THE STEAM ENGINE. 



m, a lever jointed to a system of rods by which the valves 

 v v' admitting and withdrawing the steam at the top and bottom 

 of the cylinder are opened and closed. This lever m is acted upon 

 by pins which project from the piston-rod of the air pump, and 

 which appear in the figure. When the piston descends, the 

 upper pin strikes the arm m, which closes the upper steam valve 

 and lower exhausting valve, and opens the lower steam valve and 

 upper exhausting valve, so that the steam is admitted below and 

 withdrawn from above the piston, which is accordingly driven up. 

 "When the up-stroke is nearly terminated, the lower pin on the 

 rod R strikes the arm m, driving it upwards, and closes the upper 

 exhausting valve and the lower steam valve, while it opens the 

 upper steam valve and lower exhausting valve, by which means the 

 piston is driven down. 



This method of working the valves is however at present rarely 

 used, being replaced by another expedient which we shall pre- 

 sently describe. 



s, the pipe leading from the boiler by which steam is supplied 

 to the cylinder to impel the piston. This pipe communicates with 

 both ends of the cylinder by means of a passage s', which is 

 parallel to the cylinder. 



T, the handle of a valve called the throttle valve, which is within 

 the steam pipe s, and which is turned by the handle, so as to contract 

 or widen more or less the passage for the steam. By this means 

 the supply of steam to the cylinder is increased or diminished. 



Q, a system of revolving balls called the governor, witli which 

 the handle T of the throttle valve is connected by a series of levers 

 and joints, which are so constructed, that when the balls recede 

 from the axis of the governor, the valve is more or less closed, 

 and when they fall near the axis, the valve is fully open. These 

 balls receive a motion of revolution from the main shaft upon 

 which the crank is constracted by means of a band or by toothed 

 wheels. In either case their velocity of rotation will be always 

 proportionate to that of the shaft. In all applications of the 

 engine to the purposes of manufacture and the arts, there is some 

 determinate velocity which is required to be given to the shaft. 

 If steam be supplied in too great quantity to the cylinder, the 

 motion given to the shaft will be too rapid ; and if it be supplied 

 in too small quantity, the motion will be too slow. 



Such irregularities of motion are prevented by the governor. 

 The moment the motion begins to be too rapid, the centrifugal 

 force produced by the revolution causes the balls to fly out, to 

 recede from the axis, and to close more or less the throttle valve. 

 If, on the contrary, the motion begins to be two slow, the balls 

 fall in, approach the axis, and open the throttle valve. Thus 





