696 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



5 minutes, the heat reflected from the plain mirror raised the thermometer N° 1, 

 7° ; while the change in the temperature of the screened place, indicated by N° 4, 

 amounted only to half a degree : which shows, that an open fire sends out rays that 

 are subject to the laws of reflection, and occasion heat. 



Exper. 6. Reflection of Fire-heat by a Prism. — Every thing remaining arranged 

 as in the 5th experiment, I removed the small plain mirror, and placed in its stead a 

 prism, which had one of its angles of 90 degrees, and the 

 other 2 of 45° each, fig. 3, pi. 12. It was put so as to 

 have one of the sides facing the fire, while the other was 

 turned towards the thermometer : the hypotenuse con- 

 sequently made an angle of 45° with the bars of the 

 grate. The apparatus, after having been cooled some 

 time, was exposed to the fire, and the annexed result 

 was taken. Here, in 1 l m , the rays reflected by the prism raised the thermometer 

 44 degrees ; but, the temperature of the place having undergone an alteration of 1-f- 

 degrees, we can only place 2|- to the account of reflection. The apparatus be- 

 coming now very hot, it would not have been fair to have continued the experi- 

 ment for a longer time ; but the effect already produced was fully sufficient to show 

 that even a prism, which stops a great many heat-making rays, still reflects enough 

 of them to prove, that an open fire not only sends them out, but that they are sub- 

 ject to every law of reflection. 



Exper. 7. Reflection of Invisible Solar Heat. — On a board of about 4 feet 6 

 inches long, I placed at one end, a small plain mirror, and at the other, 2 ther- 

 mometers, fig. 4, pi. 12. The distance of N° 1, from the face of the mirror, was 

 3 feet 94 inches ; and N° 2 was put at the side of it, facing the same way, but out 

 of the reach of the rays that were to be reflected by the mirror. The colours of 

 the prism were thrown on a sheet of paper having parallel lines drawn on it, at half 

 an inch from each other. The mirror was stationed on the paper; and was ad- 

 justed in such a manner as to present its polished surface, in an angle of 45 degrees, 

 to the incident coloured rays, by which means, they would be reflected towards the 

 ball of the thermometer N° 1. In this arrangement, the whole apparatus might be 

 withdrawn from the colours to any required distance, by attending to the last visible 

 red colour, as it showed itself on the lines of the paper. When the thermometers 

 were properly settled to the temperature of their situation, during which time the 

 mirror had been covered, the apparatus was drawn gently 

 away from the colours, so far as to cause the mirror, 

 which was now open, to receive only the invisible rays 

 of heat which lie beyond the confines of red. The re- 

 sult was as annexed. Here, in 10 m , the thermometer 

 N° 1 received 4° of heat, reflected to it, in the strictest optical manner, by the 

 plain mirror of a Newtonian telescope. The great regularity with which these in- 

 visible rays obeyed the law of reflection, was such, that Dr. Wilson's sensible ther- 



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