220 



T. GRAY Otf A SEISMOGRAPHIC APPARATUS. 



or vertically. The pendulum, P, is kept deflected by means of a light 

 brass tube, which passes through its centre in a direction at right 

 angles to its axis, and which carries at one end a vertical knife-edge, 

 and at the other end a very light index of aluminium foil. This 

 thin tube rests Avith its knife-edge in a flat Y-groove, which is fixed 

 to the supporting column, C ; and by sliding it more or less through 

 the tube, a greater or less deflection can be given to the pendulum. 

 The other pendulum is exactly similar to P, with the exception that 

 the aluminium index is fixed at right angles to the tube and close to 

 the knife-edge. By this arrangement the two pointers can be ad- 

 justed to lie in the same horizontal plane, and in parallel directions. 

 When so adjusted, the planes of deflection of the pendulums are at 

 right angles to each other. The point of suspension is adjusted to 

 be a little in front of, but very nearly vertically above the knife- 

 edge, so that the period of free vibration of the pendulum round a 

 vertical axis is long, compared with the period of vibration of the 

 earth. The vertical component is recorded by means of a compen- 

 sated spring seismograph (see Phil. Mag. Sept. 1881), which consists 

 of a mass M (figs. 1 & 3) attached near one end of a horizontal lever I, 



Fig. 3. — Diagram of Spring -Seismograph recording vertical 

 movements. 



which turns freely round a knife-edge at &, and is supported by the 

 flat springs S, which are attached to the lever by links working on 

 knife-edges. To the free end of the lever one end of a small box, 



