768 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



one on each side, which may be 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 intercept every visible 

 ray of the fire by which it is heated, provided that be not carried to any great ex- 

 cess. I therefore made use of an iron stove, fig. 5, having 4 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 12 bricks, placed on a stand, with casters: these bricks, fig. 6, 

 when the stand is rolled close to the stove, which must not be done till an experi- 

 ment is to begin, form an inclosure, just fitting round the sides, bottom, 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 1^ 

 inch from the hot iron. It will be necessary to remark, that on calculating the 

 transmissions for the 5th 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 precaution I had taken, of having two sets of it, that 

 one might be cooling while the other was employed, and though neither of them 

 was ever very hot, yet I found that so much heat came to the box, that when it 

 was taken out of the bricks, in order to be cooled, the thermometers continued 

 still to rise, at an average, about 2° higher than they were. I have therefore now 

 taken the 3d minute, as a much safer way to come at the truth. 



This glass stops 700 invisible rays of heat. 

 It stops 533 invisible rays of heat. 

 It stops 783 invisible rays of heat. 



It stops 625 invisible rays of heat. 



This substance stops 726 invisible rays of 

 heat. 

 At the end of 5 minutes, when the box 

 was taken out of the bricks, the talc was 

 perfectly turned into a scattering sub- 

 stance; as such, it stops 5^6 scattered invisible ravs of heat. The sun cannot be seen through it; but 

 this I find is chiefly owing to its scattering disposition. It stops however 997 scattered rays oi light. 



