388 Royal Society : — 



ensures a constant normal velocity. The break is : — in the first 

 place, the variable friction between the combination of levers and 

 the ring on the shaft on which the force is made to act ; and, in 

 the second place, a centrifugal air-fan through which more or less 

 air is allowed to pass, according to the position of the levers. 

 Both these causes tend to regulate the velocity according to the 

 same law. 



The governors designed by the Astronomer Royal on Mr. Siemens' s 

 principle for the chronograph and equatorial of Greenwich Obser- 

 vatory depend on nearly similar conditions. The centrifugal piece 

 is here a long conical pendulum, not far removed from the vertical, 

 and it is prevented from deviating much from a fixed angle by the 

 driving-force being rendered nearly constant by means of a differential 

 system. The break of the pendulum consists of a fan which dips 

 into a liquid more or less, according to the angle of the pendulum 

 with the vertical. The break of the principal shaft is worked by the 

 differential apparatus ; and the smoothness of motion of the principal 

 shaft is ensured by connecting it with a fly-wheel. 



In the third kind of governor a liquid is pumped up and thrown out 

 over the sides of a revolving cup. In the governor on this principle, 

 described by Mr. C. W. Siemens, the cup is connected with its axis 

 by a screw and a spring, in such a way that if the axis gets ahead 

 of the cup the cup is lowered and more liquid is pumped up. If 

 this adjustment can be made perfect, the normal velocity of the cup 

 will remain the same through a considerable range of driving-power. 



It appears from the investigations that the oscillations in the 

 motion must be checked by some force resisting the motion of oscil- 

 lation. This may be done in some cases by connecting the oscillating 

 body with a body hanging in a viscous liquid, so that the oscillations 

 cause the body to rise and fall in the liquid. 



To check the variations of motion in a revolving shaft, a vessel filled 

 with viscous liquid may be attached to the shaft. It will have no 

 effect on uniform rotation, but will check periodic alterations of speed. 



Similar effects are produced by the viscosity of the lubricating 

 matter in the sliding parts of the machine, and by other unavoidable 

 resistances ; so that it is not always necessary to introduce special 

 contrivances to check oscillations. 



I shall call all such resistances, if approximately proportional to 

 the velocity, by the name of "viscosity," whatever be their true 

 origin. 



In several contrivances a differential system of wheelwork is intro- 

 duced between the machine and the governor, so that the driving- 

 power acting on the governor is nearly constant. 



I have pointed out that, under certain conditions, the sudden dis- 

 turbances of the machine do not act through the differential system 

 on the governor, or vice versa. When these conditions are fulfilled, 

 the equations of motion are not only simple, but the motion itself is 

 not liable to disturbances depending on the mutual action of the ma- 

 chine and the governor. 



