470 



THE STEAM-ENGINE. 



middle part, where the fly-wheel is attached to it, but has cylindrical necks at 

 each end, which rest in sockets or bearings supported by the framing of the 

 machine, in which sockets the axis revolves freely. On the axle of the crank 

 is placed the fly-wheel, and connected with its axle is the governor Q, which 

 regulates the throttle-valve T in the manner already described. 



Let us now suppose the engine to be in full operation. The piston being at 

 the top of the cylinder, the spanner m will be raised by the lower pin on the 

 air-pump rod, and the upper steam-valve V, and the lower exhausting-valve E', 

 will be opened, while the upper exhausting-valve E and the lower steam-valve 

 V are closed. Steam will, therefore, be admitted above the piston, and the 

 steam which filled the cylinder below it will be drawn off to the condenser, 

 where it will be converted into water. The piston will, therefore, be urged 

 by the pressure of the steam above it to the bottom of the cylinder. As it ap- 

 proaches that limit, the spanner m will be struck downward by the upper pin 

 on the air-pump rod, and the valves V and E' will be closed, and at the same 

 time the lower steam-valve V and the upper exhausting-valve E will be opened. 

 Steam will, therefore, be admitted below the piston, while the steam above it 

 will be drawn off into the condenser, and converted into water. The pressure 

 of the steam, therefore, below the piston will urge it upward, and in the same 

 manner the motion will be continued. 



While this process is going on in the cylinder and the condenser, the water 

 formed in the condenser will be gradually drawn off by the operation of the 

 air-pump piston, in the same manner as explained in the single-acting en- 

 gine ; and at the same time the hot water thrown into the hot well by the air- 

 pump piston will be carried off by the hot-water pump L. 



Such are the chief circumstances attending the continuance of the opera- 

 tion of the double-acting engine. It is only necessary here to recall what 

 has been already explained respecting the operation of the fly-wheel. The 

 commencement of the motion of the piston from the top and bottom of the 

 cylinder is produced, not by the pressure of the steam upon it upward or 

 downward, which must, for the reasons already explained, be entirely in- 

 efficient ; but by the momentum of the fly-wheel, which extricates the crank 

 from those positions in which the moving power can not affect it. 



The manner in which the motion of the crank affects the connecting-rod at 

 the dead points produces an effect of great importance in the operation of the 

 engine. When the crank-pin is approaching the lowest point of its play, and 

 therefore the piston approaching the top of the cylinder, the motion of the 

 crank-pin becomes nearly horizontal, and consequently its effect in drawing 

 the connecting-rod and the working end of the beam downward and the piston 

 upward, is extremely small. The consequence of this is, that as the piston 

 approaches the top of the cylinder, its motion becomes very rapidly retarded ; 

 and as the motion of the crank-pin at its lowest point is actually horizontal, the 

 piston is brought to a state of rest by this gradually-retarded motion at the top 

 of the cylinder. In like manner, when the crank-pin moves from its dead 

 point upward, its motion at first is very nearly horizontal, and consequently its 

 effect in driving the working end of the beam upward, and the piston down- 

 ward, is at first very small, but gradually accelerated. The effect of this upon 

 the piston is, that it arrives at and departs from the top of the stroke with a 

 very slow motion, being absolutely brought to rest at that point. 



The same effect is produced when the piston arrives at the bottom of the 

 cylinder. This retardation and suspension of the motion of the piston at the 

 termination of the stroke affords time for the process of condensation to be ef- 

 fected, so that when the moving power of the steam upon the piston can come 

 into action, the condensation shall be sufficiently complete; As the piston 



