770 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



Article 6. Scattering of Solar Heat. — We are now come to a branch of our 

 inquiry which, from its novelty, would deserve a fuller investigation than we can 

 at present enter into. The scattering of heat, is a reflection of it on the rough 

 surfaces of bodies: it is therefore a principle of general influence, since all bodies, 

 even the most polished, are sufficiently rough to scatter heat in all directions. In 

 order therefore to compare the effect of rough surfaces on heat with their effect on 

 light, I have made a number of experiments, from which the following are 

 selected, for the purpose of our intended comparative view. 



The apparatus I have used for scattering solar heat, is like that which served 

 for transmissions, fig. 13, pi. 13; but here the holes through which the sun's rays 

 enter, fig. 18, are very exactly l-±- inch in diameter each; and are chamferred away 

 on the under side, that no re-scattering may take place in the thickness of the 

 covering board: the distance of the centre of the holes is 4 inches. A little more 

 than an inch below, and under the centre of the holes, are the balls of the small 

 thermometers a and b, well shaded from the direct rays of the sun, by small slips 

 of wood, of the shape of the ball, and of that part of the stem which is exposed. 

 Under each thermometer is a small tablet, fig. 10, pi. 13, on which the objects 

 intended for scattering the sun's rays are to be placed. The tablets are contrived 

 so as to bring the objects perpendicularly under the openings, and under the centre 

 of the balls of the thermometers, at the distance of exactly 1 inch from them. Every 

 thing being thus alike on both sides of the box, it is evident, from the equality of 

 the holes, that an equal number of solar rays will fall on each object, and will by 

 them be scattered back on the thermometers, at equal angles, and equal distances. 



The first 5 experiments that follow, were made with an apparatus somewhat 

 different from the one here described; and, though the result of them may not be 

 so accurate as if they had been made with the present one, I must give them as 

 they are, since time will not allow of a repetition. 



Min. Sun. Message card scattering. 

 Exper. 195. 64 6'4 Here an object of a white colour, 3.6 inches 



5 6p| 66| ... 5| : 2£ = .413 . long, and 2.6 broad, scattered in 5 minutes, 



413 rays of heat back on one thermometer, while the other received 1000, directly from the sun. 

 Now, in order the better to compare the proportion of light and heat scattered by different objects, we 

 shall put these 413 rays equal to 1000 ; or, which is nearly the same, multiply them by 2.421. Then, 

 since the message card also scatters 1000 rays of light, as will be found in a table at the end of the 

 transmission table, our present object may be made a standard for a comparison with the 4 follow- 

 ing ones. 



Min. Sun. Pink-coloured paper scattering. 

 Exper. 196. 64 64 Here a piece of pink-coloured paper, 



5 70 66$ ... 6 : 2-| = .438. of the same dimensions with the card 



of the last experiment, and placed in the same situation, scattered, as we find by the same mode of mul- 

 tiplication, 1060 rays of heat ; and, by our table, it scatters 513 of light. 

 Min. Sun. Pale-green paper scattering. 

 Exper. 197. 64J- 644- This piece of paper scatters 896 rays of 



5 69i 66| . . . 5| : 2| = .370. heat, and 549 of light. 



Min. Sun. Dark-green paper scattering. 

 Exper. 198. 64| 65^ This paper scatters 1242 rays of heat, 



5 69 J 67% . . . 4f : 21 = .513 and only 308 of light. 



