NUMBER FOUR Subsequent to the experiments of Mr. Porter, Dr. S. P. Andrews, of Perth Amboy, N. J., a gentleman well known in scientific associations, and of high reputation as a successful inventor, devoted much time and money to the subject of aerial navigation, and with partial success. Having been early acquainted with scientific principles, and had extensive experience in mechanics, his projected enterprise gained much confidence with many intelligent men, who supposed him to be more competent to accomplish tkis long desired scientific improvement than any other man ; and this confidence in his ultimate success yet remains, in the minds of many, unimpaired. But to give our to be made into lenses ; they are placed in large clay molds made of the best fine clay. When a piece has been selected of sufficient hight and size, it is put into a mold of the re: quired dimensions, and then gradually re-heated until the glass hasmelted exactly the shape of the mold. Then, when it is sufficiently annealed, it is polished by the glass cutter in the regular manner. Other kinds of glass are made for optical purposes by being blown with the iron tube of the glass maker, as other things are blown, such, for instance, as glass for magnifying purposes. The glass is ladled from the crucible, then taken from the ladle on the end of the iron tube, and blown of an uniform thickness, exactly the shape of a lady's muff. When annealed it is cut up one side with a diamond, and then exposed to considerable heat. When the heat causes the glass to open Manufacture of Optical Glass. The materials are fused in the furnace; and when nearly ready for working are stirred about with cold iron rods to break the cords and lessen the cloudiness, Sometimes the metal is ladled all from the cruibles, and thrown into cold water. This stirring and ladling has the effect of breaking the strica. It is then closed up in the crucible again until it is perfectly fused in the ordinary manner, but is not worked outas is the case at Whitefriars Glass Worksfor working either with the glass-maker's rods or the iron ladle renders it worse. When a large crucible is declared to be perfectly ready, it is allowed to cool until the whole mass is one solid piece of ordinary glass, weighing about twelve or sixteen hundredweight. This mass is sure to crack up into large boulders, and from these pieces ara selected those which are MARIOTT'S '' AVITOR” AIR SHIP. flat surface, and spread out into a large square of thick optical glass. It is again annealed, and polished to the required magnifying power. It will be easily seen from all these processes that fine optical glass must necessarily be very expensive.liJnglisA .Mechanic.