The strong-steam engineRichard Trevithick of Camborne · circa 1802
The moment Watt's patent expired, this Cornish giant did the forbidden thing: he raised the pressure. Strong steam pushes hard enough on its own that the condenser becomes optional and the engine shrinks to a fraction of a beam engine's bulk — compact enough that in 1804 one pulled the first railway load in history at Penydarren. Strengths: cheap, portable, powerful for its size; its cylindrical boiler is the ancestor of every locomotive's. Weaknesses: economy no better than Watt's unless the condenser is kept anyway — the pairing that became Cornish practice and broke the duty records — and danger: a Greenwich engine burst in 1803 when the safety valve was held shut, killing four. Trevithick answered with better valves; the trade answered by tying them down regardless. The fatal switch is provided. So is the Gazette.
Key to the plate
a. — walking beam. The beam survives into the strong-steam era on stationary engines like this one, though Trevithick’s travelling engines did without — a crank and a stout frame serve where an engine must fit on wheels.
b. — parallel motion. Watt’s straight-line linkage, inherited whole. The patent lapsed in 1800 and with it the caution: the parts stay, the pressures do not.
c. — cylindrical boiler. The shape is the invention: a cylinder resists pressure as a dome and a wagon-top never could, and cast iron replaces copper. Trevithick’s boilers bore ten times Watt’s pressures — “strong steam” — and the engine shrinks accordingly. Screw down its valve and you will learn why Greenwich, 1803, is remembered.
d. — exhaust to air. No condenser at all: the steam puffs straight to the sky, and the engine pays the atmosphere fifteen pounds of back pressure for the privilege. Strong steam makes the loss affordable, and losing the condenser, its pumps, and its water makes the engine portable — the whole point.
e. — fly-wheel. Smaller than Watt’s and turning faster. A high-pressure engine’s sharper pushes still need ironing out, and at these speeds the rim earns its keep — mind the wreck speed; the rim minds it whether you do or not.
f. — governor. The same flyballs, working the same throttle, now guarding a far fiercer engine. Cast off the load with the governor idle and a strong-steam engine finds its burst speed in under a minute.