The use of an increasing pressure of steam gave increased force and value to the improved steam-engine, but the power of constructing engines and boilers to render the increased pressure manageable was the result of a lifetime of labour.
Savery, whose engine was scarcely more than a steam-boiler, failed to control its force, and is said to have blown the roof from over his head. The mechanism of Newcomen's engine was well arranged, but suitable only for the working of pumps, and its power was limited to the weight of the atmosphere, from which it was called the atmospheric engine.
In 1756, an atmospheric engine with a cylinder of 70 inches in diameter worked at the Herland Mine, "the only objection to which was the cost of the coal, to lessen which several methods had been suggested for increasing the elasticity of the steam, and reducing the size of the boiler."[47]
[Footnote 47: Borlase's 'Natural History of Cornwall.']
In 1775 Richard Trevithick, sen., removed the flat top of a Newcomen boiler, and substituted a semicircular top, enabling it to contain stronger steam, and at the same time he improved the mechanical part of the engine by finding a better resting-place for the steam-cylinder than the top of the large boiler. Pryce gives a drawing of this engine as the best at that time in Cornwall.[48]
[Footnote 48: See drawing, vol. i., p. 25.]
"It is known as a fact that every engine of magnitude consumes 3000_l._ worth of coal every year.
"The fire-place has been diminished and enlarged again. The flame has been carried round from the bottom of the boiler in a spiral direction, and conveyed through the body of the water in a tube (one, two, or three) before its arrival at the chimney.
"Some have used a double boiler, so that fire might act on every possible point of contact, and some have built a moorstone boiler, heated by three tubes of flame passing through it.
"A judicious engineer does not attempt to load his engine with a column of water heavier than 7 lbs. on each square inch of the piston."[49]
[Footnote 49: See Pryce's 'Mineralogia Cornubiensis,' published 1778. Appendix.]
While Pryce's book was being printed, Watt in 1777 wrote of the Cornish steam-engines:--