EARTHQUAKE SHOCKS AND OTHER CONCUSSIONS. 221 



pendulum will be greater as its inertia increases, in consequence of which the bob 

 lags behind the movement of the frame to which it is attached by the elastic 

 wire, which frame carries along with it the concave surface over which the pencil 

 will therefore be dragged. To overcome the friction of the pencil, we must there- 

 fore increase the mass of the pendulum. 



II. The mass of the pendulum cannot be changed without modifying the sen- 

 sibility of the apparatus ; that is, the maximum vibration which a given shock 

 will produce. But the desired sensibility is easily maintained by the pinching 

 screw E, which must be employed to shorten the free part of the elastic wire (or 

 a thicker Avire may be introduced), until the sensibility is exactly as great as 

 may be required. 



III. Hence one and the same instrument may have any required sensibility 

 given to it, and that wholly irrespective of its dimensions. The sensibility depends 

 upon the force tending to restore the pendulum to its position of rest when the dis- 

 placement = 1. The time of vibration depends on this quantity. Hence the time 

 of vibration is the test of sensibility. As the condition of equilibrium approaches 

 to indifference, the sensibility increases without limit. 



IV. However weak the spring may be, and however great the sensibility, it 

 is plain that, on the present construction (for others might easily be suggested 

 which should give a different result), the displacement of the bob and pencil can- 

 not by possibility exceed the forward motion which the earthquake is understood 

 to communicate to the stand (which may be screwed to a floor). The inertia of 

 the pendulum cannot do more than leave the extremity B as much behind A as 

 the earthquake has shifted A forwards. If this effect be worth measuring at all, 

 the lateral vibration of the ground must be a sensible quantity, and there is no 

 difficulty in constructing an instrument on any scale, from an inch to 10 feet in 

 length, in which (the time of oscillation being the same, say one second) the maxi- 

 mum vibration shall have the same linear magnitude. The only consideration 

 is, that the range may be sufficiently great to exhibit the stronger shocks without 

 giving an inconvenient curvature to the apparatus ; and for that purpose I have 

 thought that a radius of 20 inches and a diameter of 10 inches for the spherical 

 segment is sufficient. 



V. If it be desired to magnify the scale of displacements, this may still be 

 done without any increase of dimensions. Let the bob C (Plate III. Fig. 1,) be 

 lowered upon the rod A B, so as to stand at only one-half or one-third of its 

 height : let the mass be increased so as to overcome the friction of the pencil as 

 efficaciously at this diminished leverage ; and let the spring be adjusted so as to 

 give the same sensibility as before ; the displacement of the bob will be the same 

 as at first, but the displacement of the pencil will be magnified two or three times, 

 according to their relative radii. 



In practice, the pencil will not describe precisely lines upon the sphere, 



