Refrailion of Heat. 365 



Experiment 14. RefraBion of the heat of a chimney fire. The fame burning glafs which 

 is nearly 9 inches in diameter was placed before the clear fire of a large grate, at the diftance 

 , of 3 feet from the bars, and a thermometer was placed in the fecondary focus. Another 

 thermometer was alfo placed behind a wooden fcrecn at the fame diftance from the fire. 

 The fituation of the feveral parts of the apparatus being thus adjufted the thermometers 

 were taken away to be cooled, and then reftored to their places. The ftandard thermome- 

 ter in the courfe of the firft feven minutes acquired a heat of 3-^- degrees, after which it 

 remained fteady for the fubfequent 32 minutes duration of the experiment. The other 

 thermometer was treated in the, fame manner as that which received the refrafted heat from 

 a candle in experiment 12; that is to fay, the lens was alternately placed and removed, 

 and the variations were — rife in the firft 9 minutes, 9I degrees ; in the next 5 minutes, 

 before the uncondenfed rays of the fire, fall 2^ degrees; — in the next 10 minutes rife i| 

 degrees ; in the next 5^ minutes, fall 3 degrees ; and in the next 4^ minutes, rife i-?- 

 degrees. — So that in the courfe of 35 minutes, the thermometer was raifed and deprefled 

 5 times by rays which came from the culinary fire, and were fubjeft to laws of refraction, 

 not fenfibly different from thofe which afFe£t light. 



Experiment 15. RefraBion of the heat of red hot iron. A lump of iron was forged into a 

 cylinder 2-^- inches in diameter, and 2^- inches long. This being made red hot, was ftuck 

 upon an iron handle fixed on a ftand, fo as to prefent one of its circular faces to a lens of 

 the dimenfions ufed iu the 12th experiment, at the diftance of 2,8 inches. Before the lens, 

 at fome diftance, was placed a fcreen of wood, with a hole of an inch diameter in it, to 

 limit the objeft, and to keep the heat from the thermometers. One of thefe was placed in 

 the fecondary focus of the lens, and the other, within -^^ of an inch of that focus, and at 

 the fame diftance from the kns, fo that if there was any difference in the cxpofure, it was 

 in favour of the laft thermometer, which ftood oppofite a thinner part of the lens. During 

 the experiment, the thermometers were alternately fcreened from the tSt£k of the lens, and 

 expofed to it for the fame length of time, and the efFedts were, that both underwent variations 

 of increafed and diminilhed temperature ; but that which was in the focus gained 6 de- 

 grees in the firft two minutes, whilft that near the focus gained only 4; and the firft loft 3 

 degrees in the fecond two minutes during the time the lens was fcreened, whilft the fecond' 

 loft only 2 ; and fo in three other alternations the alternations of temperature in the focus 

 were 2°, 2|°, and 1^°, whilft the correfponding changes out of the focus were i°, i^°, 

 and o\. To remove all doubt on the fubjeft, the thermometers were left in their refpeflive 

 fituations, and a plain glafs fubftituted inftead of the lens. In thefe circumftances the laft 

 related procefs of alternate covering and uncovering the glafs was repeated, and both ther- 

 mometers were found to be affefted alike. 



Experiment 16. RefraBion of fire heat by an infirument refembling a telefcope. This in- 

 ft;rument confifted of a concave mirror, which received the heat from the fire, a plain 

 mirror which reflefled it to one fide, where there was a lens at the ufual eye aperture. A 

 thermometer was placed in the focus of this lens, and another near the focus, and the fame 



methods 



