It takes a certain time for sound to travel. The rate is about 1090 feet per second when the temperature of the air is 0° Cen tigrade, and is increased when the temperature is raised. Every one who has watched the dis charge of artillery from a distance knows that the flash and smoke from the gun's mouth are per ceived a brief time before the report is heard. If the interval between the two be taken, re membering the rate at which sound travels, one may readily, make a rough estimate of the distance of the gull —allowance, of course, being made for wind.
Now if the atmosphere were visible, with the gun's discharge a tremendous disturbance would have been seen to take place in the air at the gnu's mouth at the moment the flash was seen. This disturbance would be seen spreading outward in all directions, and travelling with great rapidity. Suppose the person stood watching the advancing agitation, then, just at the moment when it reached and enveloped him, he would hear the report. So that when a person walks through a crowded and noisy thoroughfare his ear is being assailed by num berless waves of sound of all kinds and sizes and degrees of rapidity, that surge and swell in the atmosphere around him.
There are various characters of sounds which it is necessary to have some idea of for the proper understanding of the apparatus of hear ing. These characters are best exemplified by musical sounds, which are distinguished from ordinary sounds by the regular rhythmical char acter of their wave movements.
Musical Sounds differ in loudness or inten sity, in pitch, and in kind or quality.
The loudness of sounds depends on the ex tent of the vibration or movement—the large ness of the wave, so to speak.
The pitch of the sound is determined by the number of vibrations that take place in a second of time. Thus a tuning-fork whose limbs move to and fro 100 times a second will give out a sound of a certain pitch, and a tuning-fork that executes 200 movements in the same time will sound a note of a higher pitch—will, in fact, sound the octave of the former fork. This is shown in a very beautiful way by an instru ment devised by a Frenchman, Cagniard de Latour, and called a siren. It is shown in Figs. 188, 189. It consists of a metal box, the floor of which is pierced by a tube placed in connection with a large bellows. In the roof of the box is a small round opening, passing in a sloping direction. Fig. 189 shows a piece cut off the box so as to exhibit this opening. Above the fixed roof of the box, but very close to it, is a round plate, with a number of openings pierced in it, which slope in an opposite direc tion to that of the opening in the roof. This Plate turns on a fine pivot, so that one hole after another can be brought opposite the opening in the roof. Fig. 189 shows one hole in the plate
opposite the opening in the roof of the box, and the different slope of the two openings is seen. Fig. 188 shows the siren complete. Now if air be driven from a bellows into the box, it escapes by the opening in the roof ; and as it rushes out it strikes against the edge of the hole in the plate, and the "puff" of the escaping air is heard. The plate, being easily moved, is by this means made to turn, and so the opening in the roof becomes blocked; but when the plate turns a little farther a second hole in it comes opposite the opening in the roof, the air again rushes out, produces another "puff," and drives the plate round a little farther. Again the opening is blocked, and with the continued turning of the plate it is again speedily opened. If the bellows be worked hard the plate will be driven round fast, and the opening in the roof will be opened and closed very rapidly. Every time it • is opened a shock will be given to the atmos phere by the escaping puff of air. Fixed to the apparatus are two dials which mark the number of turns made by the revolving plate. If one knows how often the plate turns in a minute, and the number of holes in the plate, it is easy calculating how many shocks the air will receive in a minute. Now it is found that if the bellows be worked so feebly that the plate turns so slowly as not to open and close the hole in the roof sixteen times a second, the puffing sound is beard each time the air escapes. But if the hole in the roof of the box is opened and closed sixteen times a second, which would be effected if the plate were turning once a second, and if it were pierced with sixteen holes, then the sixteen puffs are not heard as separate sounds, but are blended together, and a low musical sound is heard. If the bellows be worked more and more quickly the plate turns faster and faster, the number of shocks given to the air in a second of time is increased, and the sound is heard to rise in pitch, until with the utmost speed of the plate it becomes a high shrill sound. It is possible, by means of the siren, to discover the number of vibra tions made in a second by the limbs of a tuning fork sounding a note of a certain pitch. Let the siren be worked till the sound produced is of the same pitch as that of the fork ; then by noting the number of times the plate is revolv ing, as marked on the dials, the calculation may be made. The instrument shows conclusively how pitch of sound depends on the number of vibrations produced by the sounding body in a second of time.