This apparatus is represented in fig. 15.; L is a perpendicular shaft or axle to which a wheel, M, with a groove is attached. A strap or rope, which is rolled upon the axle of the fly-wheel, is passed round the groove in the wheel M, in the same manner as the strap acts in a turning lathe. By means of this strap the rotation of the fly-wheel will produce a rotation of the wheel M and the shaft L, and the speed of the one will always increase or diminish in the same proportion as the speed of the other. N, N are two heavy balls of metal placed at the ends of rods, which play on an axis fixed on the revolving shaft at O, and extend beyond the axis to Q Q. Connected with these by joints at Q Q are two other rods, Q R, which are attached to a broad ring of metal, moving freely up and down the revolving shaft. This ring is attached to a lever whose centre is S, and is connected by a series of levers with the throttle-valve T. When the speed of the fly-wheel is much increased, the spindle L is whirled round with considerable rapidity, and by their natural tendency[20] the balls N N fly from the centre. The levers which play on the axis O, by this motion, diverge from each other, and thereby depress the joints Q Q, and draw down the joints R, and with them the ring of metal which slides upon the spindle. By these means the end of the lever playing on S is depressed, and the end V raised, and the motion is transmitted to the throttle-valve, which is thereby partially closed, and the supply of steam to the cylinder checked. If, on the contrary, the velocity of the fly-wheel be diminished, the balls will fall towards the axis, and the opposite effects ensuing, the supply of steam will be increased, and the velocity restored. [Footnote 20: The _centrifugal force_.] [Illustration: Pl. IV. WATT'S SINGLE ACTING STEAM ENGINE.] The peculiar beauty of this apparatus is, that in whatever position the balls settle themselves, the velocity with which the governor revolves must be the same,[21] and in this, in fact, consists its whole efficacy as a regulator. Its regulating power is limited, and it is only small changes of velocity that it will correct. It is evident that such a velocity as, on the one hand, would cause the balls to fly to the extremity of their play, or, on the other, would cause them to fall down on their rests, would not be influenced by the governor.