Page:Popular Science Monthly Volume 12.djvu/35

Rh by water boiling under atmospheric pressure, can be subjected to the action of water boiling under high pressure, and thus thoroughly "digested," or cooked.

The danger of bursting these vessels caused him, in 1681, to invent



and apply the lever safety-valve, now an indispensable appurtenance to every steam-boiler.

17. In 1690 he constructed a working model of an engine, consisting of a steam-cylinder with a piston which was raised by steam-pressure, and which descended again when the condensation of the steam produced a vacuum beneath it.

This apparatus the inventor proposed to use as a motor for working pumps and for driving paddle-wheels; but he never built a successful working machine on this plan, so far as we can ascertain; and he did not then propose a separate boiler, but made the same vessel serve at once as a boiler, steam-cylinder, and condenser, evaporating water in the cylinder itself; and, after raising the piston, removing the cylinder from the fire, or the fire from under the cylinder, to effect condensation by the gradual loss of heat by radiation.

18. The most important advance in actual construction was made by Thomas Savery.

The constant and embarrassing expense, and the engineering difficulties presented by the necessity of keeping the British mines, and