Regulus of antimony, 33 grains, on charcoal, were fus ed in 3", and II grains only remained after 195".
Fine kearsh from the cannon foundcry evaporated very fast during 120", and 36" afterwards the remainder flow ed in globules, which were attracted by the magnet when cold.
Crystal pebble of North America, five grains, con tracted in 15", Were perfectly glazed in 135", cbullesced in 150", and became of a slate colour, and semitranspa rent.
Agate, oriental flint, cornelian, and jasper, Were ren dered externally of a glossy form.
Garnet, placed upon black lead fused in 120". It be came of a darker hue, lost one-fourth of a grain, and was attracted by the magnet. Ten cut garnets from a bracelet run into one another in a few seconds.
Mr Wedgewood's pyrometrical clay run into a white enamel in a few seconds. Other seven kinds of clay sent by that gentleman were vitrified.
Limestone was sometimes vitrified and sometimes agglutinated. A globule from one of the specimens flew into a thousand pieces when put into the mouth.
Stalactites nalithus spatosus, nine grains, took a glo bular form in 60". The globule began to become clear in 148". It became perfectly transparent in 155" When cold, its transparency diminished, and it assumed a beau tiful red colour.
Lavas and other volcanic products likewise yielded to the power of this lens.
In the year 1802, Sir Joseph Banks, Dr Crawford, and sonic other members of the Royal Society, were present at an experiment for concentrating the lunar rays ; but though the most sensible thermometers were applied, it was rather thought that there was a diminution than an increase of heat.
It was not to be expected that this powerful lens, which cost such a large sum of money, could have been retain ed in the hands of Mr Parker. That ingenious artist was naturally desirous to indemnify himself for the expense of its construction. A subscription was therefore opened for purchasing the lens, as a national instrument ; but, to the disgrace of our country, this subscription completely failed, and Mr Parker was induced to sell it to Captain Mackintosh, who accompanied Lord Macartncy to China. This valuable instrument was left at Pekin, where it re mains, a monument of Chinese ignorance, and British parsimony.
From the history which we have now given of dioptric burning instruments, it evidently appears, that in every construction which bias yet been proposed, a limit is ne cessarily put to the magnitude, and consequently to the power of burning lenses. The extreme difficulty of pro curing glass proper for a large solid lens ; the trouble and expence of casting it into a lenticular form without faults and impurities ; the great increase of central thick ness which becomes necessary by increasing the diame ter of the lens ; the enormous obstruction which is thus opposed to the transmission of the solar rays ; and the augmented aberration which dissipates the rays at the focal point; arc almost insuperable obstacles to the construction of a solid lens of a size larger than that of Mr Parker.
Buffon, indeed, has very ingeniously removed one of these difficulties, by his proposal to construct the lenses of three concentric zones. The quantity of glass is ob viously diminished by this contrivance, and the ob struction of the rays in passing through the central por tions of the lens is greatly lessened ; but the outer cir cular zone must still contain as much glass as the cor responding zone in the solid lens, arid must be nearly as difficult to cast and grind as if it formed a part of the whole lens.
In order to remove these eN ils, and at the same time to diminish the expence and simplify the construction of dioptric burning instruments, the following construction has been proposed by Dr Brewster. If it he required, for example, to construct a burning lens 4 feet in dia meter, it should be composed of different pieces, as re presented in Plate CVI. Fig. 2, where ABCD is a lens of flint glass, 18 inches in diameter. This lens is sur rounded by several segments, AGID, AGEB, BELC, CLID, ground in the same tool with ABCD, but so formed with respect to their thickness at AB and GE, &c. that they may exactly resemble the corresponding portions of a solid lens. These different thicknesses can be easily calculated, and there is no difficulty in giving the segments their proper form. This zone, consisting of separate segments, is again surrounded with other segments, G NOV, MEP, l'EMQ, QM LB, 1ILKS, SKIT, '1'111 V, VI IGN, each of which is six inches broad in the direction of the radius. The section of this lens is represented in Fig. 3. where DE is the central por tion, DC n, E a F the second zone, and CA m, Nip the external zone. One of the segments is shown separately in Fig. 4. By this combination of segments, a lens four feet in diameter will be formed, and will obviously pos sess the same properties as if it consisted of solid glass. The advantages of this construction may be very shortly enumerated.