ReJleBion of inwfihk folar and culinary Heat. ' 363 



Experiment 8. ReJleBion and condetifaticn of the invifible folar rap. The fmall fteel mir- 

 ror before defcrlbed was by a fuicablc apparatus placed to receive the rays of the prifmatic 

 folar fpe£lrum perpendicularly to its diameter or fubtenfe. Half the mirror was covered 

 by a femicircular pafteboard with lines drawn thereon parallel to the diameter, by means of 

 which the laft vifible red might be made to fall at any required diftance from the uncovered 

 portion of the fpeculum. The thermometer was placed in the focus, and the whole furface 

 of the mirror kept covered till the inftrument became ftationary at the temperature of the 

 place. At this period the apparatus was moved till the laft vifible red fell on the graduated 

 pafteboard at the diftance of one-tenth of an inch from its diametrical edge, when the other 

 part of the refletking furface being uncovered for the admiflion of the invifible rays, the 

 thermometer in one minute was raifed 19 degrees. The mirror was then again covered, 

 and in 3 minutes the thermometer fell 16 degrees. — Again, the mirror was opened, and in 

 2 minutes the thermometer rofe 24 degrees. And on covering the mirror once more, in one 

 minute the thermometer fell 19 degrees. 



By this alternate rifmg and falling our author obferves, that there are three points clearly 

 afcertained. Firft, that there are invifible rays of the fun. Secondly, that thefe rays are 

 not only reflexible in the manner proved in the foregoing experiment, but that by the ftridt 

 laws of refleftion they arc capable of being condenfed. And thirdly, that by tliis condenfa- 

 tion their heat is proportionally increafed. 



Experiment 9. RefleElion of invifible culinary heat. The fteel mirror was fixed upon a 

 fmall board, and before it was a wooden fcreen, half an inch thick, which was of an height 

 to intercept the rays which might elfe have fallen on the lower half of the mirror. Behind 

 this fcreen were placed two thermometers ; one of which was in the axis of the mirror, and 

 the other, which was near it, was fcreened on the fide towards the mirror by a fmall flip 

 of pafteboard tied to the fcale. This apparatus being duly placed oppofitc a clofe ftove, 

 well heated, the invifible rays refledled from the mirror raifed the thermometer in its 

 axis 39 degrees in one minute, while the other, which had exactly the fame expofure to the 

 ftove, but was defended from the refletted rays of the mirror, rofe only one degree. 



Experiment 10. ReJleBion of the invifible rays of heat of a poker cooled from being red-hot till 

 it could no longer be feen in a dark place. A poker of a proper black heat was placed at the 

 diftance of 1 2 inches from the fteel mirror, and the effedl of its condenfed rays was received 

 upon the thermometer placed in the focus. The mirror was uncovered and covered alter- 

 nately, one minute at a time, for fix minutes, and the alternations were, rife 7°^ fall 7"; 

 rife 3° ; fall 5° ; rife i\° ; fall 6i°. 



Dr. Herfchell concludes his narration of fadts concerning refledled heat with the follow- 

 ing remarks: N 



" From thefe experiments it is now fufficiently evident, that in every fuppofedcafe of 

 ** folar and terreftrial heat, we have traced out rays that are fubjeft to the regular laws 

 " of refleftion, and are invefted with a power of heating bodies ; and this independent of 

 ** light. For though, in four cafes out of fix, we had illuminating as well as heating rays, 



3^2 "it 



