on the terrestrial Rays that occasion Heat. 301 



Here, in five minutes, the heat reflected from the plain mirror 

 raised the thermometer No. 1, 7 degrees ; while the change in 

 the temperature of the screened place, indicated by No. 4, 

 amounted only to half a degree : which shews, that an open 

 fire sends out rays that are subject to the laws of reflection, and 

 occasion heat. 



6th Experiment. 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 two 

 of 45 each.* It was put so as to have one of the sides facing 

 the fire, while the other was turned towards the thermometer : 

 the hypotenuse, consequently, made an angle of 45 degrees 

 with the bars of the grate. The apparatus, after having been 

 cooled some time, was exposed to the fire, and the following 

 result was taken. 





No. 1. 



No. 4 



0' 



6si 



62 \ 



1 



63 



6 2 | 



2 



64 



63 



4 



6*1 



% 



5 



65 



63* 



8 



65i 



63I 



10 



66\ 



63i 



11 



6 7 



6-ii 



Here, in eleven minutes, the rays reflected by the prism raised 

 the thermometer 4^- degrees ; but, the temperature of the place 

 having undergone an alteration of i| degrees, we can only 



* See Plate XII. Fig. 3. 



