SUPPOSED RADIATION AXD REFLECTION OF COLD. 343 



with which it is thrown upon it. Now, on the contrar)', 

 if a cold body, b^ Fig. 5, Pi. 9, be brought near a plane 

 reflecting surface, as particles of heat are entering into that 

 body in all directions from the surrounding air, some par- 

 ticles of heat must be entering into it in the direction ab, 

 consequently the point a of the reflecting surface must be- 

 come cooler, or, to use my former expression, a vacuity 

 of heat will be there formed : now it may be demonstrated^ 

 that this vacuity or space will not be supplied by heat mov- 

 ing in the direction a: a, y a^ or x a, but will be supplied 

 by heat moving in no other direction than c a, which heat, 

 striking against the point a, will be thrown off into the 

 body b ; the angle cad being equal to the angle 6 a e, and 

 bodies will move in the direction in which they meet with 

 the least resistance; for if heat were to come from any 

 other direction but c a, it would not be reflected towards 

 the body b, but elsewhere, and consequently, to join the 

 current of heat a b, it must again change its course. Hence 

 it follows, that, when a cold body is brought near a plane 

 reflecting surface, in proportion as the surrounding air be- 

 comes cool, heat will enter into that body in right lines 

 tending to its centre; the plane reflecting surface will have 

 its temperature lowered, and particles of heat will strike 

 upon every part of it in such directions, as to be thrown 

 off in right lines to the cool body. 



The application of this fact to the explanation of the This applied tc 

 phenomenon in question will be readily perceived, substi- f^^,. " 

 tuting concave reflecting surfaces instead of plane ones : 

 the heat enters into the cold body placed in the focus of 

 one mirror (B. fig. 6) from the surrounding air in all di- 

 rections, consequently every point of the surface of the 

 mirror, a, b, c, d, &c., becomes cooled, and those points 

 can only receive a fresh supply in parallel rays, in a direct 

 course from the opposite mirror, because only such rays 

 (striking against so many imaginary tangents a, ft, c, d, of 

 that mirror) can be thrown off towards the body B ; the 

 opposite mirror therefore becomes cool, and for the same 

 reason the whole surface of it must be supplied by heat from 

 the thermometer T, which consequently must become cooler 

 thau a body placed any where in its neighbourhood.- 



