476 Dr. Herschei/s Experiments on the solar, and 



n6tb Experiment. 



Red rays. Calcined talc. 



5 53i sH'"H : ir=^ e 8 



This substance stops 737 scattered rays of the same sort of 

 heat. 



Transmission of Fire- Heat through various Substances. 



When the same fire is to give an equal heat to two thermo- 

 meters, at some short distance from each other, it becomes 

 highly necessary that there should be a place of considerable 

 dimensions in its centre, where it may burn with an equal glow, 

 and without flame or smoke. To obtain this, I used a grate 

 19 inches broad, and 8^ high, having only three bars, which 

 divide the fire into three large openings. In the centre of the 

 middle one of these, when the grate is well filled with large 

 coals or coke, we may, with proper management, keep up the 

 required equality of radiance. 



The apparatus I have used is of the following construction.* 

 A screen of wood, 3 feet 6 inches high, and 3 feet broad, lined 

 towards the fire with plates of iron, has two holes, \ of an inch 

 in diameter, and at the distance of 2|- inches from each other, 

 one on each side of the middle of the screen, and of a height 

 that will answer to the centre of the fire. 2^ inches under 

 the centre of the holes is a shelf, about 22 inches long and 4 

 broad, on which are placed two thermometers, in opposite direc- 

 tions, fixed on proper stands, to bring the balls, quite disen- 

 gaged from the scales, directly 2 inches behind the transmitting 



* See Plate XXIV. 



