THE STEAM-ENGINE. 



471 



approaches the top of the cylinder, and its motion becomes slow, the working- 

 gear is made to open the lower exhausting-valve ; the steam enclosed in the 

 cylinder below the piston, and which has just driven the piston upward, pres- 

 ses with an elastic force of seventeen pounds per square inch on every part of 

 the interior of the cylinder, while the uncondensed Vapor in the condenser 

 presses with a force of about two pounds per square inch. The steam, there- 

 fore, will have a tendency to rush from the cylinder to the condenser through 

 the open exhausting-valve, with an excess of pressure amounting to fifteen 

 pounds per square inch, while the piston pauses at the top of the cylinder. 

 This process goes on, and when the piston has descended by the motion of 

 the fly-wheel, a sufficient distance from the top of the cylinder to call the mov- 

 ing force of the steam into action, the exhaustion will be complete, and the 

 pressure of the uncondensed vapor in the cylinder will become the same as in 

 the condenser. 



The pressure of steam in the cylinder, and of uncondensed vapor in the con- 

 denser, varies, within certain limits, in different engines, and therefore the 

 amount here assigned them must be taken merely as an example. 



The size of the valves by which the steam is allowed to pass from the cyl- 

 inder to the condenser should be such as to cause the condensation to take 

 place in a sufficiently short time, to be completed when the steam impelling 

 the piston is called into action. 



Watt, in the construction of his engines, made the exhaustion-valves with a 

 diameter which was one fifth of the diameter of the cylinder, and therefore the 

 actual magnitude of the aperture for the escape of the steam was one twenty- 

 fifth of the magnitude of the cylinder ; but the spindle of the valve diminished 

 this so that the available space for the escape of steam did not exceed one twenty- 

 seventh of the magnitude of the cylinder. This was found to produce a suffi- 

 ciently rapid condensation. 



It was usual to make the steam-valves of the same magnitude as the ex- 

 hausting-valves, but the flow of steam through the former was resisted by the 

 throttle-valve, while no obstruction was opposed to its passage through the 

 latter. 



The rapidity with which the cylinder must be exhausted by the condenser 

 will, however, depend upon the velocity with which the piston is moved in it. 

 The magnitude, therefore, of the exhausting-valves which would be sufficient 

 for an engine which acts with a slow motion would be too small where a rapid 

 motion is required. 



In the single-acting steam-engine, where the moving force always acted 

 downward on the piston, the pressure upon all the joints of the machinery by 

 which the force of the piston was conveyed to the working parts, always took 

 place in the same direction, and consequently whatever might be the mechani- 

 cal connexion by which the several joints were formed, the pins by which they 

 were connected, must always come to a bearing in their respective sockets, 

 however loosely they may have been fitted. For the same reason, however, 

 that the arch head and chain were abandoned as a means of connecting the 

 steam-piston with the beam, and the parallel motion substituted, it was also 

 necessary in the double-acting engine, where all joints whatever were driven 

 alternately in opposite directions, to fit the connecting-pins with the greatest 

 accuracy in their sockets, and to abandon all connexion of the parts by chains. 

 If any sensible looseness was left in the joints, a violent jerk would be pro- 

 duced every time the motion of the piston was reversed. Any looseness either 

 in the pivots or joints of the parallel motion of the working beam, the connect- 

 ing rod, or crank, would, at every change of stroke, be so accumulated as to 

 produce upon the machinery the effects of percussion, and would consequently 



