invented in 1765 by James Watt
On the right is seen the weight and the entrance to the mine shaft. The cylinder is on the left, the boiler underneath. A water tank to the right of the boiler contains a condenser. When the piston reaches its highest point a valve opens and releases the steam into the condenser, where it is cooled to condensation. The resulting vacuum effect drives the piston down while the cylinder and piston retain their high temperature.
Lardner, D. (1852) The Steam Engine Familiarly Explained and Illustrated. A. Hart, Philadelphia.
from http://www.chass.utoronto.ca/imago/watt.html (accessed 9 March 2004)