a 
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 
