208 



THE POPULAR EDUCATOR. 



becomes higher and higher as the disc is made to rotate more 

 rapidly. 



If we construct a disc with several concentric rings, having 

 varying numbers of apertures, we shall be able to produce dif- 

 ferent sounds according to which part we hold the jet against. 

 It is somewhat difficult to ascertain by this arrangement the 

 exact number of interruptions per second ; an apparatus was, 

 however, devised by an eminent French natural philosopher, 

 Cagniard de la Tour, which serves to register them very 

 accurately. This curious acoustic instrument, which was 

 called by him the Sirene, or Siren, because of its power of 

 emitting sounds under water, is represented in Figs. 8 and 9 in 

 the annexed illustration, the former showing the instrument 

 complete, and the latter a view of a vertical section of it. The 

 wheel-work at the upper part of the siren is for the purpose of 

 recording the revolutions 

 of the disc, but we will 

 turn our attention to the 

 other part first, o is a 

 brass cylinder into which 

 air is driven from the 

 acoustic bellows, E, ar- 

 rangements being made 

 by which the power of 

 the blast can be modified 

 at pleasure. The upper 

 end of o is closed by a 

 plate, B, of brass or 

 copper, perforated with 

 twenty holes, arranged 

 in a circle as shown in 

 Fig. 10; through these 

 apertures the air escapes. 

 We want now some means 

 of interrupting the cur- 

 rent from these so as to 

 produce a series of puffs 

 instead of a continuous 

 stream. 



This is accomplished 

 by another disc, A, similar 

 to that which closes O ; 

 this disc is mounted on a 

 spindle, T, the ends of 

 which are pointed so as 

 to turn with as little 

 friction as possible. A 

 small depression is made 

 in the centre of B to 

 carry the lower end of 

 this, and the upper end 

 turns in a cavity in the 

 end of the screw on the 

 top of the instrument. 

 The whole is very care- 

 fully constructed, so that 

 friction is reduced to a 

 minimum. The number 

 of apertures in each disc 

 is precisely the same, 

 and they correspond in position, so that all the openings are 

 opened or closed simultaneously. Only one puff is, therefore, 

 produced by the twenty holes ; the sound is, however, much 

 more powerful than if there were only one aperture. Now it 

 will be seen that for every revolution A makes, 20 distinct 

 pulsations will be produced in the air, and we therefore only 

 need some means of causing this disc to revolve rapidly, and of 

 recording its revolutions. 



The former of these is easily accomplished. The openings in 

 B, instead of being at right angles to the plane of the disc, are 

 slightly inclined, as at m (Fig. 10), so that the air issues in cur- 

 rents directed to one side. The rotating disc, A, has its apertures 

 inclined slightly in the other direction, as at n, and thus it will be 

 seen that the air as it issues strikes against the sides of these 

 apertures, and seta the disc in rotation. The force produced in 

 this way is but small, but as the disc turns very easily, it is quite 

 sufficient, and by merely increasing the pressure of the air from 

 the bellows, we can raise the pitch of the note as high as we desire. 



Now let us look at the recording portion of the apparatus. 

 On the upper end of the spindle T there is cut a screw, which 

 works in the teeth of the wheels that carry the hands. As the 

 spindle turns, these wheels are moved by it, each revolution 

 causing the wheel P to advance one tooth. The second wheel 

 makes one revolution for every 100 made by the other. By 

 pressing the stud, D, on the right-hand side of the instrument, 

 the wheels are removed from the screw, on the upper part of 

 the spindle, T, and thus cease to record ; on pressing c, on the 

 left-hand side of the instrument, contact is again renewed, and 

 thus we can let the wheels remain in action aa long as we like. 



An illustration of the manner of using the apparatus will 

 make this quite clear to the reader (Fig. 11). We will suppose 

 that we have a tuning-fork, and want to ascertain the number of 

 vibrations it produces in the course of a second. We place 



the siren in the. wind- 

 chest of the bellows, and 

 cause it to sound, having 1 

 first thrown the wheel- 

 work out of gear ; we 

 then excite the tuning- 

 fork by striking it or 

 by drawing a violin-bow 

 across it. The occur- 

 rence of beats in the 

 sound will show us that 

 the two notes are not in 

 unison ; we therefore 

 adjust the force of the 

 bellows till the beats 

 gradually become slower 

 and slower, and at last 

 vanish, showing that 

 now the siren and the 

 fork are producing the 

 same note. Keeping the 

 pressure uniform, so that 

 the same note continues 

 to be uttered, we press 

 c, and holding a watch 

 in the hand, allow the 

 wheels to record the re- 

 volutions for a given 

 period, say, exactly one 

 minute. At the expira- 

 tion of this time we press 

 the stud D, and having 

 thus stopped the wheel- 

 work, read off the number 

 of revolutions indicated. 

 Suppose we find the 

 revolutions recorded to 

 be 768, then, since at 

 every revolution the cur- 

 rent of air is interrupted 

 20 times, we must clearly 

 have produced 768 x 20, 

 or 15,360 vibrations in 

 the 60 seconds. Dividing 

 this by 60, we obtain 

 the quotient 256, which is the number of vibrations per second 

 produced by the fork. The note is that generally known aa 

 middle C. 



It haa been noticed of late years that the pitch of the tuning- 

 fork has been gradually getting higher, and, which is of more 

 importance, that it differs considerably in different cities and 

 countries. This is productive of some inconvenience, and in 

 France a commission was appointed some ten or twelve years 

 ago to inquire into the subject. After comparing the different 

 standards, they recommended the adoption in France of a 

 normal diapason. A standard tuning-fork was prepared in 

 accordance with their recommendation. The fork, which gives 

 the sound of the first A in the treble, produces 435 complete 

 vibrations in the second. On this scale middle C requires 261, 

 and the pitch is therefore slightly higher than the English. 

 There being, however, no fixed standard in England, different 

 makers vary slightly, and hence there is a want of that unifor- 

 mity which is desirable. 



