VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 763 



Crown glass ; one side rubbed on emery, 



rough side exposed. 

 49 This glass, so prepared, stops 389 scat- 



50| . . . 2 J :l-§ = .611. tered rays of the same sort of heat. 



Coach glass ; one side rubbed on emery, 

 rough side exposed. 

 52-| It stops 500 scattered rays of 



33 J . . . 1| : £ = .500. the same sort of heat. 



Crown glass 5 both sides rubbed on emery. 

 494 It stops 47 1 scattered rays of 



5 1 ... 2\ : 14 = .529. the same sort of heat. 



Coach glass j both sides rubbed on emery. 

 53f It stops 833 scattered rays of the 



34 . . . 1 £ : i = . 16"7- same sort of heat. 



Calcined talc. 



50£ This substance stops 7tf scattered rays 



5H . . • 24 : 4 = .263. 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 thermometers, 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 equal glow, and without flame 

 or smoke. To obtain this, I used a grate 1Q inches broad, and 84 high, having 

 only 3 bars, which divide the fire into 3 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, 

 fig. 3. pi. 14, 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 24- 

 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. 24- 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 directions fixed on proper stands, to bring the balls, quite 

 disengaged from the scales, directly 2 inches behind the transmitting holes. A 

 small thin wooden partition is run up between the thermometers, to prevent the 

 heat transmitted through one hole from coming to the thermometer belonging 

 to the other. The screen is fixed on a light frame, which fits exactly into the 

 opening of the front of the marble chimney-piece ; and the ends of the frame 

 are of a length which, when the screen is placed before the fire, will just bring the 

 transmitting holes to be 64- inches from the front bars of the grate. A large 

 wooden cover, also plated with iron, shuts up the transmitting holes on the side 

 next to the fire ; but may be drawn up by a string on the outside so as to open 

 them when required. Two assistant thermometers are placed on proper stands 

 to bring their balls to the same distance from the screen as those which re- 

 ceive the heat of the fire ; but removed sideways as far as necessary, to put 

 them out of the reach of any rays that pass obliquely through the transmitting 

 holes. They are to indicate any change of temperature that may take place du- 



5 e 2 



