294 THE BAROMETEE. 



forces and large factories in which fires are extensively used, it is found neces- 

 sary to command a constant and unremitting stream of air, which may be con- 

 ducted through the fuel so as to keep it in vivid combustion. This is effected 

 by bellows with three boards, the centre board being fixed and furnished with 

 a valve opening upward, the lower board being moveable with a valve also open- 

 ing upward, and the upper board being under a continual pressure by weights 

 acting upon it. When the lower board is let down, so that the chamber be- 

 tween it and the middle board is enlarged, the air included between these 

 boards being rarefied, the external pressure of the atmosphere will open the 

 valve in the lower board, and the chamber between the lower and middle boards 

 will be filled with air in its common state. The lower board is now raised by 

 the power which works the bellows, and the air between it and the middle 

 board is condensed. It cannot escape through the lower valve, because it 

 opens upward. It acts, therefore, with a pressure proportional to the working 

 power on the valve in the middle board, and it forces open this valve, which 

 opens upward. The air is driven from between the lower and middle boards 

 into the chamber between the middle and upper boards. It cannot return from 

 this chamber, because the valve in the middle board opens upward. The up- 

 per board being loaded with weights, it will be condensed while included in 

 this chamber, and will issue from the nozzle with a force proportionate to the 

 weights. While the air is thus rushing from the nozzle the lower board is let 

 down and again drawn up, and a fresh supply of air is brought into the cham- 

 ber between the upper and middle board. This air is introduced between the 

 middle and upper boards before the former supply has been exhausted, and by 

 working the bellows with sufficient speed, a large quantity of air will be col- 

 lected in the upper chamber, so that the weights on the upper board will force 

 a continual stream of air through the nozzle. 



The effect produced by a vent-peg in a cask of liquid depends on the atmo- 

 spheric pressure. If the vent-peg stop the hole in the top while the liquid is 

 discharged by the cock below, a space will remain at the top of the barrel in 

 which the air originally confined is allowed to expand and become rarefied ; 

 its pressure on the surface of the liquid above will, therefore, be less than the 

 atmospheric pressure resisting the escape of the liquid at the cock ; but still 

 the weight of the liquid itself, pressing downward toward the cock, will cause 

 the discharge to continue until the rarefaction of the air becomes so great, that 

 the excess of the atmospheric pressure is more than sufficient to resist the es- 

 cape of the liquid ; the flow from the cock will therefore be stopped. If the 

 vent-peg be now removed from the hole, air will be heard to rush in with con- 

 siderable force and fill the space above the liquid. The atmospheric pressure 

 on the surface above and on the mouth of the cock being now equal, the liquid 

 will escape from the cock by the effect of the pressure of the superior column, 

 according to the principles established in hydrostatics. If the vent-plug be 

 again placed in the hole, the flow from the cock will be gradually diminished, 

 and will at length cease. Upon the removal of the vent-peg, the same effect 

 will be observed as before. 



If the lid of a teapot be perfectly close, and fit the mouth air tight, or if the 

 interstices, as frequently happens, be stopped by the liquid which lies round 

 the edge of the mouth, then all communication between the surface of the li- 

 quid in the vessel and the external air is cut off. If we now attempt to pour 

 liquid from the teapot it will flow at first, but will immediately cease. In this 

 case the air under the lid becomes rarefied, and the pressure on the surface of 

 the liquid in the teapot is so far diminished, that the atmospheric pressure re- 

 sists its discharge at the spout. 



To remedy this inconvenience, it is usual to make a small hole somewhere 



