Photographic Begister Thermometer. 275 



place -where the gas passes from the perpendicular into the 

 horizontal tube, there is a small rectangular chamber ; and the 

 termination of the perpendicular pipe is a flat rectangular 

 piece of brass at the bottom of the chamber, having a hole in 

 its centre for the admission of the gas. On this flat surface 

 rests a small rectangular piece of iron, reaching to about yV^h 

 or aVth of ^D iiich from the top of the chamber which is closed. 

 Above the flat top of the chamber is suspended, on an axis, a 

 small horse-shoe magnet, with a lever several inches long pro- 

 jected from its upper curvature. To the end of this lever is 

 attached a thread, which proceeds to the upper limb of the 

 lever above the clock. By this attachment the magnet is 

 drawn aside from the chamber in the gas tube, and, conse- 

 quently, the piece of iron in that chamber being unaffected 

 by the magnet, rests over the orifice for the admission of the 

 gas, which it closes to such an extent as to admit only a mi- 

 nute portion, sufficient to keep up a very small flame. 



Wlien the hands of the clock approach the perpendicular 

 position, the upper hand presses against the roller at the bot- 

 tom of the lever ; and, as the hand moves round, it carries the 

 lever with it, and thus raises the small weight above men- 

 tioned, at the same time slackening the thread attached to the 

 magnet, which being thus at freedom to move, is attracted 

 towards the iron in the chamber. So soon as the poles of the 

 magnet reach the top of the chamber the piece of iron jumps 

 up towards them, and thus lets on the full flow of the gas, 

 raising the flame to its highest pitch at once. This state of 

 matters remains while the hand of the clock moves onwards 

 for about five minutes, carrying the lever with it, and so con- 

 tinuing to raise the small weight. At the expiration of the 

 five minutes the hand of the clock passes the lever, which in- 

 stantly returns to its original position, and, in consequence of 

 the accumulation of power attending the fall of the weight 

 from the height to which it has been raised, the lever returns 

 with such force as to jerk the magnet away from the chamber 

 in the gas pipe, and thus allow the piece of iron in that 

 chamber to drop into its original position, and thereby reduce 

 tlie gas flame at once to its lowest point. 



