354 CONSTRUCTION OF SIREN 



and draw the six diameters, which in the figure are not letters 

 Each of the circles is thus divided into twenty-four equal pari 

 which are further subdivided as follows : on the third circle ea< 

 twenty- fourth is divided, by trial, into three equal parts ; on t] 

 inner circle each twenty-fourth is bisected ; and on the outer circ 

 each twenty- fourth is divided into four equal parts. This gives t 

 points on the third, and 96 points on the fourth circle. The positi< 

 of the 48 holes in the inner circle is found by drawing lines (D 

 shown in the figure) between the opposite points which are mark* 

 in the figure by small crosses. The holes must be made with 

 liolloiv circular punch, about 4 mm in diameter, such as is used 1 

 saddlers for making holes in leather. Before the holes are punche 

 in order to ensure equal distances between them, small circl< 

 about 6 mm in diameter, should be drawn with a small pair of coi 

 passes round each of the marked points of division ; or a she 

 piece of wire which fits into the aperture of the punch is filed to 

 sharp point at one end, and successively placed with its point up. 

 the points of division ; the punch is then set over it and driv 

 through the cardboard. 



It is somewhat difficult to produce a uniform motion of the whi 

 ing- table, especially when air is to be blown through the tube at t 

 same time. It is therefore better that two persons should be e 

 gaged in the experiment, one giving his whole attention to the UL 

 form working of the handle, the other to the blowing and the obs<| 

 vation of the notes. For the blowing, a tube of glass or indiarubl 

 is used, 20 or 30 cm long, one end being held between the lips, t 

 other being guided by the hand. 



The interval between the notes of the different rows of holes 

 most easily observed when the end of the tube is rapidly transferi 

 from one row to another, so that the notes are heard in rapid succ* 

 sion. For blowing upon several rows simultaneously, a cork m 

 be fixed with sealing-wax into the end of a wide glass tube throu 

 which air is blown, and the jet may be directed to the several ro 

 by short glass tubes suitably bent, which pass through the cor; 

 or, if an assistant works the handle, two or three tubes may 

 held together between the lips, and their ends directed by the liau 

 For blowing at the same time upon all four rows, it is best to \\ 

 a hollow, pyramidal, or conical case made of cardboard or brass. lj 

 air is blown in at the smaller end, and four holes are made in 1i 

 base, each 3 mm wide, and the same distance apart as the rows ; 

 holes in the siren. The base should measure about 4 cm by4 mm , a ! 

 the distance from it to the narrow open end should l> about 15 

 The box is held straight over the disc of the siren, the holes in i 

 bottom close over the holes of the latter. 





