358 Messrs. Milne and Gray's Experiments 



V-grooves, the direction of the strips being at right angles to 

 each other. The outside end of each of these strips carries one 

 thread of the bifilar suspension of a mirror, the other thread of 

 the bifilar being attached to a point fixed to the earth. When 

 the strips are pushed inwards against the bob, by the earth 

 moving them, the mirrors turn, and a beam of light coming 

 from a lamp and reflected from a mirror to a scale has its 

 image permanently deflected. An instrument of this class is 

 so sensitive that, at times, it would appear as if a continual 

 earthquake w r ere going on. 



(e) Circuit-closer Seismoscope. — Underneath the centre of 

 the bob of a pendulum a vertical index is so arranged that its 

 upper end is pivoted in a universal joint near to the centre of 

 gravity of the bob of the pendulum ; about one inch beneath 

 this joint it passes through a second universal joint fixed to 

 the framework from which the pendulum is suspended. The 

 upper end of this index is attached to one pole of a battery ; 

 the lower end, which is a platinum wire, dips into a small de- 

 pression in the surface of a cup of mercury, after the fashion 

 of Palmieri's circuit-closer. A slight motion of the framework 

 causes a considerable motion of the index, and hence causes 

 the platinum wire to come into contact w T ith the mercury. With 

 this arrangement it is easy to close an electric circuit with 

 a motion of the ground equal to one twentieth of a milli- 

 metre. The index forms an automatic key, which closes an 

 electric circuit in which an electromagnet is included. The 

 electromagnet deflects a lever carrying a pencil, which rests 

 on a disk kept constantly revolving by clockwork. The posi- 

 tion of the mark upon this disk indicates the hour and minute 

 of the earthquake, or rather the earth-tremor. 



The clock is a cheap small American clock, the minute- 

 hand being replaced by a wooden disk. To avoid the pencil 

 continually tracing the same circle, the clock is mounted on 

 wheels and is caused to wind itself along a stretched string 

 which is passed round the hour-axle. 



2. Seismometers and Seismographs. 

 (a) A seismometer which we have found to act well is a 

 pendulum with an index arranged like that of the circuit- 

 closer just described, except that at the lower end of the index 

 a light sliding needle rests on the surface of a smoked glass 

 plate. On this plate the pointer gives a magnified record of 

 the motions of the earth. This instrument is especially adapted 

 to show direction, maximum amplitude of the earth's motion, 

 and the existence or non-existence of transverse vibrations. 

 We find it advisable when using pendulums as seismometers, 

 to control the motion by introducing a small amount of fric- 



