on the terrestrial Rays that occasion Heat. 311 



are subject to certain laws of refraction, which cannot differ 

 much from those affecting light. 



14^ Experiment. Refraction of the Heat of a Chimney-Fire. 



I placed Mr. Dollond's lens before the clear fire of a large 



grate.* Its distance from the bars of the grate was three feet; 



and, in the secondary focus of it was placed the thermometer 



No. 1. No. 4 was stationed, by way of standard, at a£ inches 



from the former, and at an equal distance from the fire. 



Before the thermometers was a slip of mahogany, which had 



three holes in it, T 8 ^ of an inch in diameter each. Behind the 



centre of the 1st hole, -| of an inch from the back, was placed 



the thermometer No. 1 ; and, between the 2d and 3d hole, 



guarded from the direct rays of the fire by the partition, at the 



same distance from the back, was put No. 4. Things being 



thus arranged, the situation of the apparatus which carried the 



thermometers, and that where the lens was fixed, were marked. 



Then the thermometers, having been taken away to be cooled, 



were restored to their places again, and their progress marked 



as follows. 



No. 1. No. 4. 





Burning Lens. 



Screened. 



o* 



58 



58 



H 



65 



OO 



3 



68 



6l 



5 



70 



6lX 



7 



in 



6l$ 



9 



« 



6l i 



Here, in nine minutes, the rays coming from the fire, through 

 the burning glass, gave 9J degrees of heat more to the ther- 



• See Plate XV. 

 MDCCC. S S 



