Page:The American Cyclopædia (1879) Volume XV.djvu/183

 SOUND 175 room of a house, or in a tightly closed box lined with felt, h placed a musical box. On the top of the musical box rests the end of a long light wooden rod which reaches to one of the rooms above. The rod is insulated from the floor of the rooms by India rubber. No sound.is perceived in the upper room until we place on the top of the rod a violin, a guitar, or any instrument with a sounding board, when the sounds of the musical box fill the upper room and appear to emanate from the musical instrument on the rod. That a liquid may be the medium for the transmission of sonorous vibrations is readily proved by placing on a resonant box a long cylindrical vessel filled with water, and then bringing in contact with the surface of the water a disk of wood at- tached to the foot of a vibrating tuning fork. The vibrations of this instrument are sent through the water, and reaching the top of the resonant box throw the latter into vibra- tions of the same period as those of the fork. That the air, a gaseous body, vibrates while it is transmitting sonorous pulses, can be shown jy placing in the path of these vibrations a slicate membrane strewn with a light dry >wder. The powder dances on the mem- >rane while the sound is perceived. The vi- >rations of the air can also be detected by leans of the so-called " sensitive flames," rhich are formed of jets of gas, issuing from cylindrical orifices under such great pressure that they are just on the point of flaring, or roaring. These flames are so sensitive to aerial vibrations that the slightest sound, if of the proper pitch, will cause them suddenly to con- tract greatly in their lengths, and at the same to give forth roaring sounds. These lames are generally most sensitive to acute sounds, such as a hiss or the jingling of a bunch of keys. (See PYROPHONE.) An anal- ysis of sonorous sensations reduces them to three kinds: pitch, intensity, and timbre. 1. Pitch and the Determination of the Number of Vibrations of a Sounding Body. Pitch is that quality of sound by which we distinguish the position of sounds in the musical scale, sound is thus said to be higher or lower than another. Pitch depends on the number vibrations in a second which enter the ear. The pitch rises with the increase of the num- er of vibrations. In England, Germany, and Lmerica a vibration is understood to be a mo- ion to and fro, while in France it is a motion or fro. The sound having the lowest pitch is jaused by 40 vibrations in a second ; a smaller lumber of vibrations than this does not cause continuous sonorous sensation. The highest audible sound is caused by about 40,000 vibra- ions in a second; vibrations of greater fre- quency than this are not generally audible, "lough the limit of audibility of the highest sounds is different for different persons. Thus some cannot hear the chirrup of the cricket, while others perceive sounds one or two oc- taves above it. Dr. Wollaston discovered this 752 VOL. xv. 12 FIG. 2. variation. The pitch of a sound may be deter- mined by several methods, some of the most precise of which are : 1. By means of an in- strument called a "siren," fig. 2, invented by Cagniard de Latour. It consists of a metal cylinder the bottom of which is perforated by a tube through which air is blown into the cylinder. The top of the cyl- inder is perfora- ted with a num- ber of holes. Just over this top and nearly touching it ro- tates a metallic disk on a verti- cal axis. This disk is perfora- ted with the same number of holes as are in the cylinder. The form of the holes is shown in the section in the figure. They do not pass perpendic- ularly through the plates, but slope contrary ways, so that the air when forced through the holes in the top of the cylinder impinges upon one side of the holes in the rotating plate, and thus blows it round in a definite direction. The disk in making one revolution opens and shuts the holes as many times as there are holes in the disk and cylinder, and hence the wind escapes from the cylinder in successive puffs, the frequen- cy of which depends upon the rate of rota- tion. A sound is thus produced having a pitch which rises with the increase of velocity of rotation. The vertical axis has a screw cut on it which works in a notched wheel at- tached to a dial, which shows the number of rotations of the disk. To determine the pitch of a sound by means of this instrument, we gradually increase the rotation of the disk until the sound emitted approaches the pitch of the sound the number of vibrations of which we would determine. When the two sounds are quite near in pitch, the ear will perceive distinct beats produced by the joint action of the two sounds on the air. The velocity is now cautiously increased until the beats disappear. At this moment the counter is put in operation, and the disk is allowed to run for a known number of seconds ; then the counter is thrown out of action and the num- ber of revolutions of the disk read off. On multiplying the number of revolutions of the disk by the number of its holes, and dividing this product by the number of seconds during