A Primer on the Steam Engine — nine short chapters, each with a working model. Everything here runs the same machinery as the engines in the museum; move the levers and the figures answer.
I. The push of steam
Boil water in a closed vessel and the steam presses outward on everything — including anything free to move. Give it a piston, and the pressing becomes pushing. The rule is simple bookkeeping: the force on the piston is the pressure times the piston's area. Twenty pounds on every square inch of a twenty-inch piston is three tons of push, from nothing but fire and water. Every engine in this museum, from 1712 to 1908, is a scheme for collecting that push over and over again.
II. The weight of the air
The first engines barely used the steam's push at all. Fill a cylinder with steam, spray in cold water, and the steam collapses back into a few drops — leaving nearly nothing behind. The atmosphere, which presses on everything always at about fifteen pounds the square inch, then shoves the piston into the emptiness. That is the whole of Newcomen's engine: the fire only makes room for the air to do the work. Try the injection lever — more water makes a sharper vacuum and a stronger stroke, but chills the cylinder so the next filling wastes more steam. The tension between those two is the first economy problem in engine history.
III. Reading the indicator diagram
One instrument explains every engine here, and it is worth a minute to learn. A pencil rides the piston: as the piston sweeps, the pencil moves across the paper with it, and steam pressure lifts the pencil up and down. Each stroke draws a closed loop — pressure against volume — and the area inside the loop is the work of that stroke. Tall and wide means powerful; the shape tells you where the power comes from. Watt kept the instrument a trade secret for decades. Move the cutoff and watch the loop change shape; the dot is the pencil, drawing now.
IV. Cutoff, or the art of stopping early
Steam admitted for the whole stroke pushes hard the whole way — and then is thrown out still at full pressure, its work half done. Close the valve partway instead, and the trapped steam expands, pushing as it thins, until by the stroke's end it has spent nearly everything it has. You give up some power; you keep the coal. Watch the two plates as you move the lever: earlier cutoff means fewer pounds of steam for every horsepower-hour. The whole nineteenth century is engineers finding ways to cut off earlier without the engine minding.
V. The condenser
The push on the piston is the difference between the pressure in front and the pressure behind. Exhaust to the open air and the air pushes back with its full fifteen pounds. Watt's separate condenser — a cold vessel apart from the working cylinder — turns the exhaust side into a near-vacuum of two or three pounds, and every stroke gains a dozen pounds of push for free. At the low boiler pressures of 1784 this nearly tripled the work got from each bushel of coal; it made Watt's fortune. The engine below runs without one, as engines did before 1769 — throw the switch and watch the floor of the loop fall.
VI. Dead centres and the flywheel
A piston pushes in straight lines, but mills want turning. The crank makes the conversion — at a price. Twice every revolution the rod and crank line up, and at those dead centres the steam, push as it may, turns nothing at all. The remedy is tons of iron rim: the flywheel drinks up the surplus of the strong part of the stroke and repays it through the dead points. Thin the wheel down to a ton and watch the needle surge and sag; a parked engine may even refuse to start, which is why crews barred engines round by hand to a strong position.
VII. The governor
A mill's load changes by the minute, and nobody wants to stand at the throttle all day. Watt's answer: two spinning balls that rise as the engine speeds and, in rising, close the valve. This is feedback — the machine steering itself — a century before the word. Corliss's refinement was to let the governor set the cutoff instead of strangling the steam through a half-shut throttle: full pressure always, admitted for exactly as long as the load deserves. Drag the load about and watch the cutoff answer while the speed barely stirs.
VIII. Duty, or the fireman's arithmetic
Coal cost money; the engines existed to save it. So the Cornish measured engines the way an owner would: foot-pounds of work delivered for every bushel of coal burned — the duty, printed monthly for all to see in Lean's Engine Reporter from 1811, engine against engine. It is the score in this museum too, and in the daily Trials. Note that duty counts coal burned, not coal used well: stoke beyond the engine's thirst and the surplus simply roars off at the safety valve.
IX. What came after
Everything later is refinement of these ideas. The compound engine expands the steam twice — a small high-pressure cylinder exhausting into a great low-pressure one — so each cylinder sees less of the temperature swing that condenses steam on cold iron; its indicator card is two nested loops. The uniflow of 1908 sends steam one way only, in at the hot ends and out at the cool middle, and one cylinder learns to beat the compound. But the grandest stage for these ideas was never on land at all.
X. To sea, and the end of the line
At sea the compound’s argument is made a third time: the marine triple-expansion engine spends its steam through three cylinders and two floating receivers, on three cranks set a third of a turn apart — so some cylinder is always at full push, and the engine needs no flywheel, no barring over, and, strangest of all, no governor: a propeller’s resistance grows as the square of its speed and holds the engine steady by itself. Drag the load below and watch the speed obey the water. This is the engine that drove the liners — the Titanic carried two, four storeys tall, beside a turbine that drank their exhaust. And that turbine is the end of our story: within a decade it took the engine room whole, and the beautiful machinery of pistons and cranks became a museum — this one. Each tab above is one of these chapters made iron; the question-marks beside every lever will remind you of the rest.