PNEUMATICS 



the barrel and the compressed air, by 

 opening: the valve by proper mechanism 

 provided for that purpose. A bullet 

 being placed in the barrel, and the valve 

 opened, the condensed air will press it 

 forward, and this pressure will continue 

 until the bullet leaves the mouth of the 

 barrel. 



The best construction of the air-gun is 

 Martin's. It has a lock, stock, barrel, 

 ramrod, &c., similar to a common fowl- 

 ing-piece. The magazine for condensed 

 air is a strong hollow copper ball, in 

 which air is condensed by a syringe. If 

 the air be highly condensed, a ball will 

 be projected by this instrument to the 

 distance of sixty or seventy yards. A 

 number of balls may be discharged in 

 rapid succession, without requiring any 

 further condensation in the magazine. 



CHAPTER VII. On Sounds. 



(53.) Sound is the sensation pro- 

 duced in the mind, when the organs of 

 hearing are affected by peculiar motions, 

 transmitted to them through the medium 

 of the ail* or other bodies. 



To enter into any details on the theory 

 of sound, would require a much more 

 extended discussion than would be con- 

 sistent with the limits which our plan 

 necessarily prescribes to the present 

 Treatise. We shall, therefore, in this 

 chapter, confine ourselves to the state- 

 ment and explanation of a few of the 

 most important properties connected 

 with the propagation of sound. 



When an elastic body is struck, it ac- 

 quires a tremulous or vibratory motion ; 

 this motion is communicated to the air 

 which surrounds the body, and produces 

 in it corresponding undulations, by 

 which, the ear being~aifected, the sensa- 

 tion of sound is produced. The air being 

 thus the most usual medium by which 

 we receive the sensation of sound, this 

 pail of physical science has been gene- 

 rally considered as a branch of pneu- 

 matics, but under the separate name of 

 Acoustics. 



(54.) That it is the air surrounding the 

 sonorous body which transmits the 

 sound to the ear may easily be proved. 



Let a small bell be suspended in the 

 moveable receiver of an air-pump. Be- 

 fore the process of rarefaction com- 

 mences let the receiver be shaken, so 

 that the bell may ring, and the sound will 

 be distinctly heard. As the rarefaction 

 proceeds, the sound of the bell will be 

 gradually weakened, and the process 



may be continued until it become per- 

 fectly inaudible. Upon allowing the au- 

 to return gradually into the receiver, the 

 bell will become gradually louder, until as 

 much air be admitted as was withdrawn. 



Air, however, although the most 

 usual, is neither the only nor the best 

 conductor of sound. Other elastic fluids, 

 as vapours and gasses, have this pro- 

 perty in common with air, as may be 

 proved by introducing them into the 

 exhausted receiver in place of the air 

 which has been withdrawn from it. In- 

 elastic fluids or liquids also conduct 

 sound. If two stones be struck together 

 under water the sound will be heard, the 

 ear being placed under the same water. 

 Solid bodies also conduct sound. If a 

 beam of wood, of considerable length, be 

 struck at one end, the sound will be 

 audible to an ear placed close to it at the 

 other end, although the same sound 

 would be perfectly inaudible to an ear at 

 the same distance, in any other direc- 

 tion, from the striking body. 



(55.) The propagation of sound is not 

 instantaneous ; that is to say, the sensa- 

 tion is not produced at the same instant 

 as the motion in the sonorous body 

 which causes it. If a gun or piece of 

 ordnance be discharged at a consider 

 able distance, the flash will be first seen, 

 and after a considerable interval has 

 elapsed, the explosion will be heard. In 

 like manner, lightning always precedes 

 thunder by an interval of some seconds. 

 It thus app.ears, that sound is propa- 

 gated through the ah' with a certain 

 velocity ; and to determine experiment- 

 ally this velocity has been considered an 

 interesting physical problem. 



By a comparison of the most accu- 

 rate experiments which have been made 

 on the subject, we may conclude that 

 the atmosphere, in its ordinary state, 

 conducts sound at the rate of 1130 

 feet per second. The velocity is sub- 

 ject to some slight variation, owing 

 to the change of temperature, the mois- 

 ture suspended in the air, and other 

 causes ; but 1130 feet may be taken as 

 an average rate. This rate also supposes 

 the atmosphere to be perfectly calm. If 

 there be a wind, its velocity must be 

 added to the velocity already mentioned, 

 when it blows from the sounding body 

 to the ear ; and subtracted from it when 

 it blows in a contrary direction. 



Different bodies conduct sound with 

 different velocities. A beautiful expe- 

 riment was lately instituted at Paris, to 

 illustrate this fact, by Biot. At Uie ex- 



