on the terrestrial Rays that occasion Heat. 491 



drawn out or screwed in, by way of accurately adjusting the 

 distance of the thermometer from the line of action. 



In order to procure invisible terrestrial heat, I have tried 

 many different ways, but a stove is the most commodious of 

 them. Iron is a substance that transmits invisible heat very 

 readily ; while, at the same time, it will most effectually inter- 

 cept every visible ray of the fire by which it is heated, provided 

 that be not carried to any great excess. I therefore made use 

 of an iron stove,* having four flat sides, and being constructed 

 so as to exclude all appearance of light. I had it placed close 

 to a wall, that the pipe which conveys away smoke might not 

 scatter heat into the room. . 



The thermometer box, when experiments are to be made, is 

 to be put into an arrangement of twelve bricks, placed on a 

 stand, with casters : -f these bricks, when the stand is rolled 

 close to the stove, which must not be done till an experiment 

 is to begin, form an inclosure, just fitting round the sides, bot- 

 tom, and covered part of the top of the thermometer box, and 

 completely guard it against the heat of the stove. The box is 

 then shoved into the brick opening, close to the iron side of the 

 stove, where the two front screws, coming into contact with 

 the iron plate, give the thermometers their proper distance ; 

 which, in the following experiments, has been such as to bring 

 the most advanced part of the balls to one inch and four-tenths 

 from the hot iron. 



It will be necessary to remark, that on calculating the trans- 

 missions for the fifth minute, I found that it would not be 

 doing justice to the stopping power of the glasses, to take so 

 long a time ; for, notwithstanding the use of brickwork, and the 



* See Plate XXV. Fig. 2. f See %• %• 



