WATER AND STEAM COMPARED. 387 



two taps, the one was to be opened at the moment when 

 the beam, that Newcomen had been the first to introduce 

 to such good purpose in his engines, had completed the 

 descending oscillation, and it must be shut exactly at the 

 opposite oscillation. The management of the second tap 

 must be the exact contrary. The positions of the balance 

 and of the taps are necessarily dependent on each other. 

 Potter seizes on this fact : he perceives that the beam 

 may impress on the other pieces all the motion that the 

 play of the engine required, and instantly realizes his 

 conception. Several cords are fastened to the handles of 

 the taps ; the opposite ends Potter ties to portions of the 

 beam suitably selected ; thus the purchase which this ex- 

 ercises on certain cords while rising, and those that it 

 exercises on others in descending, supplant the manual 

 efforts ; for the first time the engine works by itself ; for 

 the first time no other workman is seen near it but the 

 stoker, who from time to time goes to keep up the fire 

 under the boiler. 



For the cords of young Potter, the constructors soon 

 substituted rigid vertical rods fixed to the beam and fur- 

 nished with several pegs which pressed the heads of the 

 several taps or valves either downwards or upwards. 

 The rods themselves have been exchanged for other 

 combinations ; but however humiliating such an acknowl- 

 edgment may be, all these inventions are mere modifica- 

 tions of the mechanism suggested to a child by the wish 

 to join his little companions at play. 



WATT'S LABOURS IN THE STEAM-ENGINE. 

 In physical cabinets we find a good many machines on 

 which industry had founded great hopes, though the ex- 

 pense of their manufacture, or that of keeping them at 



