JAMES WATT, the son of a merchant, was born at Greenock, in Scotland, on the 19th of January, 1736. He received the first part of his education at a school in his native place, and completed it at home, by his own diligence. The science of mechanics, for which he afterwards became so famous, formed, at an early age, his favorite study; and, in conformity with his desire, he was, at the age of eighteen, apprenticed to a mathematical instrument-maker, in London. The bad state of his health, however, which had before retarded his progress at school, compelled him to return, after a year's stay in the metropolis, to Scotland. This was all the instruction he ever received in the business for which he was intended, yet he must have attained considerable skill, as, in 1757, he, at the recommendation of some relations, commenced the practice of it, at Glasgow, and was immediately appointed mathematical instrument-maker to the college. He continued to hold this situation till 1763, when he married, left his apartments in the university, for a house in the town of Glasgow, and commenced the profession of a general engineer. He soon acquired a high reputation; and in making surveys and estimates for canals, harbors, bridges, and other public works, was as extensively employed in his own country, as Brindley had been in England.
His attention to the employment of steam, as a mechanical agent, had been, in the first instance, excited by witnessing some experiments of his friend Mr. John (afterwards Dr.) Robison, and he had also made some experiments himself, with a view of ascertaining its expansive force. It was not, however, till 1763-4 that he began to devote himself seriously to the investigation of the properties of steam, and to ascertain those results upon which his fame was to be founded. An examination of Newcomen's engine, a model of which had been sent him to repair, revived all his former impressions respecting the radical imperfections of the atmospheric machine. to the improvements of which he now ardently devoted himself. One of machine, first discoveries was, that the rapidity with which water evaporates, depends simply upon the quantity of heat which is imbibed, and this again on the extent of the surface of the vessel containing the water, exposed to the fire. He ascertained also the quantity of coals necessary for the evaporation of any. given quantity of water, the heat at which it boils under various pressures, and many other particulars never before accurately determined.
He now proceeded to attempt a remedy of the two grand defects of Newcomen's engine - the necessity of cooling the cylinder before every stroke of the piston, by the water injected into it; and the non-employment of the machine, for a moving power, of the expansive force of the steam. On account of the first defect, a much more powerful application of heat than would otherwise have been requisite was demanded for the purpose of again heating the cylinder, when it was to be refilled with steam. To keep this vessel, therefore, permanently hot, was the grand desideratum; and Watt at length hit upon an expedient equally simple and successful. His plan was to establish a communication, by an open pipe, between the cylinder and another vessel, the consequence of which would be, that when the steam was admitted into the former, it would flow into the latter, so as to fill that also. Supposing, then, that the steam should here only be condensed, by being brought into contact with cold water, or any other convenient means, a vacuum would be produced, into which, as a vent, more steam would immediately rush from the cylinder; this steam would also be condensed; and so the process would go on, till all the steam had left the cylinder, and a perfect vacuum had been effected in that vessel, without so much as a drop of cold water having touched or entered it. The separate vessel alone, or the condenser, would be cooled by the water used to condense the steam; which, instead of being an evil, would tend to quicken and promote the condensation. Experiments fully confirmed Watt in these views; and the consequence was, not only a saving of three-forths of the fuel formerly required to feed the engine, but a considerable increase of its power.
In overcoming this difficulty, Watt was conducted to another improvement, which effected the complete removal of what we have described as the second radical imperfection of Newcomen's engine, namely, its nonemployment, for a moving power, of the expansive force of the steam. The effectual way, it occurred to him, of preventing any air from escaping into the parts of the cylinder below the piston, would be to dispense with the use of that element above the piston, and to substitute there likewise the same contrivance as below, of alternate steam and a vacuum. This was to be accomplished by merely opening communications from the upper part of the cylinder to the boiler, on the one hand, and the condenser on the other; and forming it, at the same time, into an air-tight chamber, by means of a cover, with only a hole in it to admit the rod or shank of the piston, which might, besides, without impeding its freedom of action, be padded with hemp, the more completely to exclude the air. It was so contrived, accordingly, by a proper arrangement of the cocks, and the machinery connected with them, that while there was a vacuum in one end of the cylinder, there should be an admission of steam into the other; and the steam so admitted now served, not only by its susceptibility of sudden condensation, to create the vacuum, but also, by its expansive force, to impel the piston.