SEISMOMETRY. 21 



intentionally been made loose and flexible. The indices 

 which wrote the motions of these pendulums have been 

 as various as the pendulums themselves. A small needle 

 sliding vertically through two small holes, and resting its 

 lower end on a surface of smoked glass, has on account of 

 its small amount of friction been perhaps one of the 

 favourite forms of recording pointers. 



The free pendulums w^hich have been employed, and 

 which were intended to swing, have been used for two 

 purposes : first, to determine the direction of motion from 

 the direction of swing, and second, to see if an approxi- 

 mation to the period of the earth's motion could be 

 obtained by discovering the pendulum amongst a series 

 of different lengths which was set in most violent motion, 

 this probably being the one which had its natural period 

 of swing the most nearly approximating to the period 

 of the earthquake oscillations. 



Inasmuch as all pendulums when swinging have a 

 tendency to change the plane of their oscillation, and also 

 as we now know that the direction of motion during an 

 earthquake is not always constant, the results usually 

 obtained with these instruments respecting the direction 

 of the earth's motion have been unsatisfactory. The 

 results which were obtained by series of pendulums of 

 different lengths were, for various reasons, also unsatis- 

 factory. 



Of pendulums intended to provide a steady point, from 

 which the relative motion of a point on the earth's surface 

 could be recorded, there has been a great variety. One 

 of the oldest forms consisted of a pendulum with a style 

 projecting downwards from the bob so as to touch a bed 

 of sand. Sometimes a concave surface was placed beneath 

 the pendulum, on which the record was traced by means 

 of a pencil. Probably the best form was that in which a 



