A History of the Growth of the Steam-Engine

by Robert Henry Thurston

Outside of Soho, Murdoch also found ample employment for his inventive talent. In 1792, while at Redruth, his residence before finally returning to Soho, he was led to speculate upon the possibility of utilizing the illuminating qualities of coal-gas, and, convinced of its practicability, he laid the subject before the Royal Society in 1808, and was awarded the Rumford gold medal. He had, ten years earlier, lighted a part of the Soho works with coal-gas, and in 1803 Watt authorized him to extend his pipes throughout all the buildings. Several manufacturers promptly introduced the new light, and its use extended very rapidly.

Still another of Murdoch’s favorite schemes was the transmission of power by the use of compressed air. He drove the pattern-shop engine at Soho by means of air from the blowing-engine in the foundery, and erected a pneumatic lift to elevate castings from the foundery-floor to the canal-bank. He made a steam-gun, introduced the heating of buildings by the circulation of hot water, and invented the method of transmitting packages through tubes by the impulse of compressed air, as now practised by the “pneumatic dispatch” companies. He died at the age of eighty-five years.

Fig. 37.—Hornblower’s Compound Engine, 1781.

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.

The steam beneath the piston of the large cylinder is immediately condensed, and the pressure on the upper side of that piston causes it to descend, carrying that end of the beam with it, and raising the opposite end with the pump-rods and their attachments. At the same time, the steam from the lower end of the small high-pressure cylinder being let into the upper end of the larger cylinder, the completion of the stroke finds a cylinder full of steam transferred from the one to the other with corresponding increase of volume and decrease of pressure. While expanding and diminishing in pressure as it passes from the smaller into the larger cylinder, this charge of steam gradually resists less and less the pressure of the steam from the boiler on the upper side of the piston of the small cylinder, B, and the net result is the movement of the engine by pressures exerted on the upper sides of both pistons and against pressures of less intensity on the under sides of both. The pressures in the lower part of the small cylinder, in the upper part of the large cylinder, and in the communicating passage, are evidently all equal at any given time.

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.