TerreJIrlal Rays that occajlon Heat. 261 



the contrary, came to its maximum bv alternate great ele- 

 vations and t'mall depreffions; and afterwards loll its heat by 

 great depreffions and fniail elevations. After the fiift eight 

 minutes, I changed the place of the afliftant thermometer, 

 by putting it into a ftill more deciiive iituatinn ; for it was 

 now placed by the fide of that in the focus, fo as to partici- 

 pate of the alternate fcreening, and alfo to receive a ft aall 

 fliareofone fide of the invifible heat-image, which, though 

 unfeen, we know muff, be formed in the focus of the lens. 

 Here, if our reafoning be right, the affiftant thermometer 

 mould be affected by alternate rifings and fallings; but they 

 ihould not be fo considerable as thole of the lens. 



No. 2. No. 3. "I Here the changes 



In the tocus. In the Luge of it. of the thermo- 



Both open 8' 64^- 64 meter No. % 



Both open 9 63^ 63 §• were — -p + i- 



Open 11 64*- 64 )> — ly + -| — 



Screened 12, 1 £ 65 6$\ 1 4- 1 ; and 



Open 14 63^- 63^- thofe of No. 3 



Screened 16 62^ 63 were — i 4- 



Open 18 63A 63 1 j ± — ^ + 1 — 



1 + •!• All which fo clearly confirm the effecl; of the re- 

 fraction of the lens, that it mult now be evident that there 

 are rays ifl'uing from hot iron, which, though in a ftate of 

 total invifibilitv, have a power of occafioning heat, and obey 

 certain laws of refraction, very nearly the fame with thofe 

 that affect light. 



As we have now traced the rays which occafion heat, both 

 folar and terreflrjal, through all the varieties that were men- 

 tioned in the begiuning of this paper, and have fhowu that, 

 in every (late, they are fubject to the laws of reflection and 

 of refraction, it will be eafy to perceive that I have made 

 good a proof of the three firft of my proportions. For the 

 fame experiments which have convinced us that, according 

 to our fecond and third rticles, heat is both reflexible and 

 refrangible, efl its radiant nature, and thus equally 



prove the firft of them. 



End of the Fnjl Pari. 



Slouch, near Win Ifor, 



April 26, ib . . . 



VII. Account 



