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.

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