The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

CHAPTER 1V.
ENGINE OF JAMES WATT.
Advantages of the Atmospheric Engine over that of Captain Saverv.-It;
contained no new Principle.-Papin's Engine.-James Watt. —Particulars
of his Life.-His first Conceptions of the Means of economizing Heat.Principle of his projected Improvements.
(43.) CONSIDERED practically, the engine described in the;
last chapter possessed considerable advantages over that of
Savery; and even at the present day this machine is not'
unfrequently used in districts where fuel is very abundant
and cheap, the first cost being considerably less than that of.
a modern engine. The low pressure of the steam necessary
to work it rendered the use of the atmospheric engine perfectly safe; there being only a bursting pressure of about
Llb. per inch, while in Savery's there was a bursting pressure amounting to 301bs. The temperature of the steam,
not exceeding 216~, did not weaken or destroy the materials,;
while Savery's engines required steam raised from water at
2670, which in a short time rendered the engine unable to
sustain the pressure.
The power of Savery's engines was also very limited,
both as to the quantity of water raised, and the height to
which it was elevated, (34). On the other hand, the atmospheric engine had no other limit than the dimensions of the
piston. In estimating the power of these engines, however,
we cannot allow the full atmospheric pressure as an effective
force. The condensing water, being mixed with the condensed steam, forms a quantity of hot water in the bottom
of the cylinder, which, not being submitted to the atmospheric pressure, (17), produces a vapour which resists the
descent of the piston. In practice we find that an allowance
of at least 31bs. per square inch should be made for the