140 THE SCIENTIFIC PAPERS OF 



plained, there would be no tendency for the balls to stop in their 

 movement if the governor was perfectly chronometric, but they 

 would fly at once from one extremity of their range to the other under 

 the slightest alternations of speed. The common governor was the 

 one that was least affected by change- of speed, and required the 

 greatest amount of change to bring it into action ; but it had the 

 advantage of not allowing the engine to " hunt " or beat about 

 between the extremes of speed before settling down to the proper 

 rate ; it was thus a safe governor in this respect, though a bad one 

 in others. The original invention of Watt, which had also this ad- 

 vantage, and, moreover, allowed of only a slight variation in speed, 

 occupied a sort of mean position between the common governor 

 and a parabolic one, and was therefore the best practical governor 

 hitherto applied ; and the same might now be said of the modified 

 construction of the crossed-arm governor, in which the spiral 

 spring was added as described in the paper. The addition of the 

 spiral spring, however, did away with the true chronometric 

 character of the governor, because the spring acted in conjunction 

 with gravity to depress the. balls, without being influenced by the 

 cross-suspension ; and therefore a great increase of speed was still 

 required to raise the balls, and the governor was thus rendered a 

 practical one, instead of being a mere theoretical abstraction, as 

 would be the case without the spring. A farther advantage of the 

 spring, when aiding gravity to depress the balls, was that the 

 centrifugal force on the balls was thereby increased ; if, for 

 instance, a speed of 40 revolutions per minute was sufficient to 

 maintain the balls at their mean angle when no spring was used, 

 and if with the spring 60 revolutions per minute were requisite to 

 keep them in the same position, the centrifugal force residing in 

 the balls in these two cases would be in the proportion of the 

 square of 40 to the square of 60, or as 4 to 9, and the regulating 

 power for a given variation of speed would be increased in the 

 same proportion. A smaller relative variation in speed therefore, 

 in the governor provided with the spring, would be sufficient 

 for enabling the balls to overcome the constant resistance of 

 the valve-levers and valve ; and this he believed to be one 

 of the reasons why the crossed-arm governor described in the 

 paper acted so well. The only kind of governor that was capable 

 of acting instantly upon the throttle-valve at the moment of the 



