on the terrestrial Rays that occasion Heat. 299 



Then, placing the apparatus so as to have the thermometer in 

 the red rays, but keeping the mirror covered up till the thermo- 

 meter became settled, I found it stationary at 58 . Uncovering 

 the mirror, I had as follows. 



No. 2. 

 o' 58 



2 93 



Here, in two minutes, the thermometer rose 35 degrees, by 

 reflected heat. I covered the mirror again, and, in a few minutes, 

 the thermometer, exposed to the direct prismatic red, came 

 down to 58 again. And thus the prismatic colours, if they are 

 not themselves the heat-making rays, are at least accompanied 

 by such as have a power of occasioning heat, and are liable to 

 be regularly reflected. 



4$b Experiment. Reflection of the Heat of a red-hot Poker. 



I placed the small steel mirror at 12 inches from a red-hot 

 poker, set with its heated end upwards, in a perpendicular 

 position, and so elevated as to throw its rays on the mirror** 

 The thermometer No. 2 was placed in its secondary focus, and 

 had a small pasteboard screen, to guard its ball from the direct 

 heat of the poker. 



No. 2. 



o' 54t 



H 93 



I covered the mirror. 



3' % 



Here, in if minute, the thermometer rose 38^ degrees, by 

 reflected rays; and, when the mirror was covered up, it fell in 

 the next i-§- minute, 28 degrees. On which account, we cannot 



* See Plate XII. Fig. i. 



