In 1725, Jacob Leupold invented an engine, in which the work was done by steam alone, instead of by the atmosphere, as in the engines that immediately preceded it. Leupold used two cylinders. They were open at the top to the atmosphere as in the others, but154 he used higher pressures of steam, and arranged a four-way cock between the bottoms of the two cylinders in such a way that the bottom of each cylinder, in its turn, was connected to the boiler or to the open air. Each cylinder actuated directly a separate vibrating beam, which in turn actuated the piston of a pump; the two pistons acting reciprocally, each drawing up water in its turn.


In 1765, James Watt made the very great improvement of providing a condenser separate from the cylinder of the engine, so that the great loss of heat caused by cooling the cylinder and then heating it at each stroke was wholly avoided. He covered the cylinder entirely, and surrounded it with an external cylinder kept always full of steam, that maintained the cylinder at a high temperature. The steam, instead of being condensed within the cylinder, after it had done its work, was allowed to escape into the condenser. To facilitate this action, the condenser was fitted with an air-pump that maintained a good vacuum in it.

In 1769, Watt invented an improvement that consisted mainly of means whereby the supply of steam to the cylinder could be shut off at any desired part of the stroke, and the steam allowed to complete the rest of the stroke by virtue of its expansive force. This invention increased tremendously the efficiency of the engine: that is, the amount of work done with a given amount of steam.