THE BAEOSCOPE 283 



(as indicated by the lines in the diagram) when it is placed upon 

 the top of the cylinder, into which it may be fixed with sealing-wax. 



The fact that air possesses weight may be demon- 

 strated in a more convenient manner than that em- 

 ployed in art. 24, by means of a flask of 1 or 2 litres 

 capacity, provided with a collar of brass and a stop- 

 cock capable of being screwed to the pump. When 

 the flask is exhausted, the stop-cock is closed, the 

 flask removed from the plate, suspended to one arm 

 of the balance, and counterpoised ; the stop-cock is now 

 opened very little, so that the air enters but slowly : 

 as the flask is getting filled with air it descends, in 

 consequence of the weight of the latter. 



The flask used for the experiment in art. 24 (page 220) may be 

 employed, if the student does not possess a globular flask with brass 

 collar; but for this experiment no water is required. The pinch- 

 cock is first pushed back over the india- rubber until it clamps the 

 glass tube ; the tube a of the pump is then slightly greased with 

 tallow, and the india-rubber tube is drawn over it until the end of 

 the tube a is in contact with the glass tube. After carefully ex- 

 hausting the flask the flexible tube is cautiously pushed back from 

 a until there is a sufficient space between the ends of both tubes to 

 place the pinch-cock between them. When this is done, detach 

 the flask with the flexible tube from the pump, suspend it to the 

 balance, and proceed as before. 



The fact that bodies lose weight in air may be de- 

 monstrated by means of an apparatus called the baro- 

 scope , fig. 189. A small pillar supports the beam of 

 a balance ; to the opposite ends of the beam two 

 spheres are fixed, one of brass, the other of glass, 

 hollow and air-tight. Both spheres are adjusted so 

 as to be in equilibrium in air. But the volume of 

 air displaced by the sphere of glass is larger than that 

 displaced by the sphere of metal; the glass sphere loses 

 therefore more of its weight than the brass sphere that 



