BELLOWS-PNEUMATIC INK-BOTTLE. 



than a double bellows, one forcing air into the chamber of the 

 other, and the second being urged by an uninterrupted pressure 

 produced usually by a weight suspended from the upper board. 



21. The effect produced by a vent-peg in a cask of liquid is 

 explained by the atmospheric pressure. The cask being air- 

 tight, so long as the vent-peg is maintained in its position, the 

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

 atmospheric pressure, and it can only floAV from the cock in 

 .virtue of its own weight. If the weight of the atmosphere be 

 greater than the weight of a column of the liquid, corresponding 

 with the depth of the liquid in the vessel, the liquid cannot flow 

 from the cask; but the moment the vent-peg is removed, the 

 -atmospheric pressure being admitted above the level of the liquid in 

 the cask, the liquid flows from the cock in virtue of its own weight. 



If the lid of a teapot or kettle were perfectly close, the liquid 

 would not flow from the pipe, because the atmospheric pressure 

 would be excluded from the inner surface. A small hole is there- 

 fore usually made in the lid to admit the air and allow the liquid 

 to flow freely. 



22. Ink-bottles are sometimes so constructed as to prevent the 

 inconvenience of the ink thickening and drying. Such a bottle 

 is represented in fig. 4 : A B is a close glass 

 vessel, from the bottom of which a short 

 tube B c proceeds, the depth of which is 

 sufficient for the immersion of a pen. When 

 . ink is poured in at c, the bottle being placed 

 in an inclined position, is gradually filled 

 up to the knob A. If the bottle be now placed 

 in the position represented in the figure, the 

 chambery A B being filled with the liquid, 

 the air will be excluded from it, and the 

 pressure tending to force the ink upwards in 

 the short tube c, will be equal to the weight of the column 

 of ink, the height of which is equal to the depth of the ink 

 in the bottle A B, and the bore of which is equal to the section 

 of the tube c. The ink will be prevented from rising in the tube c 

 by the atmospheric pressure, which is much greater than the 

 pressure of the column of liquid in the bottle. As the ink in the 

 short tube c is consumed by use, its surface will gradually 

 fall to a level with the horizontal tube B, a small bubble of air 

 will then insinuate itself through B, and will rise to the top 

 of the bottle A B, where it will exert an elastic pressure, which 

 will cause the surface of the ink in c to rise a little higher ; and 

 this effect will be continually repeated, until all the ink in the 

 bottle has been used. 



107 



