194 



SCIENTIFIC RECREATIONS. 



The SYREN was invented by Cagniard de Latour. It consists of a 

 metal cylinder, a tube passes through the bottom, and through the tube air 

 is blown into the cylinder. On the top a number of holes are drilled, while 

 just over the cylinder top, almost in contact with it, is a metallic disc, which 

 rotates upon a vertical axis. The disc is perforated with holes equal in 

 number to those in the cylinder top, but the holes are not perpendicular, 

 they slope in opposite directions. So when the air is forced through the 

 holes in the top of the cylinder it impinges upon one side of the holes in the 

 rotating disc, and blows it round. 



The disc in one revolution will therefore open and shut as many holes 

 as there are in the disc and cylinder, and the air blown in will escape in so 

 many puffs the number of puffs in a given time depending upon the 

 rapidity of rotation. There is an arrangement to show the number of turns. 

 By these rotations a sound is produced which rises in pitch as the revolutions 

 are increased in number. 



To determine the pitch of a certain sound we must find the number of 



Fig. 198. Sound Figures. Fig. 199. 



times the plate revolves in that time, then we shall have the number of 

 vibrations per second required to produce the note we desire. The arrange- 

 ment working in a notched wheel tells us the number of rotations of the 

 disc. Successive, and rapidly-successive puffs or beats are heard as the 

 rotation increases, and at length the two sounds will disappear, and merge 

 into one, which is perhaps that of the tuning-fork, whose note you require to 

 find the " pitch " of. By maintaining this rate for a minute or less, and 

 setting the gear to tell the revolutions, the number will be found marked on 

 the dial of the apparatus. So by multiplying the number of revolutions of 

 the disc by the number of the holes, and dividing the product by the number 

 of seconds during which the disc was in connection with the recording gear, 

 we shall have the number of vibrations per second necessary to produce the 

 pitch corresponding to the given sound. The above is the description of 

 one form of Syren ; there are others, which, however, we need not detail. 



We have seen that there are certain nodal points, or resting-places, in 

 vibrations, and this can easily be shown upon a fiddle-string, from which 

 paper discs will fall off except on the nodal point, showing that there is no 



