HYDRAULICS. 219 



side of it. /is the air-pump, which extracts the 

 air and water from the condenser. It is worked 

 by the great beam or lever, and the water brought 

 by it from the condenser, after being brought into 

 the hot- well g, is pumped up again by the pump //, 

 and is brought back again into the boiler by the 

 pipe i. k is another pump, also worked by the 

 engine itself, which supplies the cistern in which 

 the condenser is placed, with cold water. 



In the old engines, where the working-stroke 

 was only downwards, the piston-rod was attached 

 to the beam by chains, which bent round an arch 

 on the end of the beam, in order to make the 

 piston-rod move always in a perpendicular direction. 

 This may be seen in the plate of Newcomen's 

 engine. But in Mr. Watt's engines, where the 

 working-stroke is doubled, that is, both upwards 

 and downwards, chains could not answer this pur- 

 pose, as, when the piston was forced upwards, they 

 would slacken, and would not communicate the 

 motion to the beam. It was necessary, therefore, 

 that the piston-rod should be fastened to the beam 

 by inflexible bars; but that the stroke might be 

 perpendicular, a particular contrivance was invented 

 by Mr. Watt, which is exhibited in Plate 11., and 

 which answers the intended purpose admirably. 

 It is usually called the parallel-joint; and its nature 

 and construction will be easily understood from 

 the figure. In order to make the engine itself 

 open and shut the steam and eduction-valves, long 

 levers are attached to them, which are moved by 

 the piston-rod of the air-pump E F. This part of 

 the apparatus is called the working-geer, and is so 

 contrived, that the valves may be worked either 

 by hand or by the perpendicular rod. By shutting 

 these valves, the engine may be stopped in an 

 instant. 



