24 EAETHQUAKES. 



order to bring the glass into contact with the needle with- 

 out disturbance, the glass is carried on a strip of wood 

 K, hinged at the back of the box, and propped up in front 

 by a loose block of wood Y. When Y is removed the glass 

 drops down with K out of contact with the needle. The 

 box is carried on bars of wood C C, which are fixed to the 

 ground by the stakes A A. 



The great advantage of a pendulum seismograph 

 working on a stationary plate is, that the record shows 

 at once whether the direction of motion has been con- 

 stant, or whether it has been variable. The maximum 

 extent of motion in various directions is also easily ob- 

 tained. 



The disadvantage of the instrument is, that at the 

 time of a large earthquake, owing perhaps to a slight 

 swing in the pendulum, the records may be unduly 

 magnified. 



On such occasions, however, fairly good records may be 

 obtained from the friction pointers, provided that the 

 plates on which they work have been previously smoked. 

 It might perhaps be well to use two of these instruments, 

 one having a comparatively high frictional resistance, and 

 hence ' dead beat ' for large displacements. 



Many attempts have been made to use a pendulum 

 seismograph in conjunction with a record-receiving sur- 

 face, which at the time of the earthquake should be kept 

 in motion by clockwork. In this way it was hoped to 

 separate the various vibrations of the earthquake, and 

 thus avoid the greater or less confusion which occurs when 

 the index of the pendulum writes its backward and for- 

 ward motion on a stationary plate. Hitherto all attempts 

 in this direction, in which a single multiplying index 

 was used, have been unsuccessful because of the moving 

 plate dragging the index in the direction of its motion for a 



