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.

One chapter of nine. Read the whole Primer, or work the engines themselves.