Steam-Engine. 417 



rendered much longer than DF by being made crooked ; so long 

 as the point D is higher than F, the fluid will pass until it arrives 

 at D provided the height of E above D does not exceed 34 feet. 



Of the Steam-Engine. 



523. THE whole theory of the steam engine is founded upon 

 two principles, the developement of the elastic force of aqueous 

 vapour by heat, and the sudden precipitation of this vapour by 

 cooling. On account of the extensive uses of this machine in the 

 arts, we shall here treat of it at some length. 



Although it is generally sufficient in mechanics to create any 

 one force or motion, in order to be able thence to deduce all 

 sorts of motions, yet for the sake of distinctness we shall suppose 

 that it is proposed, in the first place, to draw water from a mine 

 by means of a sucking pump T' T'. Here the point in question is Fig. 251. 

 to raise the piston P '. For this purpose we attach the piston rod 

 to a chain applied to one of the extremities A' of a bent lever 

 moving about its centre C. It is evident, that if we attach to the 

 opposite arm of the lever a similar chain represented by AD, we 

 shall only have to pull this chain, in order to raise the piston P', 

 and draw the water into the body of the pump by the external 

 pressure of the atmosphere. This being done, the valves placed 

 at the bottom of the pump will close ; and the apparatus being 

 left to itself, if we suppose the weight of the piston P', together 

 with that of the frame which supports it, to exceed the total 

 weight of AD and P, it is evident that the piston P' will descend 

 into the water by its own weight and cause the water to raise 

 the valve opening through its centre ; and having reached the 

 bottom of the body of the pump, will separate this water entirely 

 from the water below. Then by pulling anew the chain AD, we 

 shall raise this water with the piston, and at the same time draw 

 more water into the body of the pump ; after which the piston 

 will descend by its own weight in the same manner as before, 

 and so on indefinitely. It remains then to give the requisite mo- 

 tion to the chain AD. For this purpose we attach its lower ex- 



Mech. 53 



