INSTRUMENTS FOR EXPERIMENTS ON SOUND. 149 



attraction on the prong of the fork, breaking contact by so 

 doing ; a fresh contact is thus made, and so the fork is kept 

 in permanent vibration. I have here an apparatus which I 

 have made for this purpose (you will excuse my mentioning 

 that much here shown is the work of my own hands). 

 When the magnet is formed, it separates the prong and lifts 

 the stile out of the mercury cup in so doing the fork is 

 now in vigorous vibration and produces a note, which at 

 present you cannot hear, but by bringing a suitable resonator 



FIG. 5. Cagniard de la Tour's Siren. 



FIG. 6. Interior view of the Siren. 



to it you will hear it distinctly. In that way Helmholtz has 

 been able to keep eight or ten forks all vibrating from one 

 principal fork. Here is one of these principal forks, sent 

 from Paris, having a mercury contact upon it, and there is 

 also a series of secondary forks which have only the electro- 

 magnet and which can be thrown into secondary vibration 

 from this ; you can thus reproduce the various vowel sounds 

 which have been explained and demonstrated by Helmholtz. 

 I have next to speak of sirens. This fanciful name was 



