Mr. Watt proposed to remedy this defect by placing a cylinder under each end of the great beam, in order that one of the pistons should be rising while the other was falling; and as both the cylinders were to be sup plied from the same boiler, and the steam condensed in the same condenser, it was probable that these two actions on the beam would be regular and uniform.
'['his plan, however, was we believe never carried into effect, and Mr. Watt resolved upon using only one cylinder, and upon giving the engine a double ac tion by introducing steam both above and below the piston, and thus forcing the piston both upwards and downwards. For this double engine he took out a patent in 1782, but as it was afterwards greatly im proved, we shall describe one of those which existed at a later date for the Albion Mills. In his specifica tion of 1782, the piston rod gave motion to the beam by a toothed rack, at the upper end of the rod, which wrought in a toothed arch, which replaced the arch head at the end of the beam, and the irregularities of this action were equalized by a fly-wheel placed near the top of the piston rod. This was, however, soon laid aside, and the contrivance adopted which is now in universal use. The following are the parts of this engine (See Plate DVIII. Fig. 1.) : The boiler 21, in which the water is converted into steam by the furnace p g, is most frequently made of iron though sometimes of copper. Its bottom is con cave, and the flame is made to circulate round its sides in the flue o o, and is sometimes conveyed through the very middle of the water by a tube o, in order that a great surface may be exposed to the ac tion of the fire. In some of Mr. Watt's engines, the fire contained in a vessel of iron was introduced into the very middle of the water.
Before proceeding to describe the operation of Mr. Watt's double engine, we must first give an account of the construction of the regulating valves, and the manner in which the plug-tree acts upon their levers in opening and shutting them. A section of the regulator box is shown in Fig. 2. Plate DVIII, by the letters HHJJ. The opening into the cylinder is shown at HH. A spindle A passes through one side of the box, and upon this as an axis moves a toothed sector B, which works into the toothed rack C fixed to the brass valve D, fitted by grinding to its seat EE, F being the guide for the stem D d of the valve. On the spindle A is fixed a lever LA, jointed at L to the rod LM, which is again jointed at M to the arm MN of a bent lever MNO, movable round the axis N.
Part of the plug-tree is shown at QY., and P is one of its pins. From this description it is obvious that the valve D will be opened or shut by the ascent or de scent of the plug-tree QR, the pins or which act upon the lever NO. When the pin P pushes down the spanner NO, the arm MN rises to the right hand, and pulls down the spanner AL by means or the uniting rod 1\ IL; these parts are so arranged that when the cock is shut, LM and MN form one straight line as in the figure. hence %%hen the spanner begins to raise the valve, its mechanical energy is almost infinitely great, and from the same cause, if any thing should try to open the valve it would be ineffectual.
By means of the steam pipe F, Plate DVIII. Fig. 1, the steam is conveyed from the boiler 21 to the cross pipe or upper steam nozle G, and by the perpendicu lar steam pipe I, to the lower steam nozle K. In the nozle G there is a valve (D. Fig. 2.) which, when open, admits steam into the cylinder above the piston B, through the horizontal square pipe at the top, of the cylinder, and in the lower steam nozle K there is another valve like that at D, which, when opened by the pin of the plug-tree, admits steam into the cylinder below the piston. In the zipper exhaustion nozle H there is a similar valve, which, when open, allows steam to pass from the cylinder above the piston into the eduction pipe J, which conveys it to the con densing vessel M, where it meets the jet of the in jection from the cock N, and is reduced to water; and in the lower exhaustion nozle L there is also a valve, which, when open, allows the steam to pass out of the cylinder below the piston into the condenser M.
The engine, says Mr. Watt, being at rest, the cylinder quite cold, and the condenser cistern full of water, when the water in the boiler begins to boil, steam will enter by the small pipe f into the space be: tween the cylinder and the heating-case E, which will expel the air contained in that space, and between the two bottoms of the cylinder, at a cock fixed in the outer bottom, which, when all the air is expelled, and the cylinder thoroughly warmed, is to be shut, and the water which may be formed in these spaces during the working of the engine, will issue by the inverted syphon e.