When the pistons have reached the bottoms of their respective cylinders, the valves at the top of the small cylinder, _B_, and at the bottom of the large cylinder, _A_, are closed, and the valves _c_ and _d_ are opened. Steam from the boiler now enters beneath the piston of the small cylinder; the steam in the larger cylinder is exhausted into the condenser, and the steam already in the small cylinder passes over into the large cylinder, following up the piston as it rises.
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.