Tare/trial Rajs that occafion Heat. 129 



No. I. No. So Here, in ten minutes, the ther- 



mometer No. I loft again the 

 four degrees it had acquired, 

 while N>. 2 full remained un- 

 altered: and this becomes there- 

 fore a mod deeifive experiment, in proof of the exifterice of 

 invilible ravs, of their being fubject to the laws of reflection, 

 and of their power of occafi'oriing heat. 



o' co 56 



5 .58 '56 



« 57 56 



5 6 5 6 



8th Experiments Rrfh-fl'ion and Condenfation of the inv'ijible 

 Solar Rajs. 



I made an apparatus for placing the fmall fteel mirror at 

 any required angle *; and, having expofed it to the prifmatic 

 fpeclrum, fo as to receive it perpendicularly, I caufed the 

 colours to fall on one half of the mirror, which, being covered 

 by a femicircular piece of pafteboard, would flop all vifible 

 rays, fo that none of them could reach the polifhed furfacei 

 On the pafteboard were drawn feveral lines, parallel to the 

 diameter, and at the di fiance of one-tenth of an inch from 

 each other; that, by withdrawing the apparatus, I might 

 have it at option to remove the laft vifible red to any required 

 dlftance from the reflecting furface. In the focus of the 

 mirror was placed the thermometer No. 2. I covered now 

 alfo the other half of the mirror, till the thermometer had 

 afiumed the temperature of its fuuation. Then, withdrawing 

 the apparatus out of the vifible fpectrum, till the laft tinge of 

 red was one-tenth of an inch removed from the edge of the 

 pafteboard, and the whole of the coloured image thus thrown 

 on the femicircular cover, I opened the other half of the 

 mirror for the admiflibh of invifible rays. The refult was ap 

 follows: . 



No. 3. -s Here, in one minute, the ther- 



In the Focus of invifible Hfeat. I mometer rofe 19 decrees, 

 o' 61 j 1 covered the mirror; 



I 80 J 



'" (Here, in three minutes, the thermometer fell 



' { 16 degrees. I opened the mirror again; 

 + 64J - ' 



• See Plate TL Gg. t. 

 Vol.. VIII. 8 No. 2. 



