91 
Steam Mngine, 
tlierefore preferred ; and is known by the common name 
of the Air-pump. There stili remained some defects mi- 
remedied in Newcomen^s cylinder. Tiie piston in that 
engine was kept tight by water ; much of which passing 
by the sides, injured the vacuum below, by its evapora- 
tion ; and this water, as well as the atmosphere v\^hich 
came into contact with the upper part of the piston and 
sides of the cylinder at every stroke, tended materially to 
cool that vessel. Mr. Watt removed these defects, by 
applying oils, wax, and fat of animals, to lubricate his 
piston and keep it tight : he put a cover on his cylinder 
(with a hole in it made air and steam tight, for the piston 
rod to pass through,) and employed the elastic force of 
steam to press upon the piston : he also surrounded the 
cylinder with a case containing steam, or a case of wood, 
or of other non-conducting substance, which should keep 
it always of an equable temperature. 
The improvement of Newcomen’s engine, so far as the 
saving of steam and fuel was concerned, was now com- 
plete in Mr. Watt’s mind ; and in the course of the fob 
lowing year, 1765, he executed a working model, the eb 
feet of which he found fully to answer his expectations. 
It worked readily with ten and an half lib. on the inch, 
and was even capable of raising fourteen lib. ; and did 
not require more than one third of the steam, used in the 
common atmospheric engine, to produce the same effect. 
Indeed, the principle of keeping the vessel, in which the 
elasticity of the steam is exerted always hot, and that in 
which the condensation is performed always cold, is in it- 
self perfect. For the steam never coming in contact with 
any substance colder than itself until it had done its office,, 
no part is condensed until the whole effect has been ob- 
tained in the cylinder ; and when it has acted there, it is 
so condensed in the separate vessel that no resistance re^ 
mains : accordingly, the barometer proves a vacuo 
