INSTEUMENTS FOE EXPEEIMENTS ON SO UND. 153 



one of these forks even while I am speaking to you, you 

 can feel the particular notes that I speak of; it seems to 

 vibrate when a certain note occurs. The same with a drum 

 while I was speaking I could feel distinct vibrations in the 

 drum as each word came out of my mouth. The same with 

 a piano if you take off the dampers and speak into it, and 

 then stop suddenly, you hear the piano singing out loudly 

 with a sort of hum, the notes which you have been speaking 

 or singing. Or even more simply than that. If any of you 

 happen to be at a musical performance such as the opera, and 

 put your finger on the top of the crown of your hat, you will 

 find it acts like a membrane to reinforce particular sounds, 

 giving a regular vibration ; and then another note to which it 



FIG. 8. Helmholtz's Kesonator. 



does not con senate comes up, and the hat is still ; so that the 

 hat itself seems to be enjoying the music after its fashion. 

 This propensity for singling out sound has been utilized by 

 Helmholtz in making his resonators. He originally made the 

 resonators, of which I have two sets here, with external 

 membranes very much like this old marimba, but afterwards 

 he found he could use the drum of the ear for the same purpose 

 by making the cavity of a particular size, so that themselves 

 speaking a certain note, they will single out that note from 

 ail others and reinforce it largely. Here is one which is suffi- 

 ciently vibratile to give when struck a note like that of a bell, 

 corresponding to the tone of E with 320 vibrations. When 

 I speak that note you will hear it, and when I go down 

 another note it ceases altogether. Here is another answering 

 to G, and another to the upper C with 512 vibrations, and 



