LIMITS OF HEARING. Aerial waves of all wave numbers do not affect the auditory nerves of the normal human ear, it being found by trial that wave-numbers less than 30 do not produce a musical tone, and wave-numbers exceeding about 20,000 do not produce sound at. all. For musi cal purposes the extremes are about 40 and 4000. To study waves whose wave-numbers exceed 10,000 (and in fact for those of much less num 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 m. less cylindrical Hanle about a foot high. When waves of a great wave•number fall upon such a Ilazne they break through the inclosing envelope separating the gas from the air. thus causing the jet to "flare" out like a fan.
VEr.ocITY OF SOUND. 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 me dium, solid, liquid, or gas, are 'ailed "sound waves" in these media. These waves spread out from the vibrating body into the surnainding 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 rano: Sion; interference. IZeference should be made to a paper by Professor It. \V. Wood in the Philo sophical .11gy azine, Volume X IXII • p. 21S, 1899, for a description of a most interesting series of experiments on these properties of aerial waves.
The best determinations of the velocity of these waves are given in the following table: 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 t 273 \Vhen 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.