SEISMOMETRY. 17 



of the earthquake seldom remains in the same azimuth 

 throughout the whole disturbance. 



An extremely delicate, and at the same time simple 

 form of seismoscope may be made by propping up strips 

 of glass, pins, or other easily overturned bodies against 

 suitably placed supports. In this way bodies may be 

 arranged, which, although they can only fall in one direc- 

 tion, nevertheless fall with far less motion than is neces- 

 sary to overturn any column which will stand without 

 lateral support. 



Projection Seismometers. — Closely related to the seis- 

 moscopes and seismometers which depend on the overturn- 

 ing of bodies. Mallet has described two sets of apparatus 

 whose indications depend on the distance to which a body 

 is projected. In one of these, which consisted of two 

 similar parts arranged at right angles, two metal balls 

 rest one on each side of a stop at the lower part of two 

 inclined \/ like troughs. In this position each of the 

 balls completes an electric circuit. By a shock the balls 

 are projected or rolled up the troughs, and the height to 

 which they rise is recorded by a corresponding interval in 

 the break of the circuits. The vertical component of the 

 motion is measured by the compression of a spring which 

 carries the table on which this arrangement rests. In the 

 second apparatus two balls are successively projected, one 

 by the forward swing, and the other by the backward 

 swing of the shock. Attached to them are loose wires 

 forming terminals of the circuits. They are caught in a 

 bed of wet sand in a metal trough forming the other end 

 of the circuit. The throw of the balls as measured in 

 the sand, and the difference of time between their suc- 

 cessive projections as indicated by special contrivances 

 connected with the closing of the circuits, enables the 

 observer to calculate the direction of the wave of shock, 



