of the invisible Rays of the Sun. 287 



No. 1. No. 2. No. 3. 



48* - 48* - - 47J 



$*f - - 4 8 l - - 47i 



Therefore, the thermometer No. 1 rose si degree, in 10 

 minutes, at i\ inch beyond the visible light of the red rays. 



I might now have gone on to the 5th line ; but, so fine a day, 

 with regard to clearness of sky and perfect calmness, was not 

 to be expected often, at this time of the year; I therefore 

 hastened to make a trial of the other extreme of the prismatic 

 spectrum. This was attended with some difficulty, as the illu- 

 mination of the violet rays is so feeble, that a precise termination 

 of it cannot be perceived. However, as well as could be judged, 

 I placed the thermometers one inch beyond the reach of the 

 violet rays, and found the result as follows, 



No. 1. 



No. 2. 



No. i 



48 



48 



47i 



48 



48 



47i 



48 



47i 



47 



4^ - 



47i 



47 



48 



48 



47f 



Here the several indications of the thermometers, two of 

 which, No. 1 and 2, were used as variable, while the 3d was 

 kept as the standard, were read off during a time that lasted 

 12 minutes ; but they afford, as may be seen by inspection, no 

 ground for ascribing any of their small changes to other causes 

 than the accidental disturbance which will arise from the motion 

 of the air, in a room where some employment is carried on. 



I exposed the thermometer now to the line of the very first 

 perceptible violet light ; but so that No. 1 and 2 might again 



mdccc. P p 



