WATT, JAMES (1736-1819), Scottish engineer, the inven tor of the modern condensing steam-engine, was born at Greenock on Jan. 19, 1736. His father was a small merchant there, who lost his trade and fortune by unsuccessful speculation. James made his way to London, at the age of nineteen, to be appren ticed to a philosophical-instrument maker, John Morgan, in whose service he remained for twelve months. The hard work and frugal living forced him at the end of a year to seek rest at home, not, however, before he had gained a fair knowledge of the trade and become handy in the use of tools. On his return to Scotland in 1756 he tried to establish himself as an instrument maker in Glasgow, but the city guilds would not recognize a crafts man who had not served the full term of apprenticeship, and Watt was forbidden to open shop in the burgh. The college, however, took him under its protection, and in 1757 he was estab lished in its precincts with the title of mathematical-instrument maker to the university.
Joseph Black, the discoverer of latent heat, then lecturer on chemistry, and John Robison, then a student, afterwards pro fessor of natural philosophy at Edinburgh, became his intimate friends, and with them he often discussed the possibility of im proving the steam-engine, the best type of which was at that time the Newcomen engine. It was then applied only to pumping water—chiefly in the drainage of mines; and it was so clumsy and wasteful in fuel that it was little used. Some early experiments of Watt in 1761 or 1762 had no direct result, but in 1764 his at tention was seriously aroused by having a model of Newcomen's engine, which formed part of the college collection of scientific apparatus, given him to repair. Having put the model in order, he was at once struck with its enormous consumption of steam, and set himself to find its cause and remedy.
In Newcomen's engine the cylinder stood vertically under one end of the main lever or "beam," and was open at the top. Steam, at a presture scarcely greater than that of the atmosphere, was admitted to the under side; this allowed the piston to be pulled up by a counterweight at the other end of the beam. Communi
cation with the boiler was then shut off, and the steam in the cylinder was condensed by injection of cold water from a cistern above. The pressure of the air on the top of the piston then drove it down, raising the counterweight and doing work. The injec tion water and condensed steam in the cylinder were drained out by a pipe leading down into a well.
After some unsuccessful efforts to remedy the difficulty Watt began a scientific examination of the properties of steam, studying by experiment the relation of its density and pressure to its temperature, and concluded that two conditions were essen tial to the economic use of steam in a condensing steam-engine. One was that the temperature of the condensed steam should be as low as possible, oo° F or lower, otherwise the vacuum would not be good ; the other was, to quote his own words, "that the cylinder should be always as hot as the steam which entered it." In Newcomen's engine these two conditions were incompatible, and not for some months did Watt see a means of reconciling them. Early in 1765, the idea struck him that, if the steam were condensed in a vessel distinct from the cylinder, the temperature of condensation could be kept low and that in the cylinder high. Let this separate vessel be kept cold, either by injecting cold water or by letting it run over the outside, and let a vacuum be main tained in the vessel. Then, whenever communication was made between it and the cylinder, steam would pass over from the cylinder and be condensed; the pressure in the cylinder would be as low as the pressure in the condenser, but the temperature of the metal of the cylinder would remain high, since no injection water would touch it. Without delay Watt put his idea to the test, and found that the separate condenser acted as he had an ticipated. To maintain the vacuum in it he added another new organ, namely, the air-pump, the function of which was to remove the condensed steam and injection-water along with any air gathered in the condenser.