The rotative engineJames Watt, with Matthew Boulton · Soho Manufactory, 1784
Watt, an instrument maker repairing a model Newcomen, saw that the wasteful part was chilling the working cylinder — so he condensed the steam in a separate vessel and kept the cylinder always hot, roughly tripling the work got from a bushel of coal. With Boulton's capital and salesmanship came the refinements that made steam a general servant rather than a pump: double action, the parallel motion he called his proudest invention, the crank and flywheel, and the spinning governor. Strengths: economy, smoothness, and rotary power for mills that had never had it. Weaknesses: Watt distrusted strong steam, so his engines stayed low-pressured, huge, and costly — and his patent held everyone else to the same caution until 1800. Throw the condenser switch to feel his fortune; move the cutoff to meet the idea that consumed the next century.
Key to the plate
a. — walking beam. The rocking beam, now worked at both ends: steam pushes as well as the vacuum pulls, so the chains of the old engines are replaced by rigid rods, and the beam drives a crank instead of a pump. The nodding becomes turning.
b. — parallel motion. A double-acting piston must push its rod as well as pull it, and a chain cannot push. Watt’s linkage of hinged bars guides the rod-end in a nearly perfect straight line — solved here exactly, by circle intersection, not drawn to look right. He called it his proudest invention.
c. — wagon boiler. Shaped like a covered wagon, worked at a few pounds of steam — Watt distrusted strong steam all his life, and his engines earn their keep by economy rather than pressure. The gauge glass and feed cocks keep its water honest.
d. — separate condenser. The idea that made the fortune. The old engine chilled its own working cylinder every stroke to make the vacuum; Watt keeps the cylinder always hot and condenses the steam in this separate vessel instead, with an air pump to draw off the water and air. Same vacuum, a third of the coal. Throw its switch and watch the indicator loop lose its floor.
e. — fly-wheel. Tons of iron ridden round by the crank. The steam pushes in pulses; the wheel stores each surplus and repays it through the dead centres, ironing twenty strokes a minute into smooth rotation a mill can trust.
f. — governor. Two brass balls on a spinning spindle, belted off the shaft. Speed flings them outward; their rise closes the throttle. It is proportional only — it holds the engine within a droop band, never exactly on the mark — and it is the reason an unloaded engine merely idles instead of racing to destruction.