338 



niences in practice. The author, therefore, proposed an inverted 

 pendulum, sustained by a steel wire, on the principle of the noddy 

 invented by Mr Hardy, for ascertaining the stability of clock 

 cases. The balance of gravity and elasticity (vFhich act, 

 the former to displace, the latter to redress, the pendulum) may 

 be rendered as nice as we choose, and hence the sensibility of 

 the instrument is wholly independent of its dimensions. 



The author has shewn, by a mathematical investigation, that 

 the extent of deviation due to a given concussion, within mode- 

 rate limits, depends solely upon the time of vibration of the pen- 

 dulum, — that, for any sudden forward motion of the machine, the 

 greatest displacement of the bob of the pendulum may become 

 equal to that motion, — but if the motion continue uniformly for 

 a short space and then cease, the displacement may be doubled 

 in amount. 



The self-registering part of the apparatus consists of a pencil 

 at the extremity of the inverted pendulum, which travels over a 

 prepared concave surface of paper, and marks at once the direc- 

 tion and extent of the displacement of the pencil, which is evi- 

 dently contrary to the movement of the ground. The author 'also 

 points out how, by varying the position of the bob upon the rod 

 of the pendulum, and at the same time altering the elasticity of 

 the spring, the deviation of the pencil may be increased in any 

 proportion to the actual movement of the ground, and this irre- 

 spectively of the dimensions of the instrument. 



Lastly, he shews how, by employing two instruments of the 

 same kind, but whose sensibility (determined by the time of one 

 vibration) differs in a knoivn proportion, the duration of a shock 

 and the extent of lateral movement of the ground may be cal- 

 culated ; and he gives a table for this purpose. It is to be un- 

 derstood, however, that this and other results of the mathemati- 

 cal investigation are only true in so far as the fundamental hypo- 

 thesis is correct, — viz. that an earthquake is a lateral movement of 

 the ground in one direction, through a short space, and with a 

 uniform velocity. 



Similar instruments might, no doubt, be applied to measure the 

 lateral concussions of railway trains. 



