22t HISTORY OF 



to a wrong cause ; namely, the operation of the light ; but the 

 real tact is, that the warmth of the sun-beams lessens and dissi- 

 pates the body of the air that goes to feed the flame ; and the 

 tire, of consequence, languishes for want of a necessary supply. 



Tlie air, while it thus kindles fire into flame, is, notwithstand- 

 ing, found to moderate the rays of light, to dissipate their vio- 

 lence, and to spread an uniform lustre over every object. Were 

 the beams of tlie sun to dart directly upon iis, without passing 

 through this protecting medium, they would either burn us up 

 at once, or blind us with their effulgence. But by going through 

 the air, they are reflected, refracted, and turned from their direct 

 course, a thousand different ways j and thus are more evenly 

 diffused over the face of nature. 



Among the other necessary benefits the air is of to us, one 

 of the principal is, its conveyance of sound. Even the vibra- 

 tions of a bell, which have the loudest effect that we know of, 

 cease to be heard when imder the receiver of an air-pump. 

 Thus all the pleasures we receive from conversation with each 

 other, or from music, depend entirely upon the air.* 



* Every sound is rendered stronger (ir M'caker, and may be lieard at a 

 greater or less distwnce, according to the density or rarity of that elastic 

 fluid by which it is propagated. According to Mr Hawksbee, who has made 

 deep researches into this branch of pliilosop'hy, when air lias acquired twice 

 its common density, it transmits sound twice as far as common air ; whenro 

 he reasonably concludes, that sound increases, not only in direct proportion 

 to the density of the air, but in proportion to the square of this density. If 

 sound was p'opagati d in an elastic fluid more dense than the air, it would 

 be carried proportionahly farther. M. Brisson has proved thi*. by putting n 

 sonorous boJy mto carbonic acid gas, the density of which is about one-third 

 more than that of atmospheric air ; the consequence was, that at that time, 

 and in that situation, the sou.id was very considerably increased. For this 

 reason the diyness of the air, %vhich increases its density, lias a considerable 

 ctl'ect in rendering sound hmder and more audible. Sound is also much in- 

 creased by the reverberation of the pulses of the air from those surrounding 

 bodies against which they strike ; whence it happens, music is so much loud. 

 er in a close apartment than in the open air. 



A knowledge of the progression of soun^ is not a subject of mere sterile cit- 

 riosity, but is in several instances useful ; for by this we are enabled to deter, 

 mine the distance of chips or other moving bodies. Suppose, for example, a 

 vessel fires a gun, the sound of which is heard five seconds after the flash is 

 seen; as sound moves 1142 English feet in one second, this number multi- 

 plied by five, gives the distance of 6710 feet. The same principle has been 

 already mentioned as applicable in storms of lightning and thunder. 



The waves or pulses of sound being reflcxible in tlieir course, «heu they 



