$16 Dr. Herschel's Experiments on the solar, and 



No. 4 was placed by the side of it, at less than half an inch 

 distance. In the experiment, the object-mirror was alternately 

 covered by a piece of pasteboard, and opened again . The ther- 

 mometers were read off every minute ; but, to shorten my 

 account, I only give the last minute of every change. 







No. 1. 



No. 4. 







In the Focus/ 



Near the Focus, 



The mirror covered 



o' 



77i 



77* 



The mirror open 



8 



84 



76 



Covered - 



16 



S6L 



79k 



Open 



21 



fei 



81 



Covered 



27 



8cj| 



8 2 £ 



Open 



37 



b'4 



«3i 



Covered 



47 



84 



77 



Here, in the first eight minutes, the thermometer exposed to 

 the effects of the fire-instrument, gained 2 degrees of heat more 

 than the other. In the next 8 minutes, the mirror being covered, 

 it gained 1 degree less than the other. The mirror being now 

 opened again, it gained, in 5 minutes, 2J degrees more than the 

 other. When covered 6 minutes, it gained 3^ degree less than 

 No. 4. In the next 10 minutes, when open, it gained \ degree 

 more ; and, in the last 10 minutes, when the fire began to fail, 

 and the mirror was again covered, it lost 1 degree more than 

 the other thermometer. All which can only be accounted for 

 by the heat which came to the thermometer through the fire- 

 instrument; and, as this experiment confirms what has been 

 said before of the refraction of culinary heat, so it also adds to 

 what has already been proved of its reflection. For, in this fire^ 

 instrument, the rays which occasion heat could undergo no less 

 than two reflections and two refractions. 



