78 



HEAT OR CALORIC. 



9. Model for illustrating the operation of concave mirrors. 



"The object of 

 the model repre- 

 Asented by this dia- 

 gram, is to explain 

 the mode in which 

 two mirrors oper- 

 ate, in collecting 

 the rays of radiant 

 heat emitted from 

 one focus, and in 

 concentrating them in another." 



" The caloric emitted by a heated body in the focus of the mirror 

 A, would pass off in radii or rays lessening their intensity, as the 

 space into which they pass enlarges; or, in other words, as the 

 squares of the distances. But those rays which are arrested by the 

 mirror, are reflected from it in directions parallel to its axis.* Be- 

 ing thus corrected, of their divergency, they may be received, with- 

 out any other loss, than such as arises from mechanical imperfections, 

 by the other mirror ; which should be so placed, that the axes of the 

 two mirrors may be coincident ; or, in other words, so that a line 

 drawn through their centres, from A to B, may at- the same time pass 

 through their foci, represented by the little balls supported by the 

 wires, WW." 



" The second mirror, B, reflects to its focus, the rays which reach 

 it from the first ; for it is the property of a mirror, duly concave, to 

 render parallel the divergent rays received from its focus, and to 

 cause the parallel rays which it intercepts, to become convergent, so 

 as to meet in its focus." 



" The strings, in the model, are intended to represent the paths, 

 in which the rays move, whether divergent, parallel, or convergent." 



10. Phosphorus^ kindled at the distance of twenty, or even at six- 

 ty feet, by an incandescent iron ball. Dr. Hare. 



" The annexed figure represents the mirrors, which I employ in 

 the ignition of phosphorus, and lighting a candle, by an incandescent 

 iron ball at the distance of about twenty feet." 



" I have produced this result at sixty feet, and it might be always 

 effected at that distance, were it not for the difficulty of adjusting the 

 foci with sufficient accuracy and expedition. I once ascertained 

 that a mercurial thermometer, when at the distance last mentioned., 

 was raised to 110 degrees of Fahrenheit." 



* " The axis of a mirror is in a line drawn from its centre through its focus." 

 t Especially if enveloped in cotton, which is a bad conductor. 



