176 SCIENTIFIC RECREATIONS. 



pendulum A a slight movement of oscillation, the point of tube T traces a 

 straight line on the paper placed in P ; but if we move pendulum B, the 

 paper also is displaced, and the point of tube T will trace curves, the shape 

 of which varies with the nature of the movement of pendulum B, the relation 

 between the oscillations of the two pendulums, etc. If the pendulums 

 oscillate without any friction the curve will be clear, and the point will pass 

 indefinitely over the same track, but when the oscillations diminish, the curve 

 also diminishes in size, still preserving its form, and tending to a point 

 corresponding with the position of repose of the two pendulums. The 

 result is therefore that the curves traced by the apparatus, of which we 



Fig. 176. M. Tisley s Harmonograph. 



produce three specimens (figs. 177, 178, 179), are traced in a continuous 

 stroke, commencing with the part of the greatest amplitude. 



By changing the relation and phases of the oscillations we obtain 

 curves of .infinitely varied aspect.. M. Tisley has a collection of more than 

 three thousand curves, which we have had occasion to glance over, in which 

 we failed to meet with two corresponding figures. The ratio between these 

 curves corresponds with some special class, of which the analyst may define 

 the general characters, but which is outside our present subject. By giving 

 the plate P a rotatory movement, we obtain spiral curves of a very curious 

 effect, but the apparatus is more complicated. Considered from this point of 

 view it constitutes an interesting mechanical apparatus, showing the combina- 

 tion of oscillations, and resolving certain questions of pure mechanics. 



