SOUND. 115 



rate at which sound travels, by observing the number of 

 seconds which intervene between the flash and the report, 

 we can always determine the distance. 



FREDERICK. 



At what rate is sound conducted 1 



MRS. F. 



Eleven hundred feet a second may be estimated as a fair 

 average, but this rate supposes the atmosphere to be perfectly 

 calm. If there be wind, the velocity of the wind must be 

 added to the velocity already mentioned, if the wind blows 

 from the sounding body to the ear; or must be subtracted, if it 

 blows in a contrary direction. 



But, to return to meteors. One which appeared five years 

 after that which I have just mentioned, emitted much louder 

 sounds, which in Devonshire and Cornwall resembled that 

 of a cannon; and the air experienced so violent a concussion, 

 as to shake the windows and doors, and even to throw a 

 looking-glass out of its frame and break it; and these effects 

 were the result of an explosion which took place sixty-seven 

 miles above the earth. With respect to the point which gave 

 rise to this conversation, what prevented Mary from hearing 

 Henrietta call her to-day, although, at another time, she heard 

 her, at a greater distance, perfectly well? The reason is this; 

 that sound, as light, is but imperfectly transmitted through 

 mixed media. Fog, falling rain, or snow, all therefore ob- 

 struct its progress; for sound moves in different velocities in 

 different bodies. When the medium through which sound or 

 light passes is of the same density, the sound or light will be 

 transmitted with the least loss and the greatest distinctness; 

 but, if the medium has different densities, or consists of dif- 

 ferent bodies imperfectly mixed, or is interrupted by empty 

 spaces, the light or sound will be either greatly diminished 

 or entirely destroyed. This effect, in the case of light, may 

 be seen if we look through a piece of cracked glass; or if we 

 add syrup to water in a glass, and, before they have quite 

 combined, hold up the glass to a candle, the candle will 



