1877.] 



the Motion of Vibrating Bodies. 



161 



axis rapidly through. 90° the bar is placed parallel to the fork, which is 

 left vibrating. This contrivance may be found useful for lecture pur- 

 poses when large forks are used to show Lissajous' figures. It may be 

 mentioned in passing that two forks without mirrors or lenses may 

 readily be brought into unison or into some simple relation by attaching 

 to one prong of each a thin piece of black paper with a fine slit. One 

 fork is placed vertically and the other horizontally, a fixed lens being 

 mounted between them so as to form an image of one slit on the other. 

 Light is then passed through one slit ; and on looking through the other 

 a square dot of light is seen, which produces~the Lissajous' figures when 

 the forks are in motion. With a large fork the influence of the additional 

 weight of the paper and attaching gum is imperceptible. 



When a slit on a fork or reed is used together with the rotating cylinder, 

 it is necessary to avoid parallax by throwing on the slit an image of the 

 lines by means of a lens, and the observation is much facilitated by the 

 employment of a second leus to view the slit. The reed is placed 

 within a box capable of travelling on a fixed bed parallel to the cylinder, 

 and the slit is soldered to the reed within the box ; the latter is pierced 

 by two holes closed with lenses. Some difficulty was at first experienced 

 with the harmonium reed, which was placed in a box small enough to be 

 readily movable aloug the slide, and to which the air had to be led by a 

 flexible tube. The reed requiring a large quantity of air at low pressure 

 to cause it to vibrate properly, it was not possible, without the use of 

 large bellows and wide con ducting-tubes, to produce the desired effect; 

 it also appears essential that the air in contact with the reed should fee 

 contained in a chest of considerable dimensions, to permit of sufficient 

 compression of the air when the tongue nearly closes the orifice. After 

 numerous failures, a method, which we believe to be novel in this applica- 

 tion, was found quite efficacious, namely, the employment of the principle 

 of the injector or jet-pump. To the box is fixed a wide brass tube, with 

 a considerable orifice in the side ; at the end of the tube away from the 

 box is fitted by a cork a glass tube, terminating in a narrow jet 1| millim. 

 in diameter ; when air is forced through the jet at a pressure about equal 

 to that of a column of water 20 or 25 centims. in height the reed 

 vibrates perfectly, the mean pressure of the air in the box being equal to 

 that of a column of water about 1| millim. high. 



Instead of, or together with, the graduated scale along which the box 

 containing the resd or tuning-fork slides, a scale may be placed on a thin 

 rule close to the cylinder, the numbers on which are visible simultaneously 

 with the figures produced by the vibrating slit. This may be of some 

 advantage when it is necessary to observe rapid changes of velocity. By 

 drawing circles round the cylinder corresponding to the divisions of the 

 scale, any possible error from slight shifting of the scale is obviated. With 

 this arrangement it is not even necessary that the reed or tuning-fork 

 should be near the rotating cylinder ; for if the slit is placed at the focus 



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