296 THE BAROMETER. 



bubble of air will then insinuate itself through the tube r, and will rise to the ( 

 top of the bottle A B ; there it will exert an elastic pressure, which will cause 

 the surface in C to rise a little higher, and this effect will be continually re- ) 

 peated until all the ink in the bottle has been used. 



The only inconvenience which has been attributed to these ink-bottles arises f 

 from sudden changes in the temperature to which they are exposed. When ; 

 the external air, having been previously warm, becomes suddenly cold, the \ 

 small quantity of air which is included in the bottle A not being cooled so fast / 

 as the external air, will exert an elastic pressure which will cause the ink to I 

 flow at C. This is an effect, however, which we have never observed, al- 

 though we have seen these bottles much used. < 



If such an ink-bottle be placed upon a marble chimney-piece, or any other 

 surface heated beyond the temperature of the air in the room, the air confined 

 in the bottle will then become heated, and acquire increased elastic force, and 

 in this case the ink will overflow. 



The fountains for supplying water to bird-cages are constructed upon the 

 same principle. 



The pneumatic trough used in the chemical laboratories, and the gas-hold- 

 ers or gasometers used in gas works, depend on the atmospheric pressure. A 

 vessel having its mouth upward, is completely filled with a liquid. The mouth 

 is then stopped, a flat piece of glass, or a smooth plate of metal, pressed 

 against it, and the vessel is inverted, the mouth being plunged in a cistern 

 filled with the same liquid. If the height of the vessel in this case be less 

 than the height of the column of the liquid which the atmospheric pressure 

 would support, the vessel will continue to be completely filled with the liquid, 

 even after the plate is removed from its mouth ; for the atmospheric pressure, 

 acting on the surface of the liquid in the cistern, will prevent the liquid con- 

 tained in the vessel from falling out of it. Any one may satisfy himself of this 

 fact. Take a wine-glass and. fill it with water, and then, having applied a 

 piece of card to its mouth so as to prevent the water from escaping, invert it, 

 and plunge the mouth downward in a basin of water. Let the card be then 

 removed, and let the glass be raised above the surface, still, however, keeping 

 the edge of its mouth below the surface. It will be observed that the glass 

 will still remain completely filled with water. Take a small quill, or a hol- 

 low piece of straw, and insert one end in the water, so that it will be im- 

 mediately below the mouth of the glass, and at the same time blow gently 



through the other end, so as to introduce air in small quantities into the water 

 ) immediately under the mouth of the glass. This air will ascend in bubbles, 

 and will find its way to the highest part of the glass, and, remaining there, 

 will expel the water from it ; and this will continue so long as air is supplied, 

 until all the water contained in the glass is expelled from it, and the glass is 

 7 filled with air. If the process be further continued, the air will begin to 

 ( escape under the edge of the glass, and rise in bubbles to the surface. 

 '' The pneumatic trough is a large cistern filled with mercury, in which is 

 placed, below the surface of the liquid, a shelf to support a receiver. By 

 plunging any vessel in the deeper part of the trough, it may be filled with mer- 

 cury, and if it be slowly raised, keeping its mouth still below the surface of 

 the liquid, it will still remain filled with mercury by the pressure of the atmo- 

 sphere acting on the surface of the mercury in the trough. The mouth of the 

 vessel may then be placed on the shelf, while the vessel itself is above the 

 surface of the mercury. 



The trough is represented in fig. 9, at A B. The shelf is placed in it at C ; 

 a receiver, R, is placed on the shelf, with its mouth downward, over an aper- 

 ture, D, which communicates with a tube, by which gas may be introduced. 



