,‘ This contrivance was applied to many engines, and possesses the great advantage of giving a double ve locity to the fly ; but is perhaps more subject to wear, and to he broken under great strains, than the crank, which is now more commonly used, although it re quires a fly-wheel of four times the weight, if fixed upon the first axis. My application of the double en gine to these rotative machines rendered unnecessary the counter-weight, and produced a more regular mo tion; so that, in most of our great manufactories, these engines now supply the place of water, wind, and horse mills ; and instead of carrying the work to the power, the prime agent is placed wherever it is most convenient to the manufacturer." But whether the rotatory motion is produced by the simple crank,* or by the sun and planet wheel, there is an irregularity in its action, which it is neces sary to correct. The force of the planet wheel to drive the sun wheel is constantly varying. In the position shown in Fig. 1, Plate DVIII, and also in the position when the planet wheel is exactly above the sun wheel, it is nothing. It then gradually in creases till the centres of the two wheels are in the same horizontal line, and from that point it again di minishes. In the two positions when the force is no thing, we might expect that the engine would stop, as the action of the steam tends only to press the axis of the planet wheel against that of the sun wheel. But as the planet wheel, when once put in motion, has a tendency to go on, it never can stop at these two po sitions of minimum force, and the instant it passes them it acquires fresh power to continue its motion. Iris obvious, however, that such a motion must be irregular, being slow in one position and quick in another.
In order to remedy this evil, a large and heavy fly wheel in is fixed on the axis of the sun wheel j. This heavy wheel, when once put in motion, equalises the irregular motion of the planet wheel in the manner which we have already fully explained in our article MEennxics, (Vol. MI. p. 666.) The other contrivances invented by Mr. Watt we shall describe in his own words.
"The Parallel Motion, in the single engines, serves in place of chains, and in the double engines, supplies the place of the rack and sector. It has been mentioned, that the ^acks and sectors were very subject to wear, and that, when perfect, they did not move with that smoothness that was wished ; and to chains there were many objections. It occurred to Mr. Watt, that if some mechanism could be devised moving upon centres, which would keep the piston-rods perpendicu lar, both in pushing and pulling, a smoother motion would be attained ; and, in all probability, that the parts would be less subject to wear. After some con sideration, it occurred to him, that if two levers of equal lengths were placed in the same vertical plane, nearly, as shown in Fig. 3 of plate DVIII, moveable on the centres B and C, and connected by a rod A D, the point E, in the middle of that rod, would de scribe nearly a straight and perpendicular line, when the ends A and D of the levers, and of that rod, moved in the segments of circles FG, and IH, provided the arch FG did not much exceed 40 degrees, and conse quently that if the top of the piston-rod were attached to that point E, it would be guided perpendicularly, or nearly so.
It necessarily followed, that if for convenience the lever CD (which represents what he called the regu lating-radius) were made only half the length of the lever AB (which represents the half length or radius of the working-beam) a point situated at one-third of the ,length of the rod AD, from the joint A, would then move in a perpendicular line. These were first ideas, but the parallel motion was soon moulded into the form in which it appears in all Boulton and Watt's engines, and in which it is seen in Plate D VIII, Fig. 1, of the second engine at the Albion Mill. A patent for the protection of this, and some other of Mr. Watt's invention, passed the seals in April 1784, but the invention was made in the latter end of 1783.
The regulation of the speed of the rotative engines, is a matter essential to their application to cotton spinning,• and many other manufactories.
It is performed by admitting the steam into the cylin der more or less freely, by means of what is called a which is commonly a circular plate of metal A, Fig. 4, having a spindle B fixed across its diameter.
This plate is accurately fitted to an aperture in a metal ring CC, of some thickness, through the edge way of which the spindle is fitted steam-tight, and the ring is fixed between the two flanches of the joint of the steam-pipe which is next to the cylinder. One end of the spindle, which has a square upon it, comes through the ring, and has a spanner fixed upon it by which it can be turned in either direction.
When the valve is parallel to the outsides of the ring, it shuts the opening nearly perfectly ; but when its plane lies at an angle to the ring, it admits more or less steam according to the degree it has opened ; consequently the piston is acted upon with more or less force. For many purposes engines are thus reg ulated by hand at the pleasure of the attendant ; but where a regular velocity is required, other means must be applied to open and shut it, without any at tention on the part of those who have the care of it. For this purpose Mr. Watt had various methods, but at last fixed upon what he calls the Governor, (shown at W. Plate DVIII, Fig. I.) consisting of a perpen dicular axis, turned by the engine : To a joint near the top of this axis are suspended two iron rods, car rying heavy balls of metal at their lower ends, of the nature of pendulums. When this axis is put in mo tion by the engine, the balls recede from the perpen dicular by the centrifugal force, and by means of a combination of levers fixed to their upper end, raise the end of a lever which acts upon the spanner of the throttle-valve, and shuts it more or less according to the speed of the engine, so that as the velocity aug ments, the valve is shut, until the speed of the en gine and the opening of the valve conic to a maximum and balance each other.