1877.] for investigating Rotary Motion. 93 



the positions corresponding to velocities of 50, 40, and 30 will 

 be found by simply dividing cd into four equal parts, and by 

 further subdivision the positions corresponding to all velocities 

 between V and v can be obtained. 



In the Cycloscope, as at present constructed, a box a (Plate III.) 

 containing a reed, to the tongue of which a piece of very thin 

 zinc with a fine slit has been soldered, traverses on a slide in 

 front of a drum carrying paper ruled in the way described. 



The motion is given by a hand-wheel 6 to a pinion c on the 

 same axis, the pinion gearing into a rack attached to the slide 

 d. A pointer p attached to the reed box indicates on a scale 

 ss the speed corresponding to any position of the reed box. 

 A vernier might be employed in place of the pointer p, by which 

 the primary divisions could be subdivided into 10 or 100 parts. 



To make the waves more visible, two small lenses e e are 

 employed, fixed in the fro-nt and back of the reed box. The 

 lens on the back of the box throws an image of the lines on the 

 slit ; that in front magnifies this image, and thus parallax is 

 avoided. If the instrument is to be used in a dark situation, 

 a small lantern is placed on the bracket t attached to the side 

 of the reed box, in order to illuminate the portion of the drum 

 under observation. 



In the early experiments it was found difficult to keep up 

 a sufficient supply of air to the reed without a considerable 

 pumping power and large conducting tubes. By the utilization 

 of the principle of the injector, or jet pump, this difficulty has 

 been entirely removed. The air is supplied through a small 

 flexible indiarubber tube from a pair of foot-bellows. This tube 

 terminates in a small glass tube drawn out to leave a narrow jet, 

 \\ mm. in diameter. This fine jet is passed through a coi-k, fitted 

 into a wdde brass tube h (Plate III.) fixed into the lower part 

 of the back of the reed box. The lower part of the brass tube 

 is cut away to allow free access to the surrounding air. The 

 arrangement is shown in enlarged section in Fig. 10, Plate II. 



Air, of pressure about equal to a column of water 20 or 25 

 cm. in height, is forced through the jet, and the reed vibrates 

 perfectly, the mean pressure of air in the reed box being only 

 about equal to a column of water 1^ mm. in height. 



The graduation of the scale should be performed after the 

 paper is mounted on the drum. The latter may be conveniently 

 arranged so that the centre line no (Fig. 8) falls opposite the 

 junction, which will then present the appearance shown in Fig. 9, 

 but this is not essential. The lines must now be counted romid 

 at any two intersections and the period of the reed (60 complete 

 vibrations per second) being known, the velocities which are 

 required to produce stationary waves at these two positions can 



