ON RADIANT, HEAT. 315 



kindled by them, had long ago been made at Vienna. The 

 work of Zahn referred to is no doubt his Ocutm artificiatis*, 

 which was published in 1685. 



I have a small tract entitled : Account of the parabolic Wooden mi*. 



wooden Mirrors, and their surprising action, which were ,orsof X reat 



. power. 



lately invented by Andrew Gaertner, Machinist, and Mo- 

 del-Master to the King of Poland and Elector of Saxony, 

 at Dresden, 1785. 



In tSiis work is the following description of an experiment. Experii 

 " I placed a live coal before the mirror, in its focus, and witil 

 immediately the mirror diffused a strong heat to the distance 

 of forty or fifty paces, which it would not do with the sun. 

 Hence I conceive, that what is told of the celebrated Ar- 

 chimedes is true: though that he did not produce so power- 

 ful a heat by the help of the solar rays, but by a fire kindled 

 for the purpose; for when I placed a small mirror, half an 

 ell [near a foot] in diameter, opposite the greater, and sixty- 

 paces from it, and the coal was made to burn bright, imme- 

 diately the little mirror lighted a candle ; which many per- 

 sons would not believe, till they had seen it. I tried also 

 whether a lighted candle would produce the same effect as 

 the coal ; but it did not, for there was not the least heat from 

 it." 



The large mirror here alluded to was tj> feet in diame- Size of the 

 ter: the largest of the author's making was five. mirrors. 



The following experiment appears to me still more sur- Other experi* 

 prising. The author says: «' if I held the mirror ten or meilt ** 

 twelve paces from a hot iron stove, immediately it kindled 

 a fire: and the same thing took place on holding it 20 or 

 24 paces from a fire in an open fire place." 



Pictet's experiment with ice, which surprised him so 

 much, is described in the same work. 



A little after the experiments I have just mentioned, the R a diantcold, 

 author continues thus, «« lf ? instead of fire, I placed cold 

 water in the focus of the mirror, it diffused an agreeable 

 coldness even in the height of summer: and if, instead of 

 water, I used ice, very considerable cold was produced at 

 the distance of ten or twenty paces. 



* More probably, I should think, his mir.uli .^Economia, or his Spe- 

 cula physieooniaihcmalicuhi$tor:ca. C, 



Dolf, 



