PNEUMATICS. 



17? 



construction is the same as Fio. 166. 



that of the exhausting sy- 

 ringe, the piston being work- 

 ed by a lever or handle, as in 

 the common pump, the valves 

 opening and closing with 

 great nicety and perfection. 



._,T, , . ,. 381. When 



VThat 18 the 



construction of the density 



the condensing ^f ^^Q ^^j^ jg 



•ynnge? 



required to 



bo increased, the condensing 

 syringe, the converse of the 

 exhausting syringe, is em- 

 ployed. It consists merely 

 of an exhausting syringe, or 

 air-pump, reversed, its valves 

 being so arranged as to force 

 air into a chamber, instead of 

 drawing it out. For this 

 purpose, the valves open 

 inward in respect to the interic of the cylinder, while in the exhausting 

 syringe and air-pump, they open o'ltward. 



382. That the air in the inside of 



vessels is the force which resists and 



counterbalances the great pressure 



of the external atmosphere, may be 



proved by the follo's^'ing experiment : 

 A strong glass vessel. Fig. 167, is provided, open 

 both at top and bottom, and having a diameter of 

 four or five inches. Upon one end is tied a bladder, 

 so as to be completely air-tight, while the other end is 

 placed upon the plate of an air-pump. Upon exhaust- 

 ing the air irom beneath tho bladder, it will be forced 

 inward by the pressure of the air outside, and when the 

 exhaustion has been carried to such an extent that the 

 Btrengtli of the bladder is less than this pressure, it vdM 

 burst with a loud report. 



What is the ^^^- '^^® air-pump was invented, in 

 experiment of the year 1654, by Otto Guericke, a Ger- 



ilmisphercsT ™'^°' ''^"^ ^* "^ ^''^''^t P^b"« exhibition of 

 its powers, made in the presence of the 

 emperor of Germany, the celebrated experiment known 

 as the " Magdeburg Hemispheres," was first shown. The 

 Magdeburg Hemispheres, so called from tho city where 

 Guericke resided, consist of two hollow hemispheres of 



8* 



What is an er- 

 perimental 

 proof of the 

 crushing force 

 of the atmos- 

 phere ? 



Fig. 167. 



Fig. 168. 



