276 Mr M. Ponton on the Photographic Thermometer. 



By these means the raising and lowering of the gas is ac- 

 complished with less strain upon the clock than it would be 

 were a stop-cock employed. 



In order to prevent the action of extraneous light during 

 the day, it has been found advisable to place between the lens 

 and the thermometer a small screen, v/hich is opened and shut 

 by the action of the same lever that raises and lowers the gas. 

 The thermometer is thus exposed only while the gas flame is 

 at its height. 



For the successful production of the images of the mercurial 

 column in the thermometer tube, every thing depends on the 

 light being made to fall upon it at the proper angle, so as to 

 obliterate the shadow from the empty part of the bore, and 

 increase the shadow produced by the fall portion. Unless this 

 position be hit the images will not be distinct. 



As it appeared desirable to have it in our power so to ar- 

 range matters that the clock might be inside of the house, 

 while the thermometer and registering cylinder stand outside, 

 I have contrived a method by which this may be accomplished. 

 It is on the same principle as that by which the gas is raised 

 and lowered, and may be managed by the same lever, so that 

 the shifting of the cylinder takes place at the same instant that 

 the gas is lowered, just after the registration has been com- 

 pleted. The only connection between the clock and the cylin- 

 der is a thread, which may pass through the window. I have 

 iiad this plan in action, but the apparatus requires some little 

 rtlteration. 



By this arrangement it is easy to cause the same clock re- 

 gister both a thermometer and a barometer, or even a barome- 

 ter and two thermometers, one placed in the sun, the other in 

 the shade ; while the registration of the barometer may be 

 either by the photographic method, or by the mode previously 

 invented by Mr Bryson. 



The Committee appointed to examine Mr Ponton's Photo- 

 graphic Thermometer, report very favourably. 



