118 



EARTHQUAKES. 



A 



B 



CD 



c 



Italian churches where the belfry tower is built into one 

 of the quoins of the main rectangular building. 



Not only have we to consider the relative vibrations 

 of the various parts of a building amongst themselves, 

 but we have to consider the relation of the natural 

 vibrations of any one of them or the vibration of the 

 building as a whole, with regard to the earth, the 

 vibrations of which it must be remembered are not 

 strictly periodic. 



Some of the more important results dependent upon 

 the principle of ' relative vibrational periods ' may be 

 understood from the following experiments : — 



In fig. 26 A, B, and C are three flat springs made out of 

 strips of bamboo, and loaded at the top with pieces of 



lead. At the bottom they are 

 fixed into a piece of board D E, 

 and the whole rests on a table 

 FG. The legs of this table 

 being slightly loose, by placing 

 the fingers on the top of it, 

 a quick short backward and for- 

 ward movement can be pro- 

 duced. The weights on A and b are the same, but they are 

 larger than the weight on C. Consequently the periods of 

 A and B are the same, but different to the period of C. 

 The dimensions of these springs are as follows : height, 

 18 inches; A and B each carry weights equal to 320 

 grammes, and they make one vibration per second ; C has 

 a weight of 199 grammes, and makes 0*75 vibrations per 

 second. 



First Experiment. — It will be found that by giving the 

 table a gentle backward and forward movement, the extent 

 of which movement may be so small that it will be diffi- 

 cult to detect it with the eye, either A and B may be 



Fig, 26. 



