Terrejlrial Rays that occajion Heat. «57 



by the fide of it, at lefs than half an inch diftance. In the 

 experiment, the objeft-mirror was alternately covered by a 

 piece of pafteboard, and opened again. The thermometers 

 were read off every minute ; but, to fliorten my account, I 

 only give the laft minute of every change. 



No. I. No. 4. "1 Here, in the 



In the Focus. Near the Focus. 



firft eicrht 



77i 



84 



86!- 



89$ 



84 



inftrument, gained 2 degrees of heat more than the other. 

 In the next 8 minutes, the mirror being covered, it gained 

 1 degree lefs than the other. The mirror being now opened 

 again, it gained, in five minutes, 2\ degrees more than the 

 other. When covered fix minutes, it gained 1^ degree lefs 

 than No. 4. In the next ten minutes, when open, it gained 

 i degree more ; and, in the laft ten minutes, when- the fire 

 began to fail, and the mirror was again covered, it loft one 

 dep-ree more than the other thermometer. All which can 

 only be accounted for by the heat which came to the ther- 

 mometer through the fire-inftrument ; and, as this experi- 

 ment confirms what has been faid before of the refraction 

 of culinary heat, fo it alfo adds to what has already been 

 proved of its reflection. For, in this fire-inftrument, the 

 ravs which occafion heat could undergo no lefs than two 

 reflections and two refractions. 



ljth JLxfxcriment. Refra&ion of the mv'iJihU Rays of 

 Solar Heat. 



I covered one-half of Mr. Dollond's burning lens with 

 pafteboard, and threw the prifmatic fpectrum upon that 

 cover*; then, keeping the laft vifible red colour one- tenth 

 of an inch from the margin of the pafteboard, I let the in- 

 vilibie rays beyond the fpectrum fall on the lens. In the 



* See Plate I. fig. i. 



Vol. VIII. L 1 focus 



