110 THE SCIENTIFIC PAPERS OF 



prevent an occasional collapse of the pendulum in opening the 

 valve. The sustaining- or driving-power of the pendulum was 

 obtained in taking advantage of the differential motion, a weight 

 being attached to a horizontal lever upon the throttle-valve 

 spindle, which, in exerting a pressure of the differential wheel 

 against the two principal wheels, caused an impulse to be given 

 to the one connected with the pendulum, whereas the third wheel 

 was driven by the engine in the direction proper to raise the 

 weight continually. The excess of driving-power imparted to the 

 pendulum was destroyed by calling into action a friction between 

 two solid substances at the moment when the angle of rotation had 

 reached its intended maximum ; or a liquid resistance was intro- 

 duced, such as a fan, rotating with the pendulum, being dipped 

 into a bath of mercury or other liquid, at the moment when the 

 extension of the pendulum was attained (see Plate 19, Fig. 2). 



The governors constructed on this principle are remarkable for 

 their instantaneous action upon the supply-valve of the engine, 

 when a sudden disturbance of the balance between load and power 

 takes place ; and they possess also, to the fullest extent, the power 

 of maintaining the regulated machine at the same speed, when the 

 load reaches its maximum, as when it is at its minimum. In the 

 hands of the Astronomer Royal the uniformity of motion obtained 

 by the use of this governor approaches indeed that of an ordinary 

 chronometer, yet it is not free from objections resulting from the 

 delicate adjustment and frequent attention requisite to maintain 

 it in good working condition. In its application to the regulating 

 of physical apparatus it has, moreover, been found that the conical 

 pendulum is apt to fall into elliptical rotation, whereby the sub- 

 divisions of time below the half second become inaccurate. 



FOUCAULT'S GOVERNOR. Monsieur Foucault, of Paris, has 

 treated the same question in a different manner, substituting for the 

 friction between solids, or of solids against fluids which we 

 employed to fix the angle of rotation of the conical pendulum, 

 the varying resistance of an air-propeller, according to the amount 

 of area for the escape of air, which area he regulates by means of 

 the angular elevation of his conical pendulum. The conical 

 pendulum he employs is mounted in the manner of a Watt's 

 governor of the best construction, which renders his apparatus 

 portable, and therefore convenient for general purposes, while, on 



