1877.] for investigating Rotary Motion. 95 



gave "OlO per cent, for each degree Centigrade as the loss oc- 

 casioned by rise of temperature. Tiie determinations of the 

 effect of temperature above alluded to were carried out by means 

 of Lissajous' figures, and it may be mentioned that the latter can 

 be produced by attaching pieces of paper with very fine slits 

 to the forks in place of mirrors, as has been the usual practice. 

 One fork is placed horizontally, the other vertically, a fixed lens 

 being mounted between them so as to form an image of one 

 slit on the other. With large forks the effect of the additional 

 weight of the paper and gum is inappreciable. 



There are several methods of determining the absolute period 

 of a reed or fork. These are described at some length in a 

 paper read by Mr A. J. Ellis, F.R.S., before the Society of Arts in 

 May, 1877. 



The principle of the Cycloscope. is now being employed in 

 the construction of an apparatus for determining the absolute 

 pitch of a fork or reed, and the experiments so far have given 

 excellent results. If a steady rotary motion capable of perfect 

 control is given to a drum carrying a ring of lines, and if the 

 motion is so regulated that any recognized wave, given by a fork 

 or reed of unknown period, is kept stationary during a measured 

 interval of time, the exact number of rotations of the drum during 

 this interval being accurately recorded, then evidently the period 

 of the fork or reed can be obtained. It is not even necessary 

 to attach a slit to the fork, as the edge of the latter is found to 

 answer equally well. 



Three determinations of the period of a 2-56 fork made by 

 Prof. McLeod gave the numbers 256-287, 256-281 and 256-287 

 vibrations per second ; a 320 fork gave 320"364< ; a 88i fork 

 384-456, and a 512 fork 512%549. These numbers must be re- 

 garded as merely preliminary, since known imperfections exist in 

 the apparatus. 



Some interesting experiments have been made with discs 

 on the principle of the thaumatrope. If a disc provided with 

 radial slits is driven at a constant speed by clockwork in front 

 of another disc driven from any machine and provided with a ring 

 of dots or symbols, and if N and n are the number of rotations 

 of the clock disc and the other disc per minute respectively, >S^, the 

 number of slits, d the number of dots or symbols ; then v/hen 

 N . 8 = n . d; d dots will be visible and stationary. Thus S and i\^ 

 being given or assumed, d can be obtained for any assigned value 

 of w. 



If the machine disc is running- a little too fast for the above 

 equation, the dots will appear to move slowly in the same direc- 

 tion as this disc ; if too slow, they will move in the opposite 

 direction. 



