on the terrestrial Rays that occasion Heat. 313 



No. 1. No. 4. 



Burning Lenta Screened. 



31'x 68 <JiJ 



35 $9* 6 ^ 



And here the thermometer received if degree of heat again ; 

 so that, in the course of 35 minutes, the thermometer No. 1 

 was alternately raised and depressed five times, by rays which 

 came from the chimney fire, and were subject to laws of refrac- 

 tion, not sensibly different from those which affect light. 



15th Experiment. Refraction of the Heat of red-hot Iron. 



I caused a lump of iron to be forged into a cylinder of 2~ 

 inches diameter, and 2-§- inches long.* This, being made red- 

 hot, was stuck upon an iron handle fixed on a stand, so as to 

 present one of its circular faces to a lens placed at 2,8 inches 

 distance; its focus being 1,4 inch, and diameter 1,1. Before 

 the lens, at some distance, was placed a screen of wood, with a 

 hole of an inch diameter in it, by way of limiting the object, 

 that its image in the focus might not be larger than necessary. 

 The screen also served to keep the heat from the thermometers. 

 No. 2 was situated in the secondary focus of the lens ; and 

 No. 3 was placed within T 3 ^ of an inch of it, and at the same 

 distance from the lens as No. 2. By this arrangement, both 

 thermometers were equally within the reach of transmitted 

 heat ; or, if there was any difference, it could only be in favour 

 of No. 3, as being behind a part of the lens which, on account 

 of its thinness, would stop less heat than the middle. Now, as 

 the experiment gives a result which differs from what would 

 have arisen from the situation of the thermometers, on a sup- 

 position of transmitted heat, we can only ascribe it to a conden- 



« See Plate XVI. Fig. i. 



Ss 2 



