284 EXPERIMENTS WITH THE AIR-PUMP 



is, the absolute weight of the glass sphere is reallj 

 greater. The loss of weight caused by the presence 

 air must cease when the air is removed; hence, whei 



FIG. 189 ( real size). 



the apparatus is placed under the receiver of an air- 

 pump, and the air is withdrawn, the glass sphere 

 descends; if the air is let in again, the apparent equi- 

 librium is restored. 



If the end of the beam which carries the brass sphere forms 

 screw, so that the brass sphere can be moved further from the fulcrum 

 it is possible to adjust the apparatus so as to be in equilibrium ii 

 vacuo, but in air the brass sphere will now descend. 



The action of the siphon depends on the pressim 

 of the atmosphere ; hence the flow of a liquid througl 

 a siphon must cease, if the air is withdrawn. It i? 

 difficult to demonstrate this fact, if a light liquid sue! 

 as water be used ; for the pressure of the atmospher<| 

 is capable of supporting a column of water 10 m high 1 

 and the pressure would have to be reduced to one-hun 

 dredth in order to stop the flow of water through s 

 siphon 10 cm high; before that degree of rarefaction i; 



