PNEUMATICS. 39 



derably rarefied the receiver will remain so fixed by 

 means of the downward pressure of the atmosphere as 

 to render it almost impossible to be removed without re- 

 admitting the air. The downward pressure is also shown 

 by putting a piece of wet bladder over the top of an open 

 receiver, and on exhausting the receiver, the bladder will 

 be pressed violently down so as ultimately to burst. 

 If a thin phial hermetically sealed be placed within the 

 receiver, and the air exhausted from the receiver, the air 

 within the phial, by means of its expansive force, will 

 burst the phial. If a cup of water, just below the boil- 

 ing point, be placed within the receiver,and the air rare- 

 fied, it will begin to boil, and the violence of the ebulli- 

 tion will be in proportion to the rarefaction of the air. 

 On this principle it is that water will boil sooner on the 

 tops of high mountains, than it will in the valleys ; as 

 also that it will take longer to boil at the bottom of a 

 deep mine, than on the earth's surface. The mean boil- 

 ing point at the surface of the earth is 21 C 2 degrees ; of 

 Fahrenheit ; on the top of a mountain, where the weight 

 of the air is diminished one-tenth, it is 207 degrees, while 

 at the bottom of a deep mine it is 1 Z14 degrees. The 

 weight of bodies is found by the air-pump not to depend 

 on the nature of the body, but on the resistance of the 

 air ; thus a piece of metal and a feather will fall with 

 equal rapidity in an exhausted receiver. 



There are various other phenomena exhibited by means 

 of the air-pump, which it is impossible here to particu- 

 larize fully. Thus in an exhausted receiver a bell will not 

 sound, a siphon will not run, vegetation stops, and ani- 

 mals in general in a very short time die. The glow-worm 

 loses its light in proportion as the air is exhausted ; gun- 

 powderand sulphurwill not burn; smoke descends instead 

 of ascending, together with many other interesting results. 



The AiR-Gux is an instrument for propelling bullets 

 by condensed air. Air-guns are constructed in various 

 ways ; the more common one is made like a fowling-piece, 

 having under the lock a round steel tube with a small 

 moveable pin in the inside, which is pushed out when the 

 trigger is pulled ; to this tube a hollow copper ball, hav- 

 ing a suitable valve, is made to screw, being perfectly 



