation 

 acted like the breaker of the sea, and heavy falls of rock 

 encumbered the western side of the valley.* 



Earthquake shadoius. — If a mountain stands upon a 

 plain through which an elastic wave is passing, which is 

 almost horizontal, the mountain is, so to speak, in the 

 shadow of such a wave. If we only consider the normal 

 motion of this wave, we see that the only motion which 

 the mountain can obtain will be a wave of elastic distor- 

 tion produced by a shearing force along the plain of the 

 base. Should, however, the wave approach the mountain 

 from below, and emerge into it at a certain angle, only 

 the portion of the mountain on the side from which the 

 wave advanced could remain in shadow, whilst the portion 

 on the opposite side would be thrown into a state of com- 

 pression and extension. Portions in shadow, however, 

 would be subject to waves of elastic distortion. In a 

 manner similar to this we may imagine that certain 

 portions of the bluff, so far as the advancing wave was 

 concerned, were in shadow, and thus saved from the 

 immediate influence of the direct shock. A hypothetical 

 case of such a shadow is shown in the accompanying 

 section, illustrating the contour of the ground at Yoko- 

 hama. The situation which might be in the shadow of 



