426 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



water the latent heat of vaporisation, the water in the condenser 

 is constantly being heated, and it is necessary to replace it as con- 

 stantly by a fresh supply of cold water ; whence the need of the 

 exhausting pump E, which is worked by a rod attached to the beam ; 

 this pump returns the condensed and warm water to the chamber K', 

 and there the feed-pump w acts, raises the water, and sends it on to 

 the boiler; Y, the rod of that pump receiving its motion from the 

 .beam. 



FIG. 299. Watts' beam engine. 



v. Steam-pipe. T. Slide-valve. J. Cylinder. H. Condenser. PB. Exhausting or air-pump. WY. 

 Feed-pnmp of the boiler UX. Feeding pump of the cistern R pZ. Governor. dd. Excentric. 

 A BCD. Parallelogram. GM. Connecting rod and crank. V. Fly-wheel. 



Lastly, we see in x the rod of the pump u, which serves to feed 

 the cistern RR with cold water. This pump, generally more powerful 

 than the other two, obtains the water from some neighbouring source, 

 such as a spring, a tank, or a river. 



This complication of parts, and accessory apparatus, which more- 

 over derive all their motion from the steam-engine, only occur in the 

 condensing engines, that is, those which work at a low pressure. 

 In engines at high pressure, whether fixed or movable, the condenser, 

 the exhausting pumps, and all the machinery connected with 



