EARTHQUAKES AND VOLCANOES. 



three successive positions of the wave are represented by ABC, 

 A' B' c', and A" B" c". A building or other object in a position 

 perpendicular to the ground is represented at r s on the slope of 

 the wave where it leans from the vertical to the right. When the 

 wave has advanced to the position A' B' c', the foot of the building 



A A' A" C C' 



is raised to the crest of the wave at r', and the building, r' s', is 

 then vertical, having in the interval been gradually raised from its 

 inclined direction. When the wave has progressed to A" B" c", 

 the foot of the building has been again lowered to its first position, 

 but being now on the slope, A" B", it will be inclined to the left of 

 the vertical, as represented at r" s". 



14. Thus by the undulations passing successively under it, such 

 a building will be alternately raised and lowered through a certain 

 vertical height, and at the same time rocked right and left through 

 a certain angle of vibration. 



The undulations which produce earthquakes are sometimes 

 rectilinear and propagated in parallel lines and in a single direc- 

 tion. In other cases they form concentric circles, and are propa- 

 gated from a certain central point, like the waves produced on the 

 surface of still water round the point at which a pebble is dropped 

 into it. 



The angle through which objects will be deflected from the per- 

 pendicular in the rocking motion imparted to them, will depend 

 on the angle of the slope of the wave, and the latter will depend 

 on the proportion which the height of the undulation bears to its 

 amplitude. The less the height, and the greater the amplitude, 

 the less will be the deflection from the perpendicular. In the 

 cases of many earthquakes, this deflection is so small, that the 

 cohesion of the materials of building is generally sufficient to 

 prevent them from falling notwithstanding their deflection. 



That a building may stand, though inclined considerably from 

 the perpendicular, is proved by examples which occur in almost 

 every city. The leaning towers of Pisa and Bologna have stood 

 for seven hundred years. 



The effect of the alternate motion, upwards and downwards in 



the vertical direction, must depend more upon the rapidity of the 



alternation than upon the range of the elevation and depression. 



It is easily conceivable that the ground may be slowly raised and 



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