Thus, at each stroke a small cylinder full of steam is taken from the boiler, and the same weight, occupying the volume of the larger cylinder, is exhausted into the condenser from the latter cylinder.

Referring to the method of operation of this engine, Prof. Robison demonstrated that the effect produced was the same as in Watt’s single-cylinder engine—a fact which is comprehended in the law enunciated many years later by Rankine, that, “so far as the theoretical action of the steam on the piston is concerned, it is immaterial whether the expansion takes place in one cylinder, or in two or more cylinders.” It was found, in practice, that the Hornblower engine was no more economical than the Watt engine; and that erected at the Tin Croft Mine, Cornwall, in 1792, did even less work with the same fuel than the Watt engines.

Hornblower was prosecuted by Boulton & Watt for infringement. The suit was decided against him, and he was imprisoned in default of payment of the royalty, and fine demanded. He died a disappointed and impoverished man. The plan thus unsuccessfully introduced by Hornblower was subsequently modified and adopted by others among the contemporaries of Watt; and, with higher steam and the use of the Watt condenser, the “compound” gradually became a standard type of steam-engine.

Arthur Woolf, in 1804, re-introduced the Hornblower or Falck engine, with its two steam-cylinders, using steam of higher tension. His first engine was built for a brewery in London, and a considerable number were subsequently made. Woolf expanded his steam from six to nine times, and the pumping-engines built from his plans were said to have raised about 40,000,000 pounds one foot high per bushel of coals, when the Watt engine was raising but little more than 30,000,000. In one case, a duty of 57,000,000 was claimed.

Fig. 38.—Bull’s Pumping-Engine, 1798.

[Large scale image] (434 kB).

The most successful of those competitors of Watt who endeavored to devise a peculiar form of pumping-engine, which should have the efficiency of that of Boulton & Watt, and the necessary advantage in first cost, were William Bull and Richard Trevithick.[42] The accompanying [illustration] shows the design, which was then known as the “Bull Cornish Engine.”

The steam-cylinder, a, is carried on wooden beams, b, extending across the engine-house directly over the pump-well. The piston-rod, c, is secured to the pump-rods, d d, the cylinder being inverted, and the pumps, e, in the shaft, f, are thus operated without the intervention of the beam invariably seen in Watt’s engines. A connecting-rod, g, attached to the pump-rod and to the end of a balance-beam, h, operates the latter, and is counterbalanced by a weight, i. The rod, j, serves both as a plug-rod and as an air-pump connecting-rod. A snifting-valve, k, opens when the engine is blown through, and relieves the condenser and air-pump, l, of all air. The rod, m, operates a solid air-pump piston, the valves of the pump being placed on either side at the base, instead of in the pump-bucket, as in Watt’s engines. The condensing-water cistern was a wooden tank, n. A jet “pipe-condenser,” o, was used instead of a jet condenser of the form adopted by other makers, and was supplied with water through the cock, p. The plug-rod, q, as it rises and falls with the pump-rods and balance-beam, operates the “gear-handles,” r r, and opens and closes the valves, s s, at the required points in the stroke. The attendant works these valves by hand, in starting, from the floor, t. The operation of the engine is similar to that of a Watt engine. It is still in use, with a few modifications and improvements, and is a very economical and durable machine. It has not been as generally adopted, however, as it would probably have been had not the legal proscription of Watt’s patents so seriously interfered with its introduction. Its simplicity and lightness are decided advantages, and its designers are entitled to great credit for their boldness and ingenuity, as displayed in their application of the minor devices which distinguish the engine. The design is probably to be credited to Bull originally; but Trevithick built some of these engines, and is supposed to have greatly improved them while working with Edward Bull, the son of the inventor, William Bull. One of these engines was erected by them at the Herland Mine, Cornwall, in 1798, which had a steam-cylinder 60 inches in diameter, and was built on the plan just described.