VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 701 



This again confirms the same, by loss of 3° of N° 1. N*4. 



, ° , . . 111 Min. Burning Lens. Screened. 



heat. I restored the lens once more, and had as an- 31 i gg g t M 



nexed. 35 69I 6l\ 



And here the thermometer received 14. degree of heat again ; so that, in the 

 course of 35 minutes, the thermometer N° 1 was alternately raised and depressed 

 5 times, by rays which came from the chimney fire, and were subject to laws of 

 refraction, not sensibly different from those which affect light. 



Exper. 15. Refraction of the Heat of Red-hot Iron. — I caused a lump of iron 

 to be forged into a cylinder of 24- inches diameter, and 24. inches long, fig. 9. 

 This, being made red-hot, was stuck on 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. N° 2 was situated in the 

 secondary focus of the lens; and N° 3 was placed within -^ of an inch of it, and 

 at the same distance from the lens as N° 2. By this arrangement, both ther- 

 mometers were equally within the reach of transmitted heat; or, if there was any 

 difference, it could only be in favour of N° 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 supposition of transmitted heat, we can 

 only ascribe it to a condensation of it by the refraction of the lens; and, in this 

 case, the thermometer N° 3, by its situation, must have been partly within the 

 reach of the heat-image formed in the focus. 

 During the experiment, the thermometers 

 were alternately screened 2 minutes from Open 

 the effects of the lens, and exposed to it for Screened 

 the same length of time; and the result Screened 

 was as annexed. Here, in the first and 2d °P en 

 minutes, N° 2 gained 2° of heat more than N° 3. In the 3d and 4th, it lost 

 1 more than N° 3. In the 5th and 6th, it gained 1 more. In the 7th and 8th, 

 it lost 14 more; and in the 9th and 10th, it gained -2- more than the other ther- 

 mometer. This plainly indicates its being acted on by g creenec j 

 refracted heat. Lest there should remain a doubt on Open 

 the subject, I now removed the lens, and, putting a open 

 plain glass in the room of it, I repeated the experiment, Screened 

 with all the rest of the apparatus in its former situation. pea 

 Here we find, that both thermometers received heat and parted with it always in 

 equal quantities, which confirms the experiment that has been given. And thus 

 it is evident, that there are rays issuing from red-hot iron, which are subject to 



