Watt found that the College possessed the model of a Newcomen engine, which had been sent to London for repair, and not returned. He prevailed upon the authorities to get it returned, and put into his own hands.

He soon had it repaired, and in working order. He discovered that nearly four-fifths of the whole steam employed was condensed on its first admission to the cylinder, which had been cooled by the injection into it of the cold water. It was only the surplus that could act upon the piston. He came to the conclusion that to make a perfect steam engine, the cylinder should always be as hot as the steam that entered it, but it was equally necessary that the steam should be condensed when the piston descended. Thus the cylinder was never to be at a less temperature than 212° (when water becomes steam), and yet at each descent of the piston it was to be less than 100° — conditions that seemed incompatible. Many lines of thought he followed out, and many experiments made. At length, all at once, the light burst upon him. But the discovery is best told in his own words.

“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 of 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 is 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 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 outlet 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