293. At the left is seen the great steam tube, admitting steam to the cylinder (Cy), by the throttle-valve (t), and a smaller tube, conveying warmed water to the boiler, from the hot well (H.) The piston is at the top of the cylinder, that end of the beam in its most elevated position, and the air-pump piston also at its highest elevation. The fixed point for the parallel motion is F, the rod proceeding from which keeps the inner extremity of the rod B at a proper distance from the head of the piston-rod. The hot-water pump is worked by the rod W, and the cold-water pump, by the rod O L; frequently, both are worked by one rod from the beam. The rod which propels the crank is in its lowest position, hanging quite perpendicular. The spindle of the governor has motion communicated to it by a cord or strap from the axis, shewn by the double line. This turns a perpendicular bevelled wheel, acting upon another, which is horizontal, to the centre of which the spindle is attached. governor here differs slightly in construction from that represented in Fig. 22, page 135, but is quite the same in principle. It acts upon the lever R, which will be observed, by tracing the course of the dotted lines connected with it, to act upon the throttle-valve t. The eccentric rod is seen passing from the axis behind the piston, to raise or lower the slidevalve. The rod and valve are similar in construction and action to those shewn in Figures 17, 18, and 20-21. The large and heavy FLY-WHEEL attached to the shaft, to equalise the motion when the force of the steam is increased or diminished, or the work to be done is increased or diminished, is observed at the right. By an endless cord or belt, moving round the shaft, and round the rod of any machinery, the motion is transmitted to the machinery. When the machinery consists of a number of separate parts, which work in

The

dependently, and require to be frequently stopped, or set in action at different times, belts or straps are provided for each, which can be slipped off or on in a moment.

294. This is the DOUBLE-ACTING engine of Watt. But his first invention was a SINGLE-ACTING enginethat is, one in which the piston was only propelled downwards by the force of steam, being pulled upwards by a weight attached to the other extremity of the beam, as in Newcomen's engine. The steam was admitted above the piston, the vacuum made below it. When the piston reached the bottom, the connections between the cylinder and boiler, and between the cylinder and condenser, were shut, and by the side tube a communication opened between the lower and upper part of the cylinder. The piston was then equally pressed in both directions by the steam, and the weight at the other end of the beam turned the balance, and pulled up the piston, the steam passing from above to below the piston during its ascent. Then the opening was formed to the condenser, steam admitted again. above the piston; and thus the motion was regularly continued.

295. These engines answered very well for raising water; and although the single-acting engine and the atmospheric engine had been adapted for machinery by the addition of a crank and fly-wheel, these were found insufficient to give that smoothness and uniformity of motion requisite. This was from the piston acting only in its descent, and a heavy fly-wheel or weight being necessary to carry the crank round during the intermission of the power. But still the atmospheric engine was in considerable use for this purpose. Mr Smeaton recommended that, instead of applying a crank and fly-wheel to the atmospheric engine, to produce a continued circular motion, the engine should be used to raise water, which should be applied in the

usual manner to turn a water-wheel, from the axis of which the continuous circular motion was procured. In 1781, he proposed the same with one of Boulton and Watt's single-acting engines, for a corn mill. Watt, in 1778, turned his attention to the subject, and, in 1782, he took out a patent for the DOUBLE-ACTING ENGINE. He says-" Having made my reciprocating engines very regular in their movements, I considered how to produce rotative motions from them, in the best manner; and, amongst various schemes which were subjected to trial, or which passed through my mind, none appeared so likely to answer the purpose as the application of the crank, in the manner of the common turning lathe ; but, as the rotative motion is produced in that machine by the impulse given to the crank in the descent of the foot only, it requires to be continued in its ascent by the energy of the wheel, which acts as a fly; being unwilling to load my engine with a fly-wheel heavy enough to continue the motion during the ascent of the piston, (or with a fly-wheel heavy enough to equalise the motion, even if a counterweight were employed to act during that ascent,) I proposed to employ two engines, acting upon two cranks, fixed upon the same axis, at an angle of 120° to one another, and a weight placed upon the circumference of the fly-wheel, at the same angle to each of the cranks, by which means the motion might be rendered nearly equal, and only a very light fly-wheel would be requisite.”

296. "The method," says Farey, "of combining two cylinders to act with two cranks formed upon the same axis, has since been brought into use, with great advantage, in the modern engines, for propelling carriages, and for steamboats. It is an excellent plan, and has also been applied to turn-mills." The atmospheric engine was also applied in this way-

two cylinders, each with a piston, being employed. The pistons had one common rod, the cylinders being placed one over the other: the air acted alternately on them, pushing one piston down, and the other up, the cylinder of the latter being inverted-closed above, and open below. Thus a double action was given to the piston. These, and other contrivances for double engines, have been superseded by Watt's beautiful invention the DOUBLE-ACTING SINGLE CYLINDER ENGINE.

297. Having now concluded our account of the chief of the inventions and improvements by Watt, and these being such as to give to the fire-engine a power and extent of usefulness quite unexampled, to confer on his engine the peculiar distinction which it still retains, of being THE steam-engine, we shall present a short sketch of the life of its illustrious author, who stands forth as one of the brightest examples of that which is alone entitled to honour and respect-talent successfully applied to produce works of utility to man.

298. JAMES WATT was born at Greenock, in the west of Scotland, in the year 1736. His father was a blockmaker and ship-chandler there, and was much respected by his townsmen. Watt was educated at the grammarschool of Greenock. After being some time with a mathematical instrument maker, he went to London in 1754, and there continued the same profession. About 1758, he returned to Scotland, and commenced business on his own account in Glasgow. The professors of the University there, being anxious to promote the art of making philosophical instruments, appointed Watt philosophical instrument maker to the University, and gave him apartments there, for carrying on his business. My attention was first directed," says Watt, "in 1759, to the subject of steam-engines by Dr Robison, then a student in the University of Glasgow, and nearly

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