136 



BURNING INSTRUMENTS. 



an interval of four lines between each, for the pur- 

 pose of allowing them a free motion in every direc- 

 tion, and also of permitting the observer to see the 

 place to which the images were to be thrown. These 

 pieces of glass were mounted in an iron frame, and 

 each of them was so fitted up with screws and springs, 

 that a motion could be given to them in any direction, 

 so that the images reflected from all the mirrors might 

 be easily thrown upon the same spot. The machinery 

 for giving motion to each mirror is represented in Fig. 

 PLATE CV. 2, 3, and 4. The greatest difficulty experienced in the 

 Fig. 2, 3, 4. formation of this mirror, was to obtain glasses suffi- 

 ciently plane. Buffon selected those that gave the 

 roundest image of the sun at the distance of 150 feet ; 

 and he had to examine more than 500 before he could 

 procure 168 that answered his purpose. With this 

 instrument the following results were obtained. 

 Results ob- March 23. 1747, Buffon set on fire, at the di-s- 

 tained with tance o f 55 f eetj a pl an k o f tarred beech wood, with 

 TO" 8 ^ mirrors only. In this experiment, the mirror 

 was not put upon its stand, and was therefore very 

 disadvantageously placed, forming with the sun an 

 angle of 20 of declination, and another of more than 

 10 of inclination. 



March 23. 174-7. The mirror being still more dis- 

 advantageously placed, a plank tarred and sulphured 

 was set on fire at the distance oif 126 feet, with 98 mirrors. 

 April 3. 1747. At 4 o'clock in the evening, when 

 the sun's rays were weak, and his light very pale, 

 and when the mirror was mounted upon its stand, 

 a slight inflammation was produced upon a plank co- 

 vered with wool cut into small pieces (laine hackee) 

 at the distance of 138 feet, with 112 of the mirrors. 

 April 4. 1747. At eleven o'clock in the morning, 

 when the sun was very pale and obscured with va- 

 pours and light clouds, 154 mirrors at the distance 

 of 150 feet, made a tarred plank smoke to such a 

 degree in two minutes, that it would have been in- 

 flamed, had not the sun quickly disappeared. 



April 5. 1747. At three o'clock in the afternoon, 

 the sun being more feeble than on the day preceding, 

 154 mirrors, at the distance of 250 feet, inflamed in 

 2^ minutes chips of fir deal sulphured and mixed with 

 charcoal. When the sun was vivid, the inflammation 

 took place in a few seconds. 



April 10. 1747. 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 was very sud- 

 den, and took place over the whole extent of the fo- 

 cus, which was about sixteen inches in diameter. 



April 10. 1747. At half past two o'clock, 148 

 mirrors, at the distance of 150 feet, set on fire a 

 plank of beech sulphured in some parts, and covered 

 in others with wool cut into small portions. The in- 

 flammation, which began in the uncovered part of the 

 wood, was so sudden and violent, that it was necessary 

 to plunge the plank in water in order to extinguish it. 

 Apru 11. 1747. TWELVE mirrors at the distance 

 of 20 feet inflamed small combustible matters. TWEN- 

 TY-ONE mirrors inflamed a plank of beech that had 

 been already partly burned. FORTY- FIVE mirrors at 

 the same distance, melted a large pewter flask that 

 weighed about six pounds. ONE HUNDRED AND SE- 

 VENTEEN mirrors melted some thin pieces of silver, 



and made red hot a piece of sheet iron, By employ- Burning 

 ing all the mirrors, Buffon imagined that he could I'i*f- 

 melt metals as easily at 50 feet distance as at 20. 



From ti number of subsequent experiments, Buffon, 

 ascertained that 4-0 or 45 feet was the most advanta- 

 geous distance for making experiments on the metals. 

 The silver plates which he melted at "this distance 

 with 224 glasses emitted a most abundant smoke, but 

 as they were very clear, he did not ascribe this smoke 

 to grease or any other substances which they had im- 

 bibed, as was supposed by some who had witnessed 

 the experiment. When the silver was quite new, it 

 smoked as before, and sometimes for about eight or 

 ten minutes before it was melted. 



At a subsequent period, Buffon burned wood at 

 the distance of 200 and 210 feet, when the sun was 

 brilliant, and he melted all the metals and metallic 

 minerals at the distance of 25, 30, and 40 feet. The 

 mirror requires about half an hour to be properly ad- 

 justed, so that all the images may coincide; but when 

 the adjustment is completed, the focus will continue 

 unaltered for more than an hour. 



The attention of Buffon was next directed to the Buffon's 

 construction of mirrors for burning at short distan- mirrors for 

 ces. He took circular plates of glass about 18 inch- bur ^'*5? at 

 es, two feet, and three feet in diameter, and having t ailces ' S 

 perforated them at the centre with an aperture two 

 or three lines in diameter, he placed them in a circle 

 of iron that was truly turned. A very fine screw 

 connected with a box stretching across the back of 

 the glass, passed through the hole in the centre into a 

 nut on the other side, so that by turning the screw, 

 the circular piece of flat glass was gradually incurva- 

 ted, till it formed a concave mirror. The glass of 

 three feet diameter, when it was bent about |ths of a 

 line, had its focus 50 feet distant, and set fire to light 

 substances ; when it was bent two lines, it burned at 

 the distance of forty feet ; and when it was bent 2-f 

 lines, its focal length was 30 feet ; but in attempting 

 to reduce its focal length to 20 feet, it was broken 

 in pieces. The glass of two feet diameter shared the 

 same fate ; but the one of 18 inches, which had a fo- 

 cal length of 25 feet, was preserved as a model of 

 this species of mirror. The accident which happen- 

 ed to the two largest of these mirrors, appears to have 

 been owing to the perforation in the centre. In order Buffon's 



to remedy this evil, Buffon proposed to place a circular mirr r '"* 

 piece of glass at the extremity of a cylindrical drum, ^ ^ 



'*>' * ' 



curvated 



-. by the pres- 



made of iron or copper, and completely air-tight. sure of the 

 The cavity being exhausted by means of an air-pump, atmo- 

 the glass at one extremity would be pressed in by sphere. 

 the weight of the atmosphere, and would have its fo- 

 cal length inversely proportional to the degree of ra- 

 refaction. This contrivance is represented in Fig. 5, P LATE 

 and a section of it in Fig. 6. 



Astillmoresimple and ingeniousmethod of exhaust- 

 ing the air in the drum was contrived by Buffon. He 

 proposed to grind the central part of the plain glass in- 

 to the form of a -:mall convex glass, and in the focus 

 of this convex portion to place a sulphur match, so 

 that when the mirror was directed to the sun, the 

 rays concentrated by the convex portion would in- 

 flame the mutch, which, being set on fire, would ab- 

 sorb the air, and thus produce a partial vacuum, and 



