ACOUSTICS.
Acoustics is the science relating to sound and hearing. Sound is heard when any shock or impulse is given to the air, or to any other body which is in contact directly or indirectly with the ear.
DIFFERENCE BETWEEN SOUND AND NOISE.
Noises are made by the cracks of whips, the beating of hammers, the creak of a file or saw, or the hubbub of a multitude. But when a bell is struck, the bow of a violin drawn across the strings, or the wetted finger turned round a musical glass, we have what are properly called sounds.
SOUNDS, HOW PROPAGATED.
Sounds are propagated on all bodies much after the manner that waves are in water, with a velocity of 1,142 feet in a second. Sounds in liquids and in solids are more rapid than in air. Two stones rubbed together may be heard in water at half a mile; solid bodies convey sounds to great distances, and pipes may be made to convey the voice over every part of the house.
TO SHOW HOW SOUND TRAVELS THROUGH A SOLID.
Take a long piece of wood, such as the handle of a hair broom, and placing a watch at one end, apply your ear to the other, and the tickings will be distinctly heard.
TO SHOW THAT SOUND DEPENDS ON VIBRATION.
Touch a bell when it is sounding, and the noise ceases; the same may be done to a musical string with the same results. Hold a musical pitch-fork to the lips, when it is made to sound, and a quivering motion will be felt from its vibrations. These experiments show that sound is produced by the quick motions and vibrations of different bodies.
MUSICAL FIGURES RESULTING FROM SOUND.
Cover the mouth of a wine glass, having a foot-stalk, with a thin sheet of membrane, over which scatter a layer of fine sand. The vibrations excited in the air by the sound of a musical instrument, held within a few inches of the membrane, will cause the sand on its surface to form regular lines and figures with astonishing celerity, which vary with the sound produced.
TO MAKE AN ÆOLIAN HARP.
This instrument consists of a long narrow box of very thin deal, about six inches deep, with a circle in the middle of the upper side of an inch and a half in diameter, in which are to be drilled small holes. On this side seven, ten, or more strings of very fine catgut are stretched over bridges at each end like the bridges of a fiddle, and screwed up or relaxed with screw pins. The strings must all be tuned to one and the same note,[5] and the instrument should be placed in a window partly open, in which the width is exactly equal to the length of the harp, with the sash just raised to give the air admission. When the air blows upon these strings with different degrees of force, it will excite different tones of sound. Sometimes the blast brings out all the tones in full concert, and sometimes it sinks them to the softest murmurs.
[5] D is a good note for it. The upper string may be tuned to the upper D, and the two lower to the lower D and D D. The “harmonics” are the sounds produced.
A colossal imitation of the instrument just described was invented at Milan in 1786, by the Abbate Gattoni. He stretched seven strong iron wires, tuned to the notes of the gamut, from the top of a tower sixty feet high, to the house of a Signor Moscate, who was interested in the success of the experiment; and this apparatus, called the “giant’s harp,” in blowing weather yielded lengthened peals of harmonious music. In a storm this music was heard at a greater distance.
THE INVISIBLE GIRL.
The facility with which the voice circulates through tubes was known to the ancients, and no doubt has afforded the priests of all religions means of deception to the ignorant and credulous. But of late days the light of science dispels all such wicked deceptions. A very clever machine was produced at Paris several years ago, and afterwards exhibited in London under the name of the “Invisible Girl,” since the apparatus was so constructed that the voice of a female at a distance was heard as if it originated from a hollow globe, not more than a foot in diameter. It consisted of a wooden frame something like a tent bedstead, formed by four pillars a a a a, connected by upper cross rails b b, and similar rails below, while it terminated above in four bent wires c c, proceeding at right angles of the frame, and meeting in a central point. The hollow copper ball d, with four trumpets t t, crossing from it at right angles, hung in the centre of the frame, being connected with the wires alone by four narrow ribbons r r. The questions were proposed close to the open mouth of one of these trumpets, and the reply was returned from the same orifice. The means used in the deception were as follow: a pipe or tube was attached to one of the hollow pillars, and carried into another apartment, in which a female was placed; and this tube having been carried up the leg or pillar of the instrument to the cross-rails, had an aperture exactly opposite two of the trumpet mouths; so that what was spoken was immediately answered through a very simple mode of communication.
VENTRILOQUISM.
This is an art by no means very difficult of acquirement, if the young reader will take the pains. It is produced by a reflection of sound within the mouth, the voice being brought to the lowest possible place in the larynx. When the art is acquired by practice, the voice may be made to appear as if coming from any part of a room, from up a chimney, or from the depths of a cellar. The celebrated Dr. Wolcott, better known as Peter Pindar, used to amuse his friends in a remarkable manner with this art. He would represent his landlady as demanding payment of her rent, and hold a colloquy with her, which would at last rise to terms of reproach and fury, and end by a noise as if the landlady had been kicked down stairs. The marvellous powers of Matthews, Le Lagg, Alexander, and, lastly, Mr. Love, are familiar to most persons. To learn the art, the young practitioner must have the power of enunciating well, and that without motion of the lips,—of disguising the voice, so as to imitate other sounds,—and of adapting the degree to the apparent source of the sound. By practice this art is attainable by any person whose organs of speech are completely and fully developed.
