Notes on a Ghronographic Apparatas. 77 



transmitted by a miniature key operated by the mechanism 

 of a clock or chronometer. 



The syphon pen really marks a continuous line, which is 

 interrupted every second by a small "offset" or "tooth" 

 and constitues the " mark ;" and an " offset" is left out once 

 in every complete revolution of the barrel, every minute 

 in fact, at the same time the little carriage carrying the pen 

 and magnet is continually progressing in the direction of 

 the length of the barrel, at the rate of about one-tenth of 

 an inch per minute, converting the continuous line into a 

 spiral on the cylinder. 



I described a chronograph to this Society about 13 or 14 

 years ago, and as the principle in this is much the same as 

 in the one then described, and very similar to other barrel 

 chronographs — such as Bond's, Hipps', &c. — it will not be 

 necessary to refer to any details except the pendulum, 

 which in this case is the only new or peculiar arrangement. 



" Let A A (Fig. 1.) be a vertical axis of rotation, which 

 can be driven by clockwork acting at the top or bottom of 

 the axis; from this axis a pendulum (P) is suspended in 

 such a way that when it hangs vertically the string (S) lies 

 wrapped over a curved surface, which forms part and parcel 

 of the vertical axis. This curve is the evolute of a para- 

 bola, whose distance from vertex to focus is half the length 

 of the required pendulum (when vertical). Now, let the 

 axis revolve, and the pendulum will fly out from its vertical 

 position, more or less, according to its weight and the driving 

 power ; the arc described by the pendulum, as it increases 

 its distance from the vertical, will be a parabola, by reason 

 of the string gradually unwrapping from the evolute (E). 

 Now, from the properties of the parabola, it follows that the 

 vertical distance between the centre of rotation of the pen- • 

 dulum (P) and the intersection of the string (S) with the 

 axis of rotation of the pendulum will remain constant ; and 

 therefore that the length of the pendulum remains constant 

 at whatever arc it may rotate. 



"To practically secure these conditions it is necessary, 

 first that the evolute shall be properly and precisely made ; 

 and secondly, that it shall be so adjusted that the axis of 

 the evolute and involute shall be coincident with the axis 

 of rotation. 



" The pendulums I had constructed are half -seconds, that 

 is, rotating once in a second. They are suspended in a hard 

 gun-metal frame (Fig. 2), pivoted at the top and bottom, the 



