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.

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.

The disc in one revolution will therefore open and shut as many holes as there are in the disc and cylinder, and the air blown in will escape in so many puffs—the number of puffs in a given time depending upon the rapidity of rotation. There is an arrangement to show the number of turns. By these rotations a sound is produced which rises in pitch as the revolutions are increased in number.

To determine the pitch of a certain sound we must find the number of times the plate revolves in that time, then we shall have the number of vibrations per second required to produce the note we desire. The arrangement working in a notched wheel tells us the number of rotations of the disc. Successive, and rapidly-successive puffs or beats are heard as the rotation increases, and at length the two sounds will disappear, and merge into one, which is perhaps that of the tuning-fork, whose note you require to find the “pitch” of. By maintaining this rate for a minute or less, and setting the gear to tell the revolutions, the number will be found marked on the dial of the apparatus. So by multiplying the number of revolutions of the disc by the number of the holes, and dividing the product by the number of seconds during which the disc was in connection with the recording gear, we shall have the number of vibrations per second necessary to produce the pitch corresponding to the given sound. The above is the description of one form of Syren; there are others, which, however, we need not detail.

We have seen that there are certain nodal points, or resting-places, in vibrations, and this can easily be shown upon a fiddle-string, from which paper discs will fall off except on the nodal point, showing that there is no vibration there. The same experiment may be made by means of plates, which will give us what are termed Chladni’s figures. Suppose we strew a glass-plate with fine sand, and stroke the edge with a fiddle-bow. The vibrations of the plates will make certain patterns, and cast the sand upon those points of repose to form nodal lines in various directions. The plates must, of course, be held or fastened, and a variety of sound figures may be produced. (See figs. 198 and 199.)

The relation between the number of segments on the plate and the pitch of the note, can be ascertained by using a circular plate clamped in the centre. “If the finger on the plate and the fiddle-bow are one-eighth of the circumference apart, the fundamental note will be produced. If one-sixteenth apart, the higher octave will be heard.”

Sensitive flames will detect air vibrations, and flames can also be made to sing. Sensitive flames were discovered by Mr. Barrett, who noticed the effect a shrill note had upon a gas flame from a tapering jet. The flame was a very long one (fourteen inches), and when the sound was produced it shortened at once, while the upper part expanded like a fan; the same effects, in a less marked degree, were observable when the shrill sound was prolonged from a distance of forty feet. Professor Tyndall was immediately interested in this discovery, and in January 1867 he lectured upon it at the Royal Institution.

If any one wish to try the experiment, a piece of glass tubing should be obtained, and let the mouth be tapered down to a small orifice one-sixteenth of an inch in diameter. Then when the highest pressure is on for the evening, light the gas and sound a shrill whistle. The flame will sink down and spread out. The illuminating power may thus be increased, and many experiments may be made. For instance, if a person be in the room and try to read, he will probably not be able to do so at a little distance; but if his friend whistle to the gas it will so expand itself as to enable him to read, so long as the whistle lasts.

A very ingenious burglar-detector was made upon the principle of the sensitive flame, which expands at a noise and heats a welded plate of gold, silver, and platinum. The plate swerves aside, the metals being unequally affected by heat, and as it is connected with a battery, rings a bell by electricity. A small high flame has been made sensitive to the chinking of coin, or even to the ticking of a watch. We will now give some explanation, derived partly from Professor Tyndall, of the cause of sensitive flames.

A sensitive flame is one just on the point of “roaring,” and about to change its aspect. “It stands,” says Tyndall, “on the edge of a precipice. The proper sound pushes it over.... We bring it to the verge of falling, and the sonorous pulses precipitate what was already imminent.” The flame is in a state of vibration, so sounds being vibrations, practically increase the pressure; and the flame acknowledges the pressure thus invisibly applied by air waves.

Singing Flames are produced by burning hydrogen in a tube; a musical note is thus produced in the same way as the air causes a note in an organ pipe. Faraday attributed the sound to rapid vibration caused by successive explosions of the burning gas. The Gas Harmonicon has been made on this principle. The air, being heated in the glass tube, ascends, and the flame is thus permitted to come up more forcibly in the tube; so violent agitation results when the air tries to get into the opening above. The size of the flame and its position in the tube will give a certain note which will be the same note as the air would emit if in a pipe, for the vibrations give the sound.

Sir Charles Wheatstone has shown by experiment how sound can be transmitted by placing a rod on a musical-box, and carrying the rod through the ceiling. When a guitar or violin was placed upon the rod, the sounds of the musical-box were distinctly heard in the upper room. A Phantom Band can be made by connecting certain instruments with others being played on under the stage. Every one will then appear to play by itself.