42 MOVEMENT 



The relationship which obtains between the lengths of 

 the two arms determines the degree to which the 

 movement is reduced. A movement can be uniformly 

 reduced by means of an indiarubber thread ; this is a 

 convenient method, and quite reliable 

 enough for most purposes.* 



Let c b (Fig. 29) be an elastic thread 

 a . as nearly homogeneous as possible. Under 



the influence of traction it will become 

 equally extended throughout its length. 

 Let us make fast one of its extremities, c, 

 by means of a nail, for instance. If we 

 exercise traction at the other end, b, so 

 as to bring it to b\ the point a near to c 

 will only travel the short distance aa'. If 

 we have fixed the tracing needle at this 

 point, it will record on the revolving 

 cylinder a curve, the amplitude of which 

 will be to that of the real movement as 

 the length of the thread ca is to the 

 length cb. 



But when the movement has to be 



immensely reduced, as happens when the 



path of a body moving through several 



Fic.29.-Pro P or- kilometres has to be traced on a strip of 



tiun a V a move" paper a few centimetres in length, the 



S ent an y "nda- movement has to be reduced by means 



rubber thread. of ft mechanical arrangement of wheels. 



A system of small pinions and large wheels will 

 effect this object ; in fact, as we know, it will indefi- 

 nitely reduce the amplitude of any movement. We 

 had recourse to this method when we obtained the 



* This method of reducing the curve by means of an elastic thread 

 was thought of by Admiral Paris and his sou. It was made use of in 

 their apparatus described under the name of "Wave Tracers." 

 — Maritime and Culunial Review, June, 1867. 



