76 THE ART OF PROJECTING. 



be demonstrated by changing the pitch of one of the 

 forks, when no change of form of the projected beam 

 will be observed. One of the conditions for the suc- 

 cess of this experiment is that both forks should rest 

 upon the same table, in order that the vibrations may 

 be conveyed through the solid wood from one fork to 

 the other. The intensity of the sound-wave in the air 

 is not sufficient to communicate a motion that will be 

 perceptible. A voice sounding the same fundamental 

 note as one of the forks, will set it vibrating, as will be 

 evident by the spot of light upon the screen being 

 drawn out into a line. 



With one of these forks Melde's experiment may be 

 shown in the most satisfactory manner. Choose a soft 

 white cord eight or ten feet long (a silk cord is best, though 

 a cotton twine will work very well), tie one end to the 

 fork at a and let the other end hang over a hook driven 

 in the wall at b. Weights varying from a pound to 



Fig. 68. 



half an ounce or less may be hung upon this free end 

 of the string, with which its tension may be varied. 

 The fork may be struck with a billet of wood, as in the 

 former experiments, when the string will be made to 

 vibrate, either as a whole, or in equal segments, the 

 number of which will be inversely proportional to the 

 stretching weight. The amplitude of these vibrations 

 of the string will be considerable, and if the string vi- 

 brates as a whole it may be eight or ten inches, or even 



