274 Mr M. Ponton o)i the 



Additional Communication. * 



Since my former communication to the Society on this sub- 

 ject, Mr Bryson has had the kindness to adapt to the photo- 

 graphic thermometer a clock movement, so as to complete the 

 arrangements necessary for a half-hourly registration of the 

 temperature. The connexion is formed by means of a spindle, 

 terminating in a small pinion, and attached to the striking 

 train of the clock. The pinion works into a horizontal toothed 

 wheel, carrying the cylinder, on which is placed the sensitive 

 paper, and which is thus, by the action of the clock, moved 

 one division every half hour. 



Farther experiment having proved, that, by a more perfect 

 concentration of the light, a svifScient effect could be produced 

 on the paper by an exposure of four or five minutes, it ap- 

 peared advantageous to limit the exposure to this extent, by 

 providing for the raising and lowering of the gas flame, so that 

 it might be raised to its full pitch for about five minutes every 

 half hour, and continue burning Avith a very small flame during 

 the rest of the time. 



In order to obtain a better concentration of the light, I em- 

 ploy a lenticular piece of glass, 4^ inches in length, 3 in width, 

 and about \ inch thick in the middle. This produces a long 

 narrow light along the stem of the thermometer. I find it 

 advantageous, in using a fish-tail burner, to turn the edge, in- 

 stead of the flat side of the flame, towards the lens. 



For the purpose of raising and lowering the gas, the clock 

 is furnished with two hands, forming a diameter of the circle. 

 Over or under the centre on which these hands turn, is placed 

 a perpendicular lever, terminating at bottom in a small roller. 

 Proceeding from one end of the lever is a thread passing over 

 a pulley at a little distance, and having a small Aveight sus- 

 pended to it, by wlich the lever is pulled a little aside from 

 the perpendicular position. 



In the gas tube a knee is formed, and immediately over the 



* Read before the Society, 12th May 184ii. 



