272 Mr M. Ponton o« the 



use. The application of the sulphate of iron to the paper may 

 be made at the distance of upwards of 24 hours from the time 

 of the first impression of the latent image, which will, not- 

 withstanding that lapse of time, come out distinctly ; and thus 

 a whole day's record may be brought out at once. The aid of 

 a little heat is sometimes necessary for the development of the 

 image. 



The mechanical arrangements are these : — A black japanned 

 cylinder of tin, about 4 inches diameter, and 4^ inches deep, 

 has a piece of the sensitive paper wrapped round it. This cy- 

 linder is intended to be moved round by a time-piece, and to 

 traverse behind the stem of the thermometer. It will be sta- 

 tionary for a half or a quarter of an hour as desired, at each 

 division of the cylinder, and then be moved a division by a 

 jerk. There Avill thus be time for the image to be completely 

 formed by the action of the light. Around the cylinder carry- 

 ing the sensitive paper is another blackened cylinder, 4^ inches 

 diameter and 4 inches deep, with a slit in it just sufficient to 

 admit the stem of the thermometer. This is intended to screen 

 from the light all that portion of the paper which is not in 

 action. A cover goes over the whole, the more effectually to 

 exclude all light except that which passes through the stem of 

 the thermometer. The stem itself is also furnished with wings 

 of black paper, to prevent the light from spreading on either 

 side. 



The thermometer should have its stem twice the length of 

 what is required for the natural range of temperature, so that 

 the bulb may be placed at a considerable distance from the 

 portion acted on by the light, in case of any increase of tem- 

 perature from that cause ; and the bulb and lower portion of 

 the stem should be completely screened from the light. This 

 is accomplished by placing the thermometer in a wooden box, 

 the bulb and the lower half of the stem being in the box, 

 which is open behind to admit the air, while the upper portion 

 of the stem, intended for use, is left standing above the box. 

 The time-piece may be placed in this box if convenient. 



The cylinders already described, are placed behind the ex- 

 posed portion of the stem, so that the image of the bore may 



