T. GRAY ON A SEISMOGRAPRTC APPARATUS. 



219 



The instrument is suited for recording motions of the earth which 

 are under one centimetre on either side of its normal position, while 

 the period of oscillation is less than one second. The instrument 

 may be adjusted to register motions having a longer period ; but it 

 is not expected to be called upon to do so. The actual motion of 

 the earth is to be derived from a record of three separate components 

 of the motion — two horizontal and at right angles to each other, and 

 the other vertical. The horizontal components are recorded by a 

 pair of conical pendulum-seismographs (see Phil. Mag. Sept. 1881), 

 one of which is shown at P, figs. 1 & 2. These instruments are con- 

 siderably simpler in form, and can be more readily adjusted, than the 

 instruments described in the paper above referred to. They consist 



Fig; 2. — Diagram of Pendulum- Seismograph recording horizontal 



movements. 



of cylinders of lead, P, supported from their centre of gravity by means 

 of a strong silk thread, which is suspended from a point which can be 

 moved by a simple screw arrangement in either direction, horizontally 



