Notes on a Cltronograjjhic Apparatus. 75 



play by the pendulum itself as its arc increases beyond a 

 certain limit ; but as giving the pendulum any work of this 

 kind to do leaves it no longer free, it becomes simply a 

 " make shift," and can onl}^ approach uniformity within 

 larger limits than should be nowadays admissible. 



The most successful " governors " of this class hitherto 

 constructed appear to be those where the motion of the 

 mechanism is rendered approximately uniform b}^ the fly, and 

 then finally controlled by a reciprocating pendulum, as in 

 " Bond's Spring Governor," or " Cook's Governor," where a 

 driven train of wheels is governed by a fly, but pulled up 

 every half-second by a vibrating pendulum ; the pulJing-up 

 being made as gradual as possible by means of a light spring 

 or weight inserted between the fly and the pendulum, allow- 

 ing the former to continue revolving with increasing resist- 

 ance until the latter allows its wheel to escape and so free 

 the fly. These are practically the best forms of chrono- 

 graphic governors in general use, but as there is a periodic 

 error of half a second inherent in them they are really im- 

 perfect. 



There is a form of governor which almost secures uniform 

 rotation, namely the vibrating spring ; and the more rapid 

 the vibrations are the more nearly perfect is the result. 

 Some chronogi'aphs have been made on this plan, and are 

 known as Hipps' Chronographs. They consist of a driven 

 train and registering barrel, governed by a flat, straight steel 

 spring, whose end just touches the ends of the teeth of a wheel, 

 but which by a little rotatory force in the wheel can be pushed 

 or bent so as to allow the teeth to pass it one after another ; 

 the rate at which the wheel rotates being governed by the 

 natural time of vibration of the spring, which is constant at 

 the same temperature, and the rotation of the train is there- 

 fore uniform, except for the small periodic error of which the 

 time of the spring's vibration is the measure. In practice, 

 however, I believe the escape-wheel sometimes sKps or runs. 

 The noise, too, caused by the vibration of the spring is 

 almost intolerable, and one of the American observers at the 

 late transit of Yenus told me he had to dig a big hole in the 

 ground, place the apparatus in it, and cover it over before he 

 could bear the din. 



Siemens proposed a "governor" where the control was 

 afforded by the varying friction of a fluid in a rotating para- 

 bolic cup. This, although theoretically excellent, does not 

 appear to have given satisfactory results in practice. 



