on the terrestrial Rays that occasion Heal. 32 1 



- 





No. 2. 



No. 3. 







In the Focus. 



Advanced sideways 

 Always open. 



Screened 



0' 



&£ 



63 



Open 



1 



%i 



64 



Screened 



2 



6s| 



64 



Open 



3 



64 



H\ 



Screened 



4 



63! 



6& 



Open 



5 



^* 



Hi 



Screened 



6 



64i 



»4i 



Open 



7 



^41 



64 



Screened 



8 



^4i 



64 



Here No. 3, being out of the reach of refraction, gradually- 

 acquired its maximum of heat, in consequence of an uniform 

 exposure to the influence of the hot cylinder; after which, it 

 began to decline. No. 2, on the contrary, came to its maxi- 

 mum by alternate great elevations, and small depressions ; and 

 afterwards lost its heat by great depressions, and small eleva- 

 tions. After the first eight minutes, I changed the place of the 

 assistant thermometer, by putting it into a still more decisive 

 situation ; for it was now placed by the side of that in the focus, 

 so as to participate of the alternate screening, and also to 

 receive a small share of one side of the invisible heat-image, 

 which, though unseen, we know must be formed in the focus 

 of the lens. Here, if our reasoning be right, the assistant ther- 

 mometer should be affected by alternate risings and fallings : 

 but they should not be so considerable as those of the lens. 



No. 2. No. 3. 





In the Focus. 



In the Edge of it 



Both open 8' 



Hi 



64 



Both open 9* 



Tt 2 



G3i 



