and the Mode of its Communication. 117 



polished metallic body would do, of any other form, 

 having the same area or quantity of surface opposed to 

 the hot mirror, and being placed at the same distance 

 from it. 



By a similar train of reasoning it may be shown that 

 the form of the hot body (that of a concave mirror) will 

 contribute nothing to the effect it will produce on the 

 cold mirror, in heating it by the calorific rays it emits ; 

 and that it will itself be cooled neither faster nor slower 

 on account of its peculiar form. 



Let us now suppose both mirrors to be at the tem- 

 perature, precisely, of the room (that of freezing water), 

 and that a bullet, or other small body of a spherical 

 form, at the temperature of boiling water, be placed in 

 the focus of one of the mirrors, which mirror we shall 

 call A. 



As the rays emitted by this hot body are sent off in 

 right lines, in all directions, in the same manner as 

 light is emitted by luminous bodies, all those rays which 

 fall on the concave polished surface of the mirror A 

 will be reflected (as is well known) in lines nearly paral- 

 lel to the axis of the mirror ; they will consequently 

 fall on the concave polished surface of the opposite 

 mirror B, and, being there again reflected, they will be 

 concentrated at the focus of the second mirror. 



If now a sensible thermometer, at the temperature of 

 the room, be placed in this focus, it will immediately 

 begin to rise, in consequence of the heat generated in it 

 by the action of these calorific rays, so accumulated in 

 that place. 



If, instead of being placed in the focus of this second 

 mirror, the thermometer be placed at a very small dis- 

 tance from that focus, on one side of it, the instrument, 



