CHAPTER XVII.
THE TUNING-FORK—THE SYREN—SOUND FIGURES—SINGING FLAMES.
We cannot close the subject of Sound without some mention of the Musical Pitch, and various instruments and experiments which have from time to time been made to discover the pitch, sound, and vibrations, and even to see Sound. To understand the vibrations or “pitch” of a musical note we may study the illustration, which shows us a tuning-fork in vibration.
You will perceive that each prong of the tuning-fork beats the air in an opposite direction at the same time, say from a to b (fig. 196). The prong strikes the air, and the wave thus created strikes again outward, and the condensation thus created travels along the back beat, rarefying the air, and both these, the rarefaction and the condensation, move with the same rapidity one behind the other.
Fig. 196. and Fig. 197.
The tuning-fork of course vibrates a very great many times in a second, every vibration generating a wave. “Pitch,” in a general sense, is the number of vibrations per second which constitute a note. For instance, the note A, the standard pitch consists of four hundred and thirty-five complete vibrations per second. Concert pitch is slightly higher, for there are a few more vibrations in the second. The lowest sound pitch is forty vibrations, the highest forty thousand. “Pitch” may be determined by an instrument termed the “Syren,” or by a tooth-wheeled apparatus.
The Syren was invented by Cagniard de Latour. It consists of a metal cylinder, a tube passes through the bottom, and through the tube air is blown into the cylinder. On the top a number of holes are drilled, while just over the cylinder top, almost in contact with it, is a metallic disc, which rotates upon a vertical axis. The disc is perforated with holes equal in number to those in the cylinder top, but the holes are not perpendicular, they slope in opposite directions. So when the air is forced through the holes in the top of the cylinder it impinges upon one side of the holes in the rotating disc, and blows it round.