Page:Popular Science Monthly Volume 12.djvu/156

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cross-beams, b b. The air-pump m l o p, the tank n, and valve-gear q r s, are quite similar to those of the beam Cornish engine. The balance-beam is seen at h i.

PSM V12 D156 Bull cornish engine 1798.jpg
Fig. 18.—Bull's Cornish Engine, 1798.

Fig. 19 represents another form of pumping-engine which belongs to the class known as the "compound" or "two-cylinder" engine. This class of engines, in which the steam exhausted from one cylinder is further expanded in the second, was first introduced by Hornblower, in 1781, and was patented, in combination with the Watt condenser, by Woolf, at a later date (1804), with a view to adopting high steam and considerable expansion.

The Woolf engine was to some extent adopted, but was not successful in competing with Watt engines where the latter were well built, and, like Hornblower's engine, was soon given up.

The compound engine has come up again within a few years, and, with what is now considered high steam and considerable expansion, and designed with more intelligent reference to the requirements of