78 Kotes on a Chronographic Apparatus. 



lower pivot resting on an end jewel, the upper pivot sup- 

 ported by a strong cast-iron bracket, and it is driven by a con- 

 trate wheel in the clock train, engaging into a pinion in the 

 lower end of a frame. The frame is open (as sho'»vn in 

 Fig. 1) to allow of the middle part of the axis of rotation 

 being clear for the evolute and the pendulum string or rod. 

 The evolute is fixed at M, and is capable of adjustment at 

 right-angles to the axis of rotation by a screw (Q), the proper 

 position of the curve in the other direction being practically 

 secured by careful workmanship, more especially in the con- 

 struction of the evolute itself 



" The pendulum consists of a spherical bob, weighing about 

 two and a half pounds, on a steel rod about one-tenth of an 

 inch thick, and suspended by a long and exceedingly thin 

 steel spring secured to the top of the evolute at N. 



" The regulation of the length of the pendulum is done in 

 the ordinary way with a nut at the bottom of the steel rod. 



" The governor thus made with ordinary care and work- 

 manship is by far the best of any of which I have had ex- 

 perience, and has furnished results better, I believe, than 

 any others used with chronographs ; at the same time it is 

 simple and inexpensive."* 



It is very necessary that the suspension-spring should 

 be of the thinnest steel possible, and T have found what is 

 known as French clock pendulum-spring to answer very 

 well. The adjustment of the evolute is a somewhat tedious 

 operation, but can be accomplished with great precision with 

 care. To get its proper position, if the tiiue of rotation 

 increases with an increase of arc — in other words, if it 

 revolves slower for increase of arc — the axis of the evolute 

 is beyond the axis of rotation (reckoning from the pen- 

 dulum side of the axis), and it is too near if it revolves 

 more rapidly for increase of arc. Of course for each alter- 

 ation of the position of the evolute a considerable alteration 

 of the length of the pendulum becomes necessary, and this 

 somewhat complicates the adjustment ; but with a barrel 

 chronograph this is easily overcome by alternately increasing 

 and diminishing the arc of the pendulum by adding to and 

 subtracting from the driving weight. 



* Extract from Monthly Notices of the Roval Astronomical Society 

 page 72, Vol. XXXVI. 



