72 
Steam Engine. 
proving a counterbalance for the atmosphere above the 
piston, the weight of the pump-rods, at the other end of 
the lever, carries that end down, and raises the piston of 
the steam- cylinder. The steam hole is then immediately 
shut, and a cock opened for injecting the cold water into 
the cylinder of steam, which condenses it to water again, 
and thus making a vacuum below the piston, the atmos- 
phere again presses it down and raises the pump-rods, as 
before ; and so on continually^ 
3. Watt’s. — The great features of improvement 
made by Mr. JEatt upon the engine of Newcomen and 
Cawley are, as Mr* Nicholson remarks, first, that the elas- 
ticity of the steam itself is used as the active power in this 
engine and secondly, that besides various other judicious 
arrangements for the economy of heat, he condenses the 
steam, not in the cylinder, but in a separate vessel. 
In the cylinder or syringe, concerning which we have 
spoken, in mentioning the engine of Newcomen, let us 
suppose the upper part to be closed, and the piston-rod 
to slide air tight through a collar of leathers. In this situ- 
ation, it is evident that the piston might be depressed by 
throwing the steam upon its upper furface, through an a- 
perture at the superior end of the cylinder. But if we 
suppose the external air to have access to the lower sur- 
face of the piston, we shall find that steam no stronger in 
its elasticity than to equal the weight of the atmosphere 
would not move the piston at all ; and consequently that 
this new engine would require much denser steam, and 
consume much more fuel than the old engine. The re- 
medy for this evil is to maintain a constant vacuum be- 
neath the piston. If such a vacuum were originally pro- 
duced by steam, it is certain that its permanency could 
^ The Piston being forced downward, not by the pressure of 
the atmosphere, as in Newcomen’s, but by the elastic force of 
Steam thrown on the top of the piston. 
