Chap. XXXTI.] 



THE SIREN. 



425 



The siren of Cagniard de la Tour (Fig. 186) permits 

 the number of vibrations producing sounds of different 

 pitch to be accurately estimated. It consists of a 

 brass air-chamber B, the top of which is perforated 

 with fifteen holes, disposed at regular intervals from 

 one another. Above the perforated lid is a circular 

 plate D, finely pivoted, capable of rotation on the 

 centre of the lid, and 

 pierced with an. equal 

 number of holes at dis- 

 tances from one another 

 corresponding to those 

 of the lid of B. In 

 particular positions the 

 holes of the two plates 

 will exactly coincide. B 

 has an inlet tube through 

 which air may be driven 

 into the chamber by a 

 bellows. Now the holes 

 of the lid and rotating 

 plate are not pierced in 

 a straight direction, but 

 obliquely, and those of 

 the plate are in an op- 

 posite direction to those 

 of the lid. Fig. 186 

 shows the section carried 

 through one hole, by 

 which the nature of the obliquity is seen. There- 

 fore, when air is blown into B, it escapes through 

 the openings in the lid, and then through those of the 

 disc D. But in pnssiiig from lid to disc, the air 

 strikes obliquely on the holes of the latter, and since 

 it is easily movable, causes it to rotate. By a 

 slight turn of D the holes of B are blocked ; but the 

 disc rotating farther, the holes again coincide, the 



Fig. 186. The Siren. 



