This mode of operation prevailed for about fifty years, and though much used to pump water from mines, was found to have great inconveniences and defects; till, in the year 1762, Mr. Watt, being employed to repair a working-model of an engine at the University of Glasgow, was led to direct his mind to the improvement of the machine; and from his experiments sprung the most essential change, viz. the condensation of the steam in the cylinder, by opening a communication with a separate vessel, into which the injection of cold water was made, thus allowing the cylinder to remain hot. On opening that communication, the steam instantly rushes to the cold, or rather is destroyed by the instant loss or reduction of its heat, and the vacuum thus made allows the piston to descend as before mentioned. [Illustration: _Pl. I._] [Illustration: _Pl. II._] [Illustration: _Pl. III._] [Illustration: _Pl. IV._] Mr. Watt soon added the airpump to the condenser, to extract the air extricated from the water in boiling, together with the water injected. The next step was to close the upper end of the cylinder, the piston-rod working through a tight packing to exclude the air, letting the steam in above, as well as below the piston, by an alternate communication, and then condensing it in both cases alternately, thus producing a double stroke; at the same time deriving some aid from the expansive force of the steam on the side of the piston opposite to the vacuum. This is essentially the form of all the engines in use at the present day. The minor parts devised by Mr. Watt, as the working of the valves, &c. were such as would readily occur to a scientific mechanician. While he was bringing the engine to its present perfection, and furnishing it for the numerous mines, manufactories, and breweries in Great Britain, variations were devised by Cartwright, by Hornblower, Woolf, and others in England, and more recently by Evans and by Ogden in America, evincing much ingenuity, but (with the exception of Evans's, which is a simple engine of high pressure) making the machine more complex. Watt and Bolton's engine, as most generally used, being properly an atmospheric engine, or working with steam so low as merely to produce a vacuum in the cylinder, became of enormous dimensions, when the power required was that of an hundred horses: a scale of estimate adapted to the comprehension of those who had before used the labour of that animal, and preferred to substitute the steam-engine.