66 THE ART OF PROJECTING. 



A sponge dipped in strong sulphuric acid was drawn 

 along upon the stretched wire, the acid gathering itself 

 up into small drops at regular distances apart. When 



Fig. 42. 



a current of electricity of sufficient strength was sent 

 through the wire it was heated red-hot, and the acid was 

 vaporized in dense fumes that, on account of its great 

 density, sunk down toward the bottom of the tube, 

 making so many gaseous strings hanging from the wire. 

 These, of course, were subject to the motions of the 

 air in the tube, and when the other end of the tube was 

 sounded by wind from a bellows, the free end partook 

 of the vibrations. The motions were then observed 

 through a revolving stroboscopic disk, described further 

 on. Not only the swaying of these gaseous threads 

 was observed, but some of the Lissajous's curves were 

 seen. 



I think it highly probable that the motions of the air 

 in such a sounding-tube can be shown to an audience, 

 by having the tube with glass sides filled with dense 

 smoke, and a strong beam of light converged in it, and 

 having the stroboscopic disk so placed that the focus of 

 the lens would be in the holes, and so permit a large 

 amount of light to be used. Where the node was 

 formed no movement would be visible ; but by giving 

 the disk a suitable velocity, at any other place than the 

 node, the vibration might be shown in its different 

 phases. 



