THE SIKEJS". 175 



orifices, the next series, twelve, the next, ten, and the inner- 

 most series, eight. Above this fixed disk, is another disk, 

 with corresponding openings, which can be made to rotate at 

 a known rate. When the openings in the two disks are in 

 exact apposition, the air passes freely, in a continuous current, 

 from a tube connected with a chamber below the lower disk, 

 out by a tube connected with the upper disk. If the perfora- 

 tions in the two disks be oblique, the direction of the infe- 

 rior openings being opposite to those in the upper disk, a cur- 

 rent of air through the apparatus will cause the upper disk 

 to rotate, when the perforations will be closed and opened 

 at regular intervals. It is not difficult to arrange a dial 

 which will register the exact number of rotations of the 

 upper disk in a given time. The rapidity of rotation depends 

 upon the force of the current of air through the apparatus ; 

 and the tone is higher with the more powerful currents. Let 

 us suppose, now, that we have the siren perfectly adjusted, 

 with a bellows that will force an equable current of air 

 through it, and a dial which records the exact number of 

 turns of the rotating disk. "We sound the note of a tuning- 

 fork and blow air through the siren, regulating the current 

 until the two notes are exactly in unison. When this is ac- 

 complished, we set the registering portion of the apparatus in 

 action, and stop it at the end of precisely one minute. The 

 conclusion of this illustrative experiment we quote from 

 Tyndall : " I suddenly push 5 and stop the clock-work ; and 

 here recorded on the dials we have the exact number of revo- 

 lutions performed by the disc. This number is 1,440. But 

 the series of holes open during the experiment numbers 16 ; 

 for every revolution, therefore, we had 16 puffs of air, or 16 

 waves of sound. Multiplying 1,440 by 16, we obtain 23,040 

 as the number of vibrations executed by the tuning-fork in a 

 minute.- Dividing this number by 60, we find the number 

 of vibrations executed in a second to be 384. 



" Saving determined the rapidity of vibration, the length 

 of the corresponding sonorous wave is found with the ut- 

 142 



