56 THE ART OF PROJECTING. 



solder to one end a piece of small copper wire, so it 

 will project from the side about one-eighth of an inch. 

 A gimlet-screw can be cut into the other end of the 

 fork, so that it can be tightly screwed into the wooden 

 weight b, if small gimlet- holes are made before. There 

 should be a number of these holes bored into , at such 

 a distance from the front edge that when the fork is in 

 one of them, the wire upon the prong will come against 

 the surface of the glass plate when the weight b is 

 raised. 



In order to use this instrument, it is necessary to 

 coat the front side of the glass c with smoke, photo- 

 graphic varnish, or a very thin coat of white wax. Fix it 

 in its place in the frame, and then raise the weight b 

 (which ought to weigh two or three pounds) until the 

 top of the tuning-fork is above the glass. Seize the 

 two prongs of the fork with the thumb and forefinger, 

 and pinching them close together, suddenly let it drop. 

 The wire finger will trace a sinuous line upon the pre- 

 pared surface, caused by the vibration of the fork 

 during its descent. The undulations will be seen to 

 increase in length as they approach the bottom ; but as 

 each one was made in the same time with every other 

 one, it is obvious that the velocity increased as it was 

 falling. In order to show this, it is merely necessary 

 to put the apparatus near the condensing lens, and 

 project the face of the glass. The line traced by the 

 fork will be seen upon the screen. It will now be well 

 to measure the lengths of the undulations, which can 

 be very well done by having a scale in millimeters 

 etched, or otherwise fixed upon another glass, which 

 can be put just in front of the first, when the number of 

 divisions of the scale to each undulation can be counted, 

 and the result stated in mathematical terms. 



