Things being in this situation to produce a com mencement of motion, the first operation is to open all the four valves, G, H, K, L; (the injection cock being shut) the steam will drive the air out of the steam and exhaustion-pipes I and J, and out of the condenser M, through the blow-pipe and its valve 0, and as soon as this is succeeded by a sharp crackling noise in the little cistern 0, the valves are to be shut until it is thought that the steam which has entered is mostly condensed.
The same operation is to be repeated, giving a longer time to cool between the times of blowing, until it is found that, upon opening the injection-cock, some water will enter, and the barometer shall show some degree of exhaustion, after which, the repetition of blowing- will soon empty the cylinder of air.
The piston being then at the top of its stroke, the valves G and L are to be opened, and the fly-wheel m turned by hand about one-eighth of a revolution, or more, in the direction in which it is intended to move; the steam which is then in the cylinder will pass by L into the condenser, when, meeting the jet of water from the injection-cock, it will be converted into water, and the cylinder thus becoming exhausted, the steam, entering the cylinder by the valve G, will press upon the piston and cause it to descend, while, by its action .upon the ‘vorking.beam through the piston rod, Re., it pulls down the cylinder end of the beam, and raises up the outer end and the connecting rod h, which causes the planet-wheel i to tend to revolve round the sun-wheel j; but the former of these wheels, being fixed upon the connecting-rod so that it cannot turn upon its own axis, and its teeth being engaged in those of the sun-wheel, the latter, and the upon whose axle or shaft it is fixed, are made to re volve in the desired direction, and give motion to the mill-work.
As the piston descends, the plug-tree Z also de scends, and a clamp, or slider q, fixed upon the side of the plug-tree, presses upon the handle 1 of the upper V-shaft, or axis, and thereby shuts the valves G and L, and the same operation, by disengaging a decent, permits a weight suspended to the arm of the lower V-shaft to turn the shaft upon its axis, and thereby to open the valves K and 11. The moment previous to the opening of these valves, the piston had reached the lowest part of its stroke, and the cy linder above the piston was filled with steam; but as soon as 11 is opened, that steam rushes by the educ tion-pipe J, into the condenser, and the cylinder above the piston becomes exhausted. The steam from the
boiler entering by I and K, acts upon the lower side of the piston, and forces it to return to the top of the cylinder. When the piston is very near the upper termination of its stroke, another slider a raises the handle 2, and, in so doing, disengages the catch which permits the upper V-shaft to revolve upon its own axis, and open the valves G and L, and the down ward stroke recommences as has been related.
When the piston descends, the buckets R, T of the air-pump P and hot water pump T also descend. The water which is contained in these pumps passes through the valves of their buckets, and is drawn up and discharged by them through the lander or trough t, by the next descending stroke of the piston. Part of this water is raised up by the pump V, for the sup ply of the holler, and the rest runs to waste." The engine now described was one of fifty horse power, and was erected along with another of the same power, to drive twenty pair of mill stones, of which twelve or more pairs, with all the machinery for dressing the flour and other purposes, were gene rally kept at work.
Although Mr. Watt's double engines were chiefly employed in producing rotatory motions, yet in the case of very deep mines they may be applied with very great advantage to work pumps, by a reciprocating motion.
For this purpose one set, or half of the pump rods, are suspended by means of a sloping rod from the working beam near the cylinder, and the other half of these rods are suspended directly from the outer end of that beam, so that the ascending motion of the piston pulls up one half of these rods, and works the pumps to which they belong, while the descending motion of the piston pulls up the other half of the rods and works their pumps.
A double engine of this kind was erected at Wheel Maid Mine in Cornwall in 1787. It had a cylinder of sixty-three inches diameter, and nine feet stroke; hut the stroke in the pumps, which were eighteen inches in diameter, was only seven feet. When it was inconvenient to divide the pump rods into two sets, the ascending motion of the piston was employ ed to raise a weight equal to one half the column of water in the pumps, and this weight acted in addi tion to the power of the engine during the descending stroke of the piston.