Ill 
Steam Engine* 
of the cylinder by a metal plate, having a hole in it through 
which the piston-rod works in a collar of leathers ; and 
instead of employing the pressure of the atmosphere to 
force down the piston, he introduces the steam above 
the piston, when the vacuum is formed underneath, 
and employs it to produce this effect : thus making the 
direct pressure of the steam the moving power, as in the 
original construction of the engine. 
The last great improvement made by Mr. Watt, was 
his giving an impulse to the piston by the steam, both in 
descending and ascending, instead of being impelled, as 
in the old engine, during the descent of the piston only. 
Having thus briefly mentioned the principal improve- 
ments made in the steam engine by Mr. Watt, we shall 
proceed to describe one of his engines on the latest con- 
struction, 
A is the boiler, to which Mr. Watt has paid very great 
attention. It is generally of an oblong form ; and the 
flame, after striking on its concave bottom, circulates 
round the sides, and sometimes returns in a pipe through 
the body of the water before it is suffered to go up into 
the chimney. In liis engines there are commonly two of 
these boilers, so that one of them may work while the 
other is repairing. B (Plate fig. 3) is the steam-pipe which 
conveys the steam to the cylinder C, which is cased, and 
closed at top by a plate, having a collar of leathers, 
through which the piston-rod D works, a and c are the 
steam-valves, through which, the steam enters into the cy- 
linder : it is admitted through a, when it is to press the 
j piston downwards, and through c when it presses it up- 
wards. b and d are the eduction- valves, through which 
the steam passes from the cylinder into the condenser e, 
which is a separate vessel placed in a cistern of cold water, 
and which has a jet of cold water continually playing up 
in the inside of it. f is tlie air pump, which extracts the air 
