Terrejlrial Rajs that occajion Heat. 133 



eccentric pivot, 00 which it might be turned away from its 

 place, and returned to the fame fituation again, at pleafure. 

 This arrangement being made, the thermometer was for a 

 few moments expofed to the rays of the candle, tillit had 

 alfumed the temperature of its fituation. Then the lens was 

 turned on its pivot fo as to intercept the direcl: rays which 

 palled through the opening in the pafteboard fcreen, and to 

 refraef. them to the focus, in which the thermometer was 

 fituated. 



Here, in three minutes, the thermometer re- 

 ceived 2 8 - degrees of heat, by the refraction 

 of the lens. The lens was now turned 



away. 



Here, in three minutes, the thermometer loft 

 > 2 1 degrees ot heat. The lens was now re- 

 turned to its fituation. 



And, in three minutes, the thermometer re- 

 gained the 2- degrees of heat. A greater 

 effect may be obtained by a different ar- 

 rangement of the diftances. Thus, if the 

 fens be placed at 3 1 inches from a wax candle, and the ther- 

 mometer fituated, as before, in the fecondary focus, we (hall 

 be able to draw from 5 to 8 degrees of heat, according to the 

 burning of the candle, and the accuracy of the adjullment of 

 the thermometer to the focus. The experiment we have rc r 

 lated (hows evidently that rays inverted with a power of heat- 

 ing bodies ifiue from a candle, and are fubjeei to laws of 

 refraction, nearly the lame with thufe reflecting light. 



l$tb Experiment. Rejraclion of the Heat that accompanies 

 the coloured Par! of the Pnj'matic Spectrum. 



I covered a burning lens of Mr. Dollond's, which is nearly 

 9 inches in diameter, and very highly polifiied, with a piece 

 of pafteboard, in which there was an opening of a fufficient 

 fize to admit all the coloured part of the prifmatic fpe&rum*. 

 In the focus of the glafs was placed the thermometer No. 3; 

 and, when eVcry thing was arranged properly, I covered the 



' Sec P! t te IV. tig 4. 



