WIECHERT SEISMOMETERS AT RIVERVIEW COLLEGE. 



391 



Fig. 



Italian instruments of more modern date have been mostly 

 of the "boom-pendulum" ("horizontal" or "conical" 

 pendulum) type, in some form or other, the underlying 

 principle of which is due to the German physicist Zollner. 

 Such are the seismographs designed (or modified) by Rebeur- 

 Paschwitz, Ehlert, Milne, Omori, Hecker, Grablowitz and 

 others. The Wiechert instrument is an inverted pendulum, 

 the centre of gravity being vertically above, instead of 

 below, the point of support, stability of equilibrium within 

 certain limits being obtained by steel springs of very high 

 quality, on the elasticity of which partly depends the period 

 of vibration of the whole pendulum. 



1. 



From the accompany- 

 ing diagram (fig. 1) some 

 idea may be formed as to 

 the mode of action of the 

 more essential parts of 

 the instrument. The 

 large stationary mass M, 

 weighing 1000 kilograms 

 (1 ton) is moveable about 

 a point K, where it is 

 supported by an iron base 

 S resting on a concrete 

 foundation. At its upper 

 part, it has a projecting 

 pillar N, passing through 

 a hole in the table T,which 

 is rigidly fixed to the concrete block below. The two rods 

 A B, A B\ placed respectively in the N.S. and E.W. 

 azimuths, transmit the corresponding components of the 

 oscillations of the point A to the recording apparatus in 

 the following way. The rod A B is connected at B with a 

 vertical lever C C D, moveable about on axis C C, with 



