“I have made a very large model of the new substitute for racks and sectors, which seems to bid fair to answer. The rod goes up and down quite in a perpendicular line without racks, chains, or guides. It is a perpendicular motion derived from a combination of motions about centres--very simple, has very little friction, has nothing standing higher than the back of the beam, and requires the centre of the beam to be only half the stroke of the engine higher than the top of the piston-rod when at lowest, and has no inclination to pull the piston-rod either one way or another, only straight up and down.... However, don’t pride yourself on it--it is not fairly tried yet, and may have unknown faults.”[263] [263] The parallel motion was first put in practice in the engine erected for Mr. Whitbread; Watt informing Boulton (27th October, 1785) that “the parallel motion of Whitbread’s answers admirably.” [Illustration: THE GOVERNOR.] Another of Watt’s beautiful inventions of the same period, was the Governor, contrived for the purpose of regulating the speed of the engine. This was a point of great importance in all cases where steam-power was employed in processes of manufacture. To modify the speed of the piston in the single-acting pumping-engine, Watt had been accustomed to use what is called a throttle valve, which was regulated by hand as occasion required. But he saw that to ensure perfect uniformity of speed, the action of the engine must be made automatic if possible, and with this object he contrived the Governor, which has received no improvement since it left his hand. Two balls are fixed to the ends of arms connected with the engine by a moveable socket, which plays up and down a vertical rod revolving by a band placed upon the axis or spindle of the fly-wheel. According to the centrifugal force with which the balls revolve, they diverge more or less from the central fixed point, and push up or draw down the moveable collar; which, being connected by a crank with the throttle-valve, thereby regulates with the most perfect precision the passage of the steam between the boiler and the cylinder. When the pressure of steam is great, and the tendency of the engine is to go faster, the governor shuts off the steam; and when it is less, the governor opens the throttle-valve and increases the supply. By this simple and elegant contrivance the engine is made to regulate its own speed with the most beautiful precision.