UNCLE IN ENGLAND. 59 



speculum, which is now used in a reflecting 

 telescope, is composed of about two parts of 

 copper and one of tin ; but what metals were 

 employed by the ancients in their burning mirrors 

 is not known." 



"You allude, I suppose, papa," said Frederick, 

 "to the famous concave mirrors with which 

 Archimedes destroyed the Roman fleet." 



" Long before his time," my uncle replied, 

 " concave mirrors had been constructed, by which 

 the sun's rays were so concentrated as to burn 

 substances placed in the focus : but those used 

 by Archimedes were not concave, they had plane 

 or flat surfaces, and it was by the combination of a 

 great number that the effect was produced. For 

 you can readily conceive that whatever portion 

 of the solar heat can be conveyed by reflection 

 from a single plane surface, the effect will be 

 doubled if the rays from another plane surface 

 be directed to the same spot. Five or six times 

 the direct heat of the sun would set dry wood 

 on fire ; but as more than half the heat is 

 dissipated by reflection and by other causes, 

 we may say that eighteen or twenty small plane 

 mirrors would be quite sufficient for that pur- 

 pose. The Count de BufFon tried a great many 

 valuable experiments on this subject ; with 154 

 mirrors he succeeded in burning wood at the 

 distance of seventy yards, and in fusing several 

 metals at eight, ten, and even twelve yards, 



