551.22 233 



I. 



The Horizontal Pendulum. 



SEISMOGRAPHS based on accurate and scientific principles have 

 now been in use for about fifteen years, and we have learnt 

 much from them regarding the movements of the ground during an 

 earthquake shock. Far beyond the disturbed area, however, deflec- 

 tions of delicate instruments, such as magnetometers and levels, have 

 occasionally been observed, and the times at which these disturbances 

 have occurred leave little doubt as to their connection with severe but 

 distant earthquakes. Chiefly within the last six years, instruments 

 of still greater sensitiveness have been constructed, and by their aid 

 it seems not impossible that we may be able to register the pulsations 

 of violent earthquakes to whatever part of the world they may extend. 

 One of the most valuable of these instruments is that known as the 

 " horizontal pendulum," the fundamental principle of which has been 



O 



Fig. i. — Hengeller's Pendulum. 



independently discovered no less than eight times within a period of 

 sixty years. 



The first occasion seems to have been in 1832, when the instru- 

 ment represented in Fig. 1 was constructed by L. Hengeller, a 

 student in Munich. The following is the description of it by his 

 teacher, the astronomer Gruithuisen (14): " It consists of a hori- 

 zontal lever, A B, of brass, on which is fixed at one end a brass ball, 

 C, as a weight ; D is a fine wire by which the lever is suspended ; 

 instead of the counterpoise, the other arm of the lever is fastened to 

 the floor by the wire E ; and the instrument becomes the more 

 delicate the nearer the wire D comes to the wire E. The ball, C, can 

 oscillate only horizontally, and," though the statement should perhaps 

 be received with caution, " is visibly (according to Hengeller's 

 experiments) attracted by a cannon ball." 



Thirty years later, in 1862, a similar pendulum was devised by 



