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2. The true nature of sound can only be discovered by 

 observation. We must know something about what we 

 have to explain. The experimental facts divide themselves 

 into three species. First, those relating to the manner in 

 which the sounding body makes the sound. Secondly, 

 those relating to the manner in which the sound when 

 formed is conveyed to the ear. Thirdly, those relating to 

 the manner in which, when the sound is thus conveyed, 

 we hear it. In regard to the first, it is universally true 

 that all sounding bodies are tremulous bodies. All vi 

 brating bodies, however, do not give sound. In regard to 

 the second, it is observed, (1.) that sound cannot be con 

 veyed through a vacuum. Hanksbee suspended a bell in 

 the receiver of an air-pump ; the sound died away gradually 

 as the air was removed (Phil. Trans. 1705.) (2.) Sound 

 is not instantaneously conveyed ; the report of a gun is not 

 heard until after the flash has been seen. But all sounds 

 travel with equal velocity. Thus the various notes of any 

 piece of music played at a distance reach us in perfect 

 order. The velocity is found to vary slightly with the 

 temperature, and at 62 Fahrenheit travels at the rate of 

 1125 feet per second. (3.) Sound, unlike light, does not 

 travel necessarily in a straight line, but on entering through 

 an orifice, as a window, spreads out in all directions, but, 

 nevertheless, it is heard with greatest distinctness in front of 

 the orifice. Thus the sound of a carriage can be heard 

 round a corner, but a change in the loudness is perceptible 

 the moment when the carriage goes behind the intervening 

 walls. (4.) Sound can be conveyed by solid bodies, and 

 other fluids besides air, but in passing from one to another 

 a considerable portion of its intensity is lost. (5.) The 

 intensity of sound diminishes as the distance from the 

 sounding body increases. But when sound travels along a 

 tube, as a speaking pipe, the gradual diminution of the sound 



