Page:The New International Encyclopædia 1st ed. v. 01.djvu/127

ACOUSTICS. ber) the best instrument is the "sensitive flame," which consists ordinarily of an ignited jet of gas escaping from a small circular orifice under high pressure, thus giving a more or less cylindrical flame about a foot high. When waves of a great wave-numher fall upon such a flame they break through the inclosing envelope separating the gas from the air, thus causing the jet to "flare" out like a fan.

. The waves produced in the air by vibrating bodies are often called "sound waves," although the name is not a good one. Similarly, compressional waves in any medium, solid, liquid, or gas, are called "sound waves" in these media. These waves spread out from the vibrating body into the surrounding medium with a velocity called the "velocity of sound," which depends alone upon the elasticity of the medium with respect to a compression and upon its density, if the medium is homogeneous. Like all waves, they may experience reflection, e.g., echoes: refraction, as when passing from cold air to hot air, or dense air to rare; dispersion; interference. Reference should be made to a paper by Professor R. W. Wood in the Philosophical Magazine, Volume XLVIII., p. 218, 1899, for a description of a most interesting series of experiments on these properties of aërial waves.

The best determinations of the velocity of these waves are given in the following table:

Gases at 0". C. Air (dry) 331.:j(i meters ])er second Hydrogen 1286. " .xvgen 317. Carbon dio.xide. 2(i2. .S'o//(/.s' and Lliiuids. Aluminium.....' 5104. " " " Steel 499(t. " " " Glass, about .... uoOO. " " " Water 1435.

The velocity of compressional waves varies greatly with the temperature. For a gas the velocity at t° C. equals that at 0° C. multiplied by

73 + t 273

When waves pass from a region where the air is cold into one where it is warm, reflection takes place at the bounding surface, and thus the entering waves are not only refracted but also weakened in intensity. The presence of fog by itself in the air has very little effect upon the waves, unless there are currents or layers of hot or cold air. The velocity of waves in air is practically independent of the intensity of the vibration, although the waves produced by a sud- den explosion travel at first slightly faster than do ordinary waves.

. When an organ-pipe or any elastic body is sounded in a room and then suddenly stopped, it is noticed that the sound does not instantly cease, but continues for several seconds. This is called reverberation; and the acoustic success of a room depends largely upon its duration. It should not exceed two seconds by more than a few tenths of a second if the room is to be used as a music hall or opera house. It is found that the reverberation in a given room is practically independent of the place where the vibrating body is situated, or of the position of the hearer; it depends upon the volume of the room, upon the material of the walls and floors, upon the cushions, the audience, etc., and to a certain extent upon the intensity of the sound. The following approximate formula has been developed by Professor Sabine of Harvard University:

(a + 6, a-, + 6, a-. + etc.) r = 0.IG4r Where a is a constant depending upon the ab- scubing i)ower of the walls of the room. b is a coellicieiit of "absorption" for one square meter of a definite material put anywhere in the room, the standard of comparison being the absor])tion of one .square meter of open window. w is the number of square nu'tcrs of the material. t is the duration of reverberation. F is the volume of the room in cubic meters. The absorption coefficients for some substances are as follows: Hard [due wood sheathing O.OGl Plaster on woo<l lath 0.034 Plaster on wire lath 0.033 Audience (per square meter) .... 0.96 Isolated woman 0.54 Isolated man 0.48 Carpet rugs 0.20 House plants 0.11 Upholstered chairs 0.30 Hair cushions (per seat) 0.21 The duration of reverberation in certain nmsic halls and auditoriums is as follows: Old Music Hall, Boston, Mass 2.44 New Music Hall, Boston, Mass 2.31 (iewiindhaus. Leipzig, Germany 2.30 Sanders' Theatre, Cambridge, Mass. . . . 3.42

. Rayleigh, Theory of Sound, 2 volumes (London, 1896), a mathematical treatment, but with several descriptive chapters; Helmholtz. Sensations of Tone, translated by Ellis (London, 1895), the standard authority on harmony and music; Sabine. Architectural Acoustics (Boston, 1900), which contains the only satisfactory treatment of this important question; Thomson and Poynting, Sound (London. 1899), a text-book for schools and colleges, and a storehouse of accurate information.

ACQUI. .-i'kwe (Ancient Aquæ Staticellæ). An epi.scopal city of northern Italy, on the left bank of the Bormida, 37 miles northwest of Genoa (Map: Italy, C 3). Every winter more than 6000 persons take the cure at the hot and cold sulphur springs that gave it its name. It has a Gothic cathedral of the eleventh century, a seminary, a college, and the ruins of a Roman aqueduct. The chief trade is in silk, lace, rope, and wine. Pop., 1901, 13,786.

AC'QUISI'TION. In law, a term which has the double meaning of the acquirement of territory by the state, and of title to real or personal property by the individual. In the case of the state it is effected in three ways: (1) By occupation, (2) by treaty and convention, and (3) by conquest (q.v. ). As referring to the origin of title to lands or goods, acquisition is either original or derivative. The former comprehends occupation, accession, and prescription