on the terrestrial Rays that occasion Heat. 303 





No. 1. 



No. 2 



0' 



56 



56 



— 



57 



56 



— 



59 



56 



7 



60 



56 



10 



60 



56 



Here, in ten minutes, the thermometer No. 1 received four 

 degrees of heat, reflected to it, in the strictest optical manner, 

 by the plain mirror of a Newtonian telescope. The great 

 regularity with which these invisible rays obeyed the law of 

 reflection, was such, that Dr. Wilson's sensible thermometer 

 No. 2, which had been chosen on purpose for a standard, and 

 was within an inch of the other thermometer, remained all the 

 time without the least indication of any change of temperature 

 that might have arisen from straggling rays, had there been any 

 such. I now took away the mirror, but left every thing else in 

 the situation it was. The effect of this was thus. 





No. 1. 



No. 2. 



0' 



60 



5^ 



5 



' 58 



56 



8 



57 



56 







56 



56 



Here, in ten minutes, the thermometer No. 1 lost again the 

 4 degrees it had acquired, while No. 2 still remained unaltered; 

 and this becomes therefore a most decisive experiment, in proof 

 of the existence of invisible rays, of their being subject to the 

 laws of reflection, and of their power of occasioning heat. 



mdccc R r 



