118 
Steam Engine. 
The greatest of Mr. Watt’s improvements consists in 
his employing the steam both to elevate and depress the 
piston. In the engines of Newcomen and Beighton, the 
steam was not the impelling power : it was used merely 
for producing a vacuum below the piston, which was 
forced down by the pressure of the atmosphere, and ele- 
vated by the counterweight at the farther extremity of the 
great beam. The cylinder, therefore, was exposed to the 
external air at every descent of the piston, and a conside- 
rable portion of its heat being thus abstracted, a corres- 
ponding quantity of steam was of consequence destroyed. 
In Mr. Watt’s engines, however, the external air is ex- 
cluded by a metal plate at the top of the cylinder, which 
has a hole in it for admitting the piston-rod ; and the pis- 
ton itself is raised and depressed merely by the force of 
steam. 
When these improvements are adopted, and the engine 
is constructed in the most perfect manner, there is 
not above ^ part of the steam consumed in heating the ap- 
paratus ; and, therefore, it is impossible that the engine 
can be rendered ^ more powerful than it is at present. It 
would be very desirable, however, that the force of the 
piston could be properly communicated to the machinery 
without the intervention of the great beam. This, in- 
deed, has been attempted by Mr. Watt, who has employ- 
ed the piston-rod itself to drive the machinery ; and Mr. 
Cartwright has, in his engine, converted the perpendicular 
motion of the piston into a rotatory motion, by means of 
two cranks fixed to the axis of two equal wheels which 
work in each other. Notwithstanding the simplicity of 
these methods, none of them have come into general use ; 
and Mr. Watt still prefers the invention of the great beam, 
which is generally made of hard oak, with its heart taken 
out, in order to prevent it from warping. A considera- 
ble quantity of power, however, is wasted by dragging, 
