Such is a general description of the new engine as improved by Mr. Watt, but some of its individual parts still remain to be noticed.
The heat of these contrivances is the .Sun and Planet Wheel, which is shown in Fig. 1, Plate DVIII, where the whe'el j is called the Sun Wheel, and i the Planet Wheel. The sun wheel j is fixed in a horizontal axis, to which it is required to communicate a continuous rotatory motion by the reciprocating motion of the rod h, suspended from one extrt'm'oy of the great beam. The planet wheel i is fixed to the lower end of the rod h, so that it cannot turn round its axis. ne sun and planet wheels are of the same size, move in the same vertical plane, and have the same number of teeth; and their centres are connected together by an iron strap, which prevents them from quitting each other. If we suppose the rod it to rise with the end of the beam which carries it, the teeth on the inner or left hand side of the planet wheel i will obviously work in the teeth of the inner or left hand side of the sun wheel j. When the centre of the planet wheel comes on a level with the centre of the sun wheel, it will then have performed one-fourth of a revolution, so that the two wheels will have made ore quarter of a revolution in respect to one another; but the planet wheel will also have made one-fourth of a revolution in its orbit round the sun wheel, and have carried the sun wheel along with it, so that the latter will have completed half a revolution. In like manner when the centre of the planet wheel is above the centre the sun wheel, the former will have completed half a revolution, and the latter a whole revolution. This double velocity of the sun wheel arises from the mo tion of the planet wheel around it. If the planet wheel were twice the size of the sun wheel, the latter would perform three revolutions for one of the former. See Mnenvxics, Vol. XII., p. 660.
The following is the history of this beautiful con trivance as given by Mr. Watt himself : Having made my reciprocating engines very re gular in their movements, I considered how to pro duce rotative motions from them in the best manner; and amongst various schemes which were subjected to trial, or which passed through my mind, none ap peared so likely to answer the purpose as the applica tion of the crank in the manner of the common turn ing lathe, (an invention of great merit, of which the humble inventor, and even its aera, are unknown).
But, as the rotative motion is produced in that ma chine by the impulse given to the crank in the descent of the foot only, and behoves to be continued in its as cent by the momentum of the wheel which acts as a fly, and being unwilling to load my engine with a fly heavy enough to continue the motion as cent of the piston, (and even were a counter-weight employed to act during that ascent, of a fly heavy enough to equalize the motion),I proposed to employ two engines acting upon two cranks fixed on the same axis at an angle of 120 degrees to one another, and a weight placed upon the circumference of the fly at the same angle to each of the cranks, by which means the motion might be rendered nearly equal, and a very light fly would only be requisite. This had occurred to me very early, but my attention being fully em ployed in making and erecting engines for raising water, it remained in petto until about the year 1778 or 9, when Mr. Wasbrough erected one of his ratchet wheel engines at Birmingham, the frequent breakages and irregularities of which recalled the subject to my mind, and I proceeded to make a model of my method, which answered my expectations: but having neglect ed to take out a patent, the invention was communi cated by a workman employed to make the model to some of the people about Mr. Wasbrough's en gine, and a patent was taken out by them for the ap plication of the to steam-engines. This fact the said workman' confessed, and.the engineer who di rected the works acknowledged it, but said, never theless, the same idea had occurred to him prior to his hearing of mine, and that he had even made a model of it before that time, which might be a fact, as the application to a single crank was sufficiently obvious. In these circumstances I thought it better to endeavour to accomplish the same end by other means, than to enter into litigation, and, if successful, by demolishing the patent, to lay the matter open to every body. 'Accordingly, in 1781, I invented and took out a patent for several methods of producing rotative motions from reciprocating ones, amongst which was the method of the sun and planet wheels.