on the terrestrial Rays that occasion Heat. 315 



Here we find, that both thermometers received heat and 

 parted with it always in equal quantities, which confirms the 

 experiment that has been given. And thus it is evident, that 

 there are rays issuing from red-hot iron, which are subject to 

 laws of refraction, nearly equal to those which affect light ; and 

 that these rays are invested with a power of causing heat in 

 bodies. 



16th Experiment. Refraction of Fire-beat, by an Instrument 

 resembling a Telescope. 



It occurred to me, that I might use a concave mirror, to 

 condense the heat of the fire in the grate of my chimney, and, 

 reflecting it sideways by a plain mirror, I might afterwards 

 bring it to a secondary focus by a double convex lens; and 

 that, by this construction, I should have an instrument much 

 like a Newtonian telescope.* The thermometer would figura- 

 tively become the observer of heat, by being applied to the 

 place where, in the real telescope of the same construction, the 

 eye is situated to receive light. Having put together the different 

 parts, in such a way as I supposed would answer the end, I 

 tried the effect by a candle, in order to ascertain the proper 

 distance of the object-mirror from the bars of the chimney-grate. 

 The front of the apparatus was guarded by an iron plate, with 

 a thick lining of wood; and the two thermometers which I 

 used, were parted from the mirrors and lens by a partition, 

 which screened them from the heat that was to be admitted 

 through a proper opening in the front plate, to come at the 

 object-mirror. In the partition was likewise an opening, of a 

 sufficient diameter to permit the rays to come from the eye-glass 

 to their focus, on the ball of the thermometer No. 1 ; while 



• See Plate XVI. Fig. 2. 



