414 



THE STEAM-ENGINE. 



which leads to the feeding cistern L, into which the condensed steam is driven. 

 That the piston should continue to be air-tight, it was necessary to keep a 

 constant supply of water over it ; this was done by a cock similar to M, which 

 allowed water to flow from the pipe M on the piston. 



Soon after the first construction of these engines, an accidental circumstance 

 suggested to Newcomen a much better method of condensation than the applica- 

 tion of cold water on the external surface of the cylinder. An engine was ob- 

 served to work several strokes with unusual rapidity, and without the regular 

 supply of the condensing water. Upon examining the piston, a hole was found 

 in it, through which the water, which was poured on to keep it air-tight, flowed, 

 and instantly condensed the steam under it. 



On this suggestion Newcomen abandoned the external cylinder, and intro- 

 duced a pipe H, furnished with a cock Q, into the bottom of the cylinder, so 

 that, on turning the cock, the pressure of the water in the pipe H, from the 

 level of the water in the cistern N, would force the water to rise as a jet into 

 the cylinder, and would instantly condense the steam. This method of con- 

 densing by injection formed a very important improvement in the engine, and 

 is still used. 



Having taking a general view of the parts of the atmospheric engine, let us 

 now consider more particularly its operation. 



When the engine is not working, the weight of the pump-rod D (fig. 4) 

 draws down the beam A, and draws the piston to the top of the cylinder, 

 where it rests. Let us suppose all the cocks and valves closed, and the 

 boiler filled to the proper depth. The fire being lighted beneath it, the water 

 is boiled until the steam acquires sufficient force to lift the valve V. When 

 this takes place, the engine may be started. For this purpose the regulating 

 valve R is opened. The steam rushes in, and is first condensed by the cold 

 cylinder. After a short time the cylinder acquires the temperature of the 

 steam, which then ceases to be condensed, and mixes with the air which filled 

 the cylinder. The steam and heated air, having a greater force than the 

 atmospheric pressure, will open a valve placed at the end X of a small tube in 

 the bottom of the cylinder, and which opens outward. From this (which is 

 called the blowing valve*) the steam and air rush in a constant stream, until all 

 the air has been expelled, and the cylinder is filled with the pure vapor of 

 water. This process is called blowing the engine preparatory to starting it. 



When it is about to be started, the engine-man closes the regulator R, and 

 thereby suspends the supply of steam from the boiler. At the same time he 

 opens the condensing valve H ;t and thereby throws up a jet of cold water into 

 the cylinder. This immediately condenses the steam contained in the cylinder, 

 and produces the vacuum. (The atmosphere cannot enter the blowing valve, 

 because it opens outward, so that no air can enter to vitiate the vacuum.) 

 The atmospheric pressure above the piston now takes effect, and forces it down 

 in the cylinder. The descent being completed, the engine-man closes the 

 condensing valve H, and opens the regulator R. By this means he stops the 

 play of the jet within the cylinder, and admits the steam from the boiler. The 

 first effect of the steam is to expel the condensing water and condensed steam 

 which are collected in the bottom of the cylinder, through the tube Y, contain- 

 ing a valve which opens outward (called the eduction valve), which leads to the 

 hot cistern L, into which this water is therefore discharged. 



When the steam admitted through R ceases to be condensed, it balances 

 the atmospheric pressure above the piston, and thus permits it to be drawn to 



* Also called the snifting valve, from the peculiar noise made by the air and steam escaping 

 from it. 

 t Also called the injection valve. 



