116 



EAETHQUAKES. 



160 feet high, was observed instrumentally to vibrate at 

 the top nearly 5 inches.^ 



At the time of a severe earthquake it does not seem 

 impossible but that a building may be swung completely 

 The accompanying illustration, fig. 24, taken from 



over. 



a photograph,^ apparently indicates a movement of this 

 description. 



Fig. 24.— Stud Mill at Haywards, California. Oct. 21, 18G8. 



Principle of relative Vibrational Period, — If a lath 

 or thin pole loaded at one end with a weight fixed to 

 the ground, so as to stand vertically, be shaken by an 

 earthquake it will be caused to rock to and fro like 

 an inverted pendulum. The period of its swing will be 

 chiefly dependent on its dimensions, its elasticity, and its 

 load. In a building we have to consider the vibration of 

 a number of parts, the periods of which, if they were 



* Mallet, Dijyiamics of EarthqvaJtes. ^ Stud Mill at Haj^wards. 



