346 THE SIREN. 



follow in rapid and regular succession. In its most 

 simple form the apparatus may be constructed by fixing 

 to the whirling table a circular sheet of mill-board, in 

 which several concentric rows of holes are punctured in 

 circles round the centre, as shown in fig. 213. To cor- 

 respond with the size of the whirling table previously 

 described, the disc should have four rows of 48, 60, 72, 

 and 96 holes respectively. If air is blown by the 

 mouth into one end of a small tube, having the same 

 internal width as the aperture of the holes, while- the 

 other end is held opposite to the line of openings close 

 to the disc, the current will be interrupted and little 

 air will flow out, when the card-board is against the 

 jet, but it will pass wherever an aperture comes opposite 

 it. If the whirling table is turned, the current will 

 be stopped and opened as many times in each second as 

 there are apertures which pass the end of the tube 

 in the same time. Thus if the row of 48 holes be used, 

 and the handle of the whirling table be worked so as 

 to make the disc rotate six times in one second, then 

 6 x 48 = 288 apertures will pass the jet, and the 

 current will be interrupted 288 times during one 

 second; 288 puffs of air follow each other rapidly and 

 regularly, and produce on our ear the sensation of f 

 musical sound, though no doubt somewhat rough anc 

 impure, because it is accompanied by the whizzing 

 sound produced by the air which strikes the card-boarc 

 between the holes. Sirens are constructed which ar< 

 more convenient for use and produce a purer soun< 

 than this contrivance, but they are rather complicate* 

 and expensive. 



The mouth of the tube being held close to one of th 



