THE STEAM-ENGINE. 469 



on the one hand, or the exhausting-pipe on the other, and the lower part of 

 the cylinder. The four valves V, E, V, and E', are connected by a system 

 of levers with a handle or spanner m, which, being driven downward or up- 

 ward, is capable of opening or closing the valves in pairs, in the manner al- 

 ready described. The condensers, the air-pump, and the hot-water pump, are 

 in all respects similar to those already described in the single-acting engine, 

 except that the condensing-jet is governed by a lever I, by which it is allowed 

 to play continually in the condenser, and by which the quantity of water ad- 

 mitted through it is regulated. The cold-water pump N is worked by the en- 

 gine as already described in the single-acting engine, and supplies the cistern in 

 which the air-pump and condenser are submerged, so as to keep down its temper- 

 ature to the proper limit. On the air-pump rod R are two pins properly placed, 

 so as to strike the spanner m, upward and downward, at the proper times, when 

 the piston approaches the termination of the stroke at the top or bottom of the 

 cylinder. The pump L conducts the warm water drawn by the air-pump from 

 the condenser to a proper reservoir for feeding the boiler. The vertical mo- 

 tion of the piston-rod in a straight line is rendered compatible with the circular 

 motion of the end of the beam by the parallel motion already described. The 

 point b, on the beam, moves upward and downward in a circular arch, of which 

 the axis of the beam is the centre. In like manner, the point d of the rod d c 

 moves upward and downward in a similar arch, of which the fixed pivot c is the 

 centre. The joint or bar d b, which joins these two pivots, will be moved so 

 that its middle point e will ascend and descend nearly in a straight line, as has 

 been already explained ; opposite this point e is attached the piston-rod of the 

 air-pump, which is accordingly guided upward and downward by this means. 

 The jointed parallelogram b d gf is attached to the beam by pivots ; and, as 

 has been explained, the point g will be moved upward and downward in a 

 straight line, through twice the space through which the point e is moved. To 

 the point g the rod of the steam-piston is attached. Thus, the rods of the 

 steam-piston and air-pump are moved by the same system of jointed bars, 

 and moved through spaces which are in the proportion of two to one. 



Although this system of jointed rods forming the parallel motion, appears in 

 the figure to consist only of one parallelogram b d gf, and one rod c d, called 

 the radius rod, it is, in fact, double, a similar parallelogram and radius rod be- 

 ing attached to corresponding points, and in the same manner on the other side 

 of the beam ; but from the view given in the cut, the one set of rods hides the 

 other. The two systems of rods thus attached to opposite sides of the beam 

 at several inches asunder, are connected by cross rods, the ends of which form 

 the pivots or joints, and extend between the parallelograms. The ends of these 

 rods are only visible in the figure. It is to the middle of one of these rods, the 

 end of which is represented at e, that the air-pump piston-rod is attached ; and 

 it is to the middle of another, the end of which is represented at g, that the 

 steam piston-rod is attached. These two piston-rods, therefore, are driven, not 

 immediately by either of the parallelograms forming the parallel motion, but by 

 the bars extending between them. 



To the working end of the beam H is attached a rod of cast-iron 0, called 

 the connecting-rod, the lower end of which is attached to the crank by a pivot. 

 The weight of the connecting-rod is so made, that it shall balance the weight 

 of the piston-rods of the air-pump and cylinder on the other side of the beam ; 

 and the weight of the piston-rod of the cold-water pump N nearly balances the 

 weight of the piston-rod of the hot-water pump L. Thus, so far as the weights 

 of the machinery are concerned, the engine is in equilibrium, and the piston 

 would rest in any position indifferently in the cylinder. 



The axis of the fly-wheel on which the crank is formed is square in the 



