A History of the Growth of the Steam-Engine

by Robert Henry Thurston

He says:[37] “I had gone to take a walk on a fine Sabbath afternoon. I had entered the Green by the gate at the foot of Charlotte street, and had passed the old washing-house. I was thinking upon the engine at the time, and had gone as far as the herd’s house, when the idea came into my mind that, as steam was an elastic body, it would rush into a vacuum, and, if a communication were made between the cylinder and an exhausted vessel, it would rush into it, and might be there condensed without cooling the cylinder. I then saw that I must get rid of the condensed steam and injection-water if I used a jet, as in Newcomen’s engine. Two ways of doing this occurred to me: First, the water might be run off by a descending pipe, if an offlet could be got at the depth of 35 or 36 feet, and any air might be extracted by a small pump. The second was, to make the pump large enough to extract both water and air.” “I had not walked farther than the Golf-house, when the whole thing was arranged in my mind.”

Referring to this invention, Watt said to Prof. Jardine:[38] “When analyzed, the invention would not appear so great as it seemed to be. In the state in which I found the steam-engine, it was no great effort of mind to observe that the quantity of fuel necessary to make it work would forever prevent its extensive utility. The next step in my progress was equally easy—to inquire what was the cause of the great consumption of fuel. This, too, was readily suggested, viz., the waste of fuel which was necessary to bring the whole cylinder, piston, and adjacent parts from the coldness of water to the heat of steam, no fewer than from 15 to 20 times in a minute.” It was by pursuing this train of thought that he was led to devise the separate condenser.

Fig. 25.—Watt’s Experiment.

On Monday morning Watt proceeded to make an experimental test of his new invention, using for his steam-cylinder and piston a large brass surgeon’s-syringe, 13∕4-inch diameter and 10 inches long. At each end was a pipe leading steam from the boiler, and fitted with a cock to act as a steam-valve. A pipe led also from the top of the cylinder to the condenser, the syringe being inverted and the piston-rod hanging downward for convenience. The condenser was made of two pipes of thin tin plate, 10 or 12 inches long, and about one-sixth of an inch in diameter, standing vertically, and having a connection at the top with a horizontal pipe of larger size, and fitted with a “snifting-valve.” Another vertical pipe, about an inch in diameter, was connected to the condenser, and was fitted with a piston, with a view to using it as an “air-pump.” The whole was set in a cistern of cold water. The piston-rod of the little steam-cylinder was drilled from end to end to permit the water to be removed from the cylinder. This little model ([Fig. 25]) worked very satisfactorily, and the perfection of the vacuum was such that the machine lifted a weight of 18 pounds hung upon the piston-rod, as in the sketch. A larger model was immediately afterward constructed, and the result of its test confirmed fully the anticipations which had been awakened by the first experiment.

Having taken this first step and made such a radical improvement, the success of this invention was no sooner determined than others followed in rapid succession, as consequences of the exigencies arising from the first change in the old Newcomen engine. But in the working out of the forms and proportions of the details of the new engine, even Watt’s powerful mind, stored as it was with happily-combined scientific and practical information, was occupied for years. In attaching the separate condenser, he first attempted surface-condensation; but this not succeeding well, he substituted the jet. Some provision became at once necessary for preventing the filling of the condenser with water.

Watt at first intended adopting the expedient which had worked satisfactorily with the less effective condensation of Newcomen’s engine—i. e., leading a pipe from the condenser to a depth greater than the height of a column of water which could be counterbalanced by the pressure of the atmosphere; but he subsequently employed the air-pump, which relieves the condenser not only of the water, but of the air which also usually collects in considerable volume in the condenser, and vitiates the vacuum. He next substituted oil and tallow for water in the lubrication of the piston and keeping it steam-tight, in order to avoid the cooling of the cylinder incident to the use of the latter. Another cause of refrigeration of the cylinder, and consequent waste of power in its operation, was seen to be the entrance of the atmosphere, which followed the piston down the cylinder at each stroke, cooling its interior by its contact. This the inventor concluded to prevent by covering the top of the cylinder, allowing the piston-rod to play through a “stuffing-box”—which device had long been known to mechanics.

He accordingly not only covered the top, but surrounded the whole cylinder with an external casing, or “steam-jacket,” and allowed the steam from the boiler to pass around the steam-cylinder and to press upon the upper surface of the piston, where its pressure was variable at pleasure, and therefore more manageable than that of the atmosphere. It also, besides keeping the cylinder hot, could do comparatively little harm should it leak by the piston, as it could be condensed, and thus readily disposed of.

When he had concluded to build the larger experimental engine, Watt determined to give his whole time and attention to the work, and hired a room in an old deserted pottery near the Broomielaw. Here he worked with a mechanic—John Gardiner, whom he had taken into his employ—uninterruptedly for many weeks. Meantime, through his friend Dr. Black, probably, he had made the acquaintance of Dr. Roebuck, a wealthy physician, who had, with other Scotch capitalists, just founded the celebrated Carron Iron-Works, and had opened a correspondence with him, in which he kept that gentleman informed of the progress of his work on the new engine.