THE STEAM-ENGINE. 



431 



condenser, the steam rushing into it will be partially condensed by the cold 

 surfaces to which it will be exposed ; but if the boiler supply it through the 

 pipe S in sufficient abundance, it will rush with violence through the cylinder 

 and all the passages, and its pressure in the condenser D, combined with that 

 of the heated air with which it is mixed, will open the valve M, and it will 

 rush through, mixed with the air, into the air-pump barrel N. It will press the 

 valves in the air-pump piston upward, and, opening them, will rush through, 

 and will collect in the air-pump barrel above the piston. It will then, by its 

 pressure, open the valve K, and will escape into the cistern B. 



Throughout this process, the steam which mixed with the air fills the cylin- 

 der, condenser, and air-pumps, will be only partially condensed in the last two, 

 and it will escape, mixed with the air, through the valve K ; and this process 

 will continue until all the atmospheric air which at first filled the cylinder, 

 tubes, condenser, and air-pump barrel, shall be expelled through the valve K, 

 and these various spaces shall be filled with pure steam. When that has hap- 

 pened, let us suppose all the valves closed. In closing the valve I, the flow 

 of steam to the condenser will be stopped, and the steam contained in it will 

 speedily be condensed by the cold surface of the condenser, so that a vacuum 

 will be produced in the condenser, the condensed steam falling in the form of 

 water to the bottom. In like manner, and for like reasons, a vacuum will be 

 produced in the air-pump. The valve M, and the valves in the air-pump pis- 

 ton, will be closed by their own weight. 



By this process, which is called blowing through, the atmospheric air, and 

 other permanent gases, which filled the cylinder, tubes, condenser, and air- 

 pump, are expelled, and these spaces will be a vacuum. The engine is then 

 prepared to be started, which is effected in the following manner : The upper 

 steam-valve G is opened, and steam allowed to flow from the boiler through 

 the passage leading to the top of the cylinder. This steam cannot pass to the 

 bottom of the cylinder, since the lower steam-valve H is closed. The space 

 in the cylinder below the piston being therefore a vacuum, and the steam press- 

 ing above it, the piston will be pressed downward with a corresponding force. 

 When it has arrived at the bottom of the cylinder, the steam-valve G must be 

 closed, and at the same time the valve H opened. The valve I leading to the 

 condenser being also closed, the steam which fills the cylinder above the pis- 

 ton is now admitted to circulate through the open valve H below the piston, so 

 that the piston is pressed equally upward and downward by steam, and there 

 is no force to resist its movement, save its friction with the cylinder. The 

 weight of the pump-rods on the opposite end of the beam being more than 

 equivalent to overcome this, the piston is drawn to the top of the cylinder, and 

 pushes before it the steam which is drawn through the tube T, and the open 

 valve H, and passes into the cylinder C below the piston. 



When the piston has thus arrived once more at the top of the cylinder, let 

 the valve H be closed, and at the same time the valves G and I opened, and 

 the condensing-cock E also opened, so as to admit the jet to play in the con- 

 denser. The steam which fills the cylinder C below the piston, will now rush 

 through the open valve I into the condenser which has been hitherto a vacuum, 

 and there encountering the jet, will be instantly converted into water, and a 

 mixture of condensed steam and injected water will collect in the bottom of the 

 condenser. At the same time, the steam proceeding from the boiler by the 

 steam-pipe S to the upper steam-box A, will pass through the open steam-valve 

 G to the top of the piston, but cannot pass below it because of the lower 

 steam-valve H being closed. The piston, thus acted upon above by the pres- 

 sure of the steam, and the space in the cylinder below it being a vacuum, its 

 downward motion is resisted by no force but the friction, and it is therefore 



