Among the most active and formidable of Watt's business rivals was JONATHAN HORNBLOWER, the patentee of the "compound" or double-cylinder engine. A sketch of this engine, as patented by Hornblower in 1781, is here given (Fig. 37). It was first described by the inventor in the "Encyclopædia Britannica." It consists, as is seen by reference to the engraving, of two steam-cylinders, _A_ and _B_--_A_ being the low and _B_ the high pressure cylinder--the steam leaving the latter being exhausted into the former, and, after doing its work there, passing into the condenser, as already described. The piston-rods, _C_ and _D_, are both connected to the same part of the beam by chains, as in the other early engines. These rods pass through stuffing-boxes in the cylinder-heads, which are fitted up like those seen on the Watt engine. Steam is led to the engine through the pipe, _G Y_, and cocks, _a_, _b_, _c_, and _d_, are adjustable, as required, to lead steam into and from the cylinders, and are moved by the plug-rod, _W_, which actuates handles not shown. _K_ is the exhaust-pipe leading to the condenser. _V_ is the engine feed-pump rod, and _X_ the great rod carrying the pump-buckets at the bottom of the shaft. The cocks _c_ and _a_ being open and _b_ and _d_ shut, the steam passes from the boiler into the upper part of the steam-cylinder, _B_; and the communication between the lower part of _B_ and the top of _A_ is also open. Before starting, steam being shut off from the engine, the great weight of the pump-rod, _X_, causes that end of the beam to preponderate, the pistons standing, as shown, at the top of their respective steam-cylinders. The engine being freed from all air by opening all the valves and permitting the steam to drive it through the engine and out of the condenser through the "snifting-valve," _O_, the valves _b_ and _d_ are closed, and the cock in the exhaust-pipe opened. [Illustration: FIG. 37.--Hornblower's Compound Engine, 1781.]