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.

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