other on exciting the fork and moving the mirror at the proper rate, we obtain this beautiful figure produced by

The

the interlacing of the two sinuous lines, fig. 22. beauty of the figure, however, is not to be compared with the rippling of the actual lines of light over the screen.

FIG. 22.

How are we to picture to ourselves the condition of the air through which this musical sound is passing? Imagine one of the prongs of the vibrating fork swiftly advancing; it compresses the air immediately in front of it, and when it retreats it leaves a partial vacuum behind, the process being repeated by every subsequent advance and retreat. The whole function of the tuning-fork is to carve the air into these condensations and rarefactions, and they, as they are formed, propagate themselves in succession through the air. A condensation with its associated rarefaction constitutes, as already stated, a sonorous wave. In water the length of a wave is measured from crest to crest; while in the case of sound the wave-length is given by the distance between two successive condensations. In fact, the condensation of the sound-wave corresponds to the crest, while the rarefaction of the sound-wave corresponds to the sinus of the water-wave. Let the dark spaces, a, b, c, d, fig. 23, represent the condensations, and the light ones, a', b', c', d', the rarefactions of the waves issuing from the

WAVE-LENGTH DEFINED.

63

fork AB: the wave-length would then be measured from a to b, from b to c, or from c to d.

Pitch has been shown to depend upon rapidity of vibration. When two notes from two distinct sources are of the same pitch, their rates of vibration are the same. If, for example, a string yield the same note as a tuningfork, it is because they vibrate with the same rapidity; and if a fork yield the same note as the pipe of an organ, or the tongue of a concertina, it is because the vibrations of the steel in the one case are executed in precisely the same time as the vibrations of the column of air, or of the tongue in the other. The same holds good for the human voice. If a string and a voice yield the same note, it is because the vocal chords of the singer vibrate in the same time as the string vibrates. Is there any way of determining the actual number of vibrations corresponding to a musical note? Can we infer from the pitch of a string, of an organ-pipe, of a tuning-fork, or of the human. voice, the number of waves which it sends forth in a second? This very beautiful problem is capable of the most complete solution.

I have shown you, by the rotation of a perforated pasteboard disc, that a musical sound is produced by a quick succession of puffs. Had we any means of registering the number of revolutions accomplished by that disc in a

minute, we should have in it a means of determining the number of puffs per minute due to a note of any deter

minate pitch. The disc,

however, is but a cheap substitute for a far more perfect apparatus which I now bring before you; which requires no whirling table, and which registers its own rotations with the most perfect accuracy. I will take the instrument asunder, so that you may see its various parts. A brass tube t, fig. 24, leads into this brass cylinder c, closed at the top by a brass plate ab. This plate is perforated with four

series of holes, placed along four concentric circles. The innermost series contains 8, the next 10, the next 12, and the outermost 16 orifices. When I blow into the tube t, the air escapes through the orifices, and the problem now before us is to convert these continuous currents into discontinuous puffs. This is accomplished

by means of a brass disc, d e, also perforated with 8, 10, 12 and 16 holes, at the same distances from the centre and with the same intervals between them as those in the top of the cylinder c. Through the centre of the disc passes a steel axis, the two ends of which are

MECHANISM OF THE SYREN.

65

smoothly bevelled off to the points p and p'. My object now is to cause this perforated disc to rotate over the perforated top a b of the cylinder c. You will understand how this is done by observing me put the instrument together. In the centre of a b, fig. 24, is a depression x sunk in steel, smoothly

polished and intended to receive the end p' of the axis. I place the end p' in this depression, and holding the axis upright, bring down upon its upper end p a steel cap, finely polished within, which holds the axis at top and bottom, the pressure being so gentle, and the polish of the touching surfaces SO smooth, that the disc can rotate with an exceedingly small amount of friction. At c, fig. 25, is the cap which fits on to the upper end of the axis pp'. In this figure the disc, de, is shown covering the top of the cylinder c. I would ask you to neglect for

FIG. 25.

Turning the

the present the wheelwork of the figure. disc de thus slowly round, I can cause its perforations to coincide or not coincide with those of the cylinder underneath. As the disc turns, its orifices come alternately over the perforations of the cylinder, and over the spaces

F

between the perforations. Hence it is plain that if air were urged into c, and if the disc could be caused to rotate at the same time, we should accomplish our object, and carve into puffs the streams of air. In this beautiful instrument, the disc is caused to rotate by the very air currents which it has to render intermittent. This is done by the simple device of causing the perforations to pass obliquely through the top of the cylinder c, and also

Another moment will make you acquainted with the recording portion of the instrument. At the upper part of the steel axis pp', you observe a screw s, working into a pair of toothed wheels, seen when the back of the instrument is turned towards you, as in fig. 25. You notice that as the disc and its axis turn, these wheels rotate. In front you simply see these two graduated dials, fig. 26, each furnished with an index like the hand of a clock. These indexes record the number of revolutions executed by the disc in any given time. By pushing the button a or b it is in my power to throw this wheelwork into or out of action, and thus to start or to suspend, in a moment, the process of recording. Here, finally, is a series of pins, m, n, o, p, by which any series of orifices in the top of the cylinder c can be opened or