V. VIBRATIONS AND WAVES. 



149 



539c. Tisley's Compound Pendulum Apparatus, for 



combining two rectangular harmonic vibrations producing figures 

 known as Lissajous' curves. See Report of the British Associa- 

 tion 1873 ; Engineering, Feb. 6, 1874 ; also " Sound," 2nd ed., 

 Tyndall. Tisley and Spiller. 



539d. Donkin's Karmono graph, for compounding two 



parallel harmonic motions. 



See Proceedings, Royal Society, 1874. 

 Tisley and Spiller. 



V. VIBRATIONS AND WAVES. 



54O. Apparatus for the Composition of two parallel 

 simple vibrations. Dr. F. G. Groneman, Groningen. 



1. Principle. If the point C moves with constant velocity on the circum- 

 ference of the circle HCG, it is known that its pro- 

 jection B upon the diameter GH performs a motion, 



which is called a simple vibration. 



If to this variable point B a string is fixed, which 

 is passed over the pulley D, and from this hangs 

 down to E, so that BD + DE is the constant length of 

 the string, it is easy to see that the point E will per- 

 form the same motion as B. 



If this string is not attached to B, but to the 

 moving point C itself, E will have a motion which is 

 not strictly, but nearly that of B, the difference re- 

 sulting from the difference of the .two variable lines 

 BD and CD. This difference can be diminished to 

 any degree, by increasing the distance between the 

 pulley and the centre A. 



2. Two discs of the same diameter are placed in the 

 same vertical plane, and can turn round horizontal 

 axes, one of which has a handle. The motion of the 

 first disc is communicated to the second by means 

 of crown wheels and cog wheels. One of the latter 

 can be fixed at any point of its axis, so that it may 

 be made to gear with each of the six crown wheels of 

 the second disc. By this arrangement the ratio of the 

 velocity of the second disc to that of the first can be 

 made successively equal to 1, i, , f, , &. For 

 changing this velocity, or for changing the difference 

 of the phases, when the velocities of the discs are 

 the same, the washer at the end of the axis of the 

 second disc must be a little loosened, and this axis 

 pushed forward. 



3. The discs have one knob each. If the distance of the knob of the first 

 disc from the centre is called 1 , that of the knob of the second disc can 

 be made successively 1, ^, or -$. From the knobs proceed two strings, 

 which pass through the pulleys on the top of the instrument, and are 

 attached to the hooks A and B (Fig. 2). 



