- The capacities for heat of iron, copper, and of some sorts of wood, as compared with water.
- The bulk of steam compared with that of water.
- The quantity of water evaporated in a certain boiler by a pound of coal.
- The elasticity of steam at various temperatures greater than that of boiling water, and an approximation to the law which it follows at other temperatures.
- How much water in the form of steam was required every stroke by a small Newcomen engine, with a wooden cylinder 6 inches in diameter and 12 inches stroke.
- The quantity of cold water required in every stroke to condense the steam in that cylinder, so as to give it a working power of about 7 pounds on the square inch.
James Watt Writes
According to James Watt: "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 off jet 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, James Watt said: "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."
James Watt had invented his all-important separate condenser. He proceeded to make an experimental test of his new invention, using for his steam cylinder and piston a large brass surgeon's syringe, 14 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 Watt fitted with a piston, with a view to using it as an "air pump."
The whole thing 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 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 making such a radical improvement, the success of this invention was followed by more. All the result of improving the old Newcomen engine.
