236 PHYSIOLOGY. 



being reflected from cloud to cloud, and through strata of 

 air of different densities, though it also arises from a dis- 

 charge of electricity, through a wide extent of air. In this 

 case, as the sound from the point nearest the hearer reaches 

 his ear first, and some moments elapse before that from the 

 more distant arrives, there must consequently be a continued 

 peal. Some of our large public houses and manufactories 

 are fitted with pipes for conveying intelligence to distant 

 apartments, attention being attracted by ringing a bell. 



20. 1 It has been doubted whether sound can be propagated 

 from one medium to another, as from air to water. It is 

 now known, however, that if a musket is discharged over a 

 person who is under water, he will hear the report. The 

 question has also arisen, whether sound can be propagated 

 from water to the air again. This is easily proved by strik- 

 ing two stones together under water, although we are told 

 that persons in a diving-bell under water, could not hear a 

 musket discharged immediately over it. But here the sound 

 had to be communicated from jthe air to the water, and from 

 the water to the air again, itound travels at the rate of 

 eleven hundred and forty-two feet in a second, or a mile in 

 four seconds. As light travels much faster, we see the flash 

 of a gun before we hear the report. This will enable us to 

 tell in a thunder storm how far we are from a thunder-cloud ; 

 as we have only to allow eleven hundred feet for each se- 

 cond, between the time when the flash is seen and the report 

 heard, and one beat of the pulse for a second. In this way, 

 too, the distance of a ship of war at sea is often ascertained 

 by those on board of the vessel she is in pursuit of. Solids 

 and fluids convey sound not only more perfectly, but also 

 more rapidly than air. It is found that the velocity of 

 sound in water, is about four thousand nine hundred feet in 

 a second, being between four and five times more rapid than 

 it is through air. Sound passes through tin at the rate of 

 eight thousand one hundred and seventy-five feet, and through 

 iron, glass and wood, eighteen thousand five hundred and 



