VIBRATORY CURVES. 173 



the aid of oxy-hydrogen light, the vibratory curves traced by one of the 

 prongs of a tuning-fork. We can perform without difficulty a very similar 

 experiment with the humble assistance of the common knitting-needle. 



Fix the flexible steel needle firmly in a cork, which will give it 

 sufficient support ; fasten then at the upper extremity a small ball of seal- 

 ing wax, or a piece of paper about the size of a large pea. If the cork in 

 which the needle is fixed be held firmly in one hand, and you cause the 

 needle to vibrate by striking it, and then letting it sway of itself, or with a 

 pretty strong blow with a piece of wood, you will perceive the little pellet of 

 wax or paper describe an ellipse more or less elongated, or even a circle 



Fig. 173. Experiment showing vibration of sound waves. 



will be described if the vibrations be frequent. The effect is much enhanced 

 if the experiment be performed beneath a lamp, so that plenty of light may 

 fall upon the vibrating needle. In this case, the persistence of impressions 

 upon the retina admits of one seeing the vibrating circle in successive 

 positions, and we may almost fancy when the needle is struck with sufficient 

 force, that an elongated conical glass, like the old form of champagne glass, 

 is rising from the cork, as shown in the illustration annexed (fig. 173). 



Acoustics may be studied in the same way as other branches of 

 physical science. We will describe an interesting experiment, which gives 

 a very good idea of the transmission of sounds through solid bodies. A 

 silver spoon is fastened to a thread, the ends of which are thrust into both 



