NATURE OF SOUND. 273 



against the bell. Readmitting the air, and again shaking the globe, 

 the sound is plainly heard. (See Fig. 215.) 



(6.) A small music box, or a clock-work arrangement for striking 

 a bell (Fig. 216), may be supported upon a thick cushion of felt or 

 cotton-batting, and placed under the capped receiver of an air-pump. 

 When the receiver is exhausted, and the machinery started by the 

 rod g, the motion may be seen but hardly any sound will be heard. 

 If the support were perfectly inelastic and the exhaustion complete, 

 no sound would be audible. The experiment may be made more 

 perfect by filling the exhausted receiver with hydrogen and again 

 exhausting the gas. 



425. Velocity of Sound ill Air. It is a familiau 

 fact that the transmission of sound is not instantaneous. 

 The blow of a hammer is often seen several seconds before 

 the consequent sound is heard ; steam escaping from the 

 whistle of a distant locomotive becomes visible before the 

 shrill scream is audible; the lightning precedes the thunder. 

 As we shall see further on, the time required for the 

 propagation of light through terrestrial distances is inap- 

 preciable. Hence the interval between the two sensations 

 of seeing and hearing is required for the transmission of 

 the sound. This interval being observed and the distance 

 being known, the velocity is easily computed. By such 

 means it has been found that the velocity of sound in 

 air at the freezing temperature is about 332 m., or 

 1090 ft. per second. There is some reason for believing 

 that very loud sounds travel somewhat more rapidly than 

 sounds of ordinary loud ness. With this exception it may 

 be said that, in a given medium, all sounds travel with the 

 same velocity. 



426. Velocity in Other Media. The velocity 

 of sound depends upon two considerations the 

 elasticity and the density of the medium. It varies 

 directly as the square root of the elasticity, and 



