VELOCITY OF SOUND. 



been briefly noticed by way of illustration. The most extensive 

 and accurate system of experiments which have been made with 

 this object, were those made at Paris by the Board of Longitude 

 in the year 1822. The sounding bodies used on this occasion, 

 were pieces of artillery charged with from two to three pounds of 

 powder, which were placed at Villejuif and Montlhery. The ex- 

 periments were made at midnight, in order that the flash might 

 be more easily and accurately noticed. They were conducted by 

 MM. Prony, Arago, Mathieu, Humboldt, Gay Lussac, and Bouvard. 

 The result of these experiments was, that when the barometer was 

 at 29 -8 inches, and the thermometer at 61, the velocity of sound 

 was 1118*4 feet per second. 



According to the theory of Laplace, the velocity of sound in- 

 creases at the rate of I'll feet per second for every degree in the 

 rise, and Decreases at the same rate for each degree in the fall of 

 the thermometer. Hence it appears that the velocity of sound at 32 

 is 1086-2 feet per second. For all practical purposes, it is sufficiently 

 exact to take 1120 feet as the velocity of sound at 62, and allow 

 thirteen inches for every variation of a degree in temperature. 



11. The production of sound is in many cases attended with the 

 evolution of light, as, for example, in fire-arms and explosions 

 generally, and in the case of atmospheric electricity. In these 

 cases, by noting the interval between the flash and the report, 

 and multiplying the number of seconds in each interval by the 

 number of feet per second in the velocity of sound, the distance 

 can be ascertained with great precision. Thus, if a flash of lightning 

 be seen ten seconds before the thunder which attends it is heard, 

 and the atmosphere be in such condition that the velocity of sound 

 is 1120 feet per second, it is evident that the distance of the 

 cloud in which the electricity is evolved must be 11200 feet. 



12. The same sounding body will produce a louder or lower 

 sound, according as the density of the air which surrounds it is 

 increased or diminished. In the experiment already explained, 

 in which the alarum was placed under an exhausted receiver, the 

 sound increased in loudness as more and more air was admitted 

 within the receiver. If the alarum had been placed under a 

 condenser, and highly compressed air collected round it, the 

 sound would be still further increased. 



When persons descend to any considerable depth in a diving- 

 bell, the atmosphere around them is compressed by the weight of 

 the column of water above them. In such circumstances a 

 whisper is almost as loud as the common voice in the open air, 

 and when one speaks with the ordinary force it produces an effect 

 so loud as to be painful. 



On the summit of lofty mountains, where the barometric column 



189 



