MEMOIR OF BUFFON. 25 



mirrors, at a distance of 250 feet, inflamed in two mi- 

 nutes and a half, minute chips of deal, sulphured and 

 mixed with charcoal. When the sun was vivid, the in- 

 flammation took place in a few seconds. On the 1 Oth 

 April, after mid-day, with a clear sun, 128 mirrors, at 

 the distance of 150 feet, set fire to a tarred plank of fir ; 

 the inflammation being very sudden. Same day, at 

 half-past two, 148 mirrors, at 150 feet, fired a plank 

 of beech sulphured in some parts, and in others cover- 

 ed with wool cut into small pieces ; the inflammation 

 was so sudden and violent that it was necessary to 

 plunge the plank into water in order to extinguish it. 

 April 11, twelve mirrors, at twenty feet, inflamed 

 small combustible matters ; forty- five, at twenty feet, 

 melted a large pewter flask that weighed six pounds ; 

 and 117 made some thin pieces of silver and iron red- 

 hot. Having satisfied himself upon this point, he fol- 

 lowed out the subject, and constructed mirrors upon 

 various plans. Perhaps the most remarkable were 

 those formed by bending glass upon moulds of a sphe- 

 rical form by means of a furrow ; he was thus able 

 to make them of very considerable size, but they re- 

 quired great caution in the cooling and grinding after- 

 wards ; and, out of twenty-four which he made, only 

 three were able to be preserved. One of these, forty- 

 six inches in diameter, was presented to the King of 

 France, and was regarded as the most powerful burn- 

 ing mirror in Europe. 



A few years before the commencement of these ex- 



