July 30th, 1796. L——, who had just returned from Edinburgh, and had taken down in two large volumes, Dr. Black's Lectures, used to read to us part of them, for about a quarter of an hour, every morning after breakfast. He was frequently interrupted (which interruptions he bore with heroic patience) by Mr. ——'s explanations and comments. When he came to the expansive power of steam, and to the description of the different steam engines which have been invented, Mr. —— stopped to ask B——, C——, and S——, to describe the steam engine in their own words. They all described it in such a manner as to show that they clearly understood the principle of the machine. Only the general principle had been explained to them. L——, after having read the description of Savary's and Newcomen's steam engines, was beginning to read the description of that invented by Mr. Watt; but Mr. —— stopped him, that he might try whether any person present could invent it. Mr. E—— thus stated the difficulty: "In the old steam engine, cold water, you know, is thrown into the cylinder to condense the steam; but in condensing the steam, the cold water at the same time cools the cylinder. Now the cylinder must be heated again, before it can be filled with steam; for till it is heated, it will condense the steam. There is, consequently, a great waste of heat and fuel in the great cylinder. How can you condense the steam without cooling the cylinder?"
S——. "Let down a cold tin tube into the cylinder when you want to condense the steam, and draw it up again as soon as the steam is condensed; or, if you could put a cylinder of ice up the great tube."
Some of the company next asked, if an horizontal plate of cold metal, made to slide up the inside of the cylinder, would condense the steam. The edges of the plate only would touch the cylinder; the surface of the plate might condense the steam.
"But," said Mr. ——"how can you introduce and withdraw it?"
C—— (a girl of 12) then said, "I would put a cold vessel to condense the steam at the top of the cylinder."
Mr. ——. "So as to touch the cylinder, do you mean?"
C——. "No, not so as to touch the cylinder, but at some distance from it."
Mr. ——. "Then the cold air would rush into the cylinder whilst the steam was passing from the cylinder to your condenser."
C——. "But I would cover in the cold vessel, and I would cover in the passage to it."
Mr. ——. "I have the pleasure of informing you, that you have invented part of the great Mr. Watt's improvement on the steam engine. You see how it facilitates invention, to begin by stating the difficulty clearly to the mind. This is what every practical inventor does when he invents in mechanics."