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 21 mi nutes chips of fir deal sulphured and mixed with char coal. When the sun was vivid, the inflammation took place in a few seconds.
?pri/ 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 sudden, and took place over the whole extent of the focus, which was about sixteen inches in diameter.
?/aril 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 inflammation, 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.
April II. 1747. TWELVE, mirrors at the distance of 20 feet inflamed small combustible matters. W ENT V ON LI mirrors inflamed a plank of beech that had been already partly burned. Fo RTV-F F. Mirrors at the same distance, melted a large pewter flask that weighed about six pOUDdS. ONE. HUNDRED AND SEVENTEEN 11111TOr5 melted some thin pieces of silver, and made red hot a piece of sheet iron. By employing all the mirrors, Buffo]] imagined that he could melt metals as easily at 50 feet distance as at 20.
Front a number of subsequent experiments, Billion ascertained that 40 or 45 feet was the most advantageous distance for making experiments on the metals. The silver plates which he melted at this distance with 224 glasses emitted a most abundant smoke, hut as they were very clear, he did not ascribe this smoke to grease or any other substances which they had imbibed, as was sup posed 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, Bunn 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 adjusted, so that all the images may coincide ; hut when the adjustment is completed, the focus will continue unaltered for more than an hour.
The attention of Bunn was next directed to the con struction of mirrors for burning at short distances. He look circular plates of glass about 18 inches, two feet, and three feet in diameter, and having perforated them at the centre with an aperture two or three lines in dia meter, he placed them in a circle of iron that was truly turned. A very fine screw connected with a box stretch ing across the back of the glass, passed through the hole in toe centre into a nut on the other side, so that by turn ing the screw, the circular piece of flat glass was gra dually incurvated, till it formed a concave mirror. The glass of three feet diameter, when it was bent about pis 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 21 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 or Is inches, which had a focal length of 25 feet, was preserved as a model of this species of mirror. The accident As hich happened to the two largest of these mirrors, appears to have been owing to the perforation in the centre. In order to remedy this evil, Baron pro pest d to place a circular piece of glass at the extremity of a t. ylindrical drum, made of iron or copper, and corn tely air-tight. The cavity being exhausted by means of an air-pump, the glass at one extremity would be pressed in by the weight of the atmosphere, and would have its focal length inversely proportional to the degree of rarefaction. This contrivance is represented in Plate CV. Fig. 5, and a se( Lion of it in Fig. 6.
A still more simple and ingenious method of exhaust ing the air in the drum was «mtrived by Bunn. lie pro posed to grind the central part of the plain glass into the form of a small convex glass, and in the focus of thi:, 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 inflame the match, which, be ing set on fire, would absorb the air, and thus produce a partial vacuum, and consequently an incurvation of the plain glass.' See Plate CV. Fig. 7 and 8.