PNEUMATICS. 



179 



a preponderance is allowef^. lo it as is sufficient to give the gas contained in 

 it the compression necessary to drive it through the pipes to the remotest part 

 of the district to be illuminated. 



Fig. 171. 



Why will not 

 a liquid flow 

 from a tight 

 cask with only 

 one opening? 



385. A liquid will not flow continuously from a tight cask 

 after it has been tapped or pierced, unless another opening 

 is made as a vent-hole, in the upper part of the cask. The 

 cask being air-tight, with the exception of a single opening 

 the surface of the liquid in the vessel will be excluded from 

 the atmospheric pressure,- and it can only flow out in virtue of its own 

 weight But if the weight of the liquid be less than the force of the air press- 

 ing upon the mouth of the opening, the liquid can not flow from the cask ; the 

 moment, however, that the air is enabled to act through the vent-hole in the 

 upper part of the cask, the pressure below is counterbalanced, and the hquid 

 descends and runs freely tlirough the opening by its own weight,. 



If the lid of a tea-pot or kettle be air-tight, the liquid wUl not flow freely 

 from the spout, on account of the atmospheric pressure. This is remedied hj 

 making a small hole in the Ud, which allows the air to enter from without. 

 The Pneumatic Ink-stand, de- 

 signed to prevent tlie ink from 

 thickening, by the exposure of a 

 small surface only to the air, is 

 constructed upon the principles 

 of atmospheric pressure. It consists of a close 

 glass vessel, represented in Fig. 172, from the 

 bottom of wliich a short tube proceeeds, the 

 depth of which is sufficient for the immersion 

 of the pen. By filling the ink-stand in an inclined position, we exclude the 



What is the 

 principle and 

 construction of 

 the Pneumatic 

 Ink-Etand ? 



Fig. 172. 



