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 (120.); [Pg219] opposite this point _e_ is attached the piston-rod of the air-pump, which is accordingly guided upwards and downwards by this means. The jointed parallelogram _b d g f_ is attached to the beam by pivots; and, as has been explained (120.), the point _g_ will be moved upwards and downwards 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 g f_, and one rod _c d_, called the _radius rod_, it is, in fact, double, a similar parallelogram and radius rod being 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 O, 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 [Pg220] square in the 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.