be required, the blowing-iron must be held perpendicularly, (with the Glass at the top of it,) so that the shape may be depressed partly by its own gravity, and partly by rapid rotation. Should the Glass vessel be required of an elongated form, the blowing end of the iron must be held uppermost, the Glass downward, and oscillated like a pendulum, or swung rapidly round vertically, thus, again uniting the principle of gravitation to that of centrifugal force.

Before describing the manipulations of moulding Glass or making it by hand, a short account of experiments upon Optic Flint plate may be interesting. For may years subsequent to the time of the celebrated Dollond, English Flint Glass was almost the only heavy glass used for telescopes, both at home and on the continent. It was generally made from the usual mixture of Flint Glass, with about ten per cent. increase of lead; but still more often of the ordinary mixture (of lead and other materials), and of the specific gravity of about 3.250 to 3.350. The process is as follows:—A ladle, in the form of a sugar-loaf, about five inches in diameter and seven inches deep, is dipped carefully into the metal, which has been previously skimmed; when filled, it is taken out of the pot, and suffered to get partially cool; to the large end of the sugar-loaf shaped piece of Glass thus produced, a Glass-blowing iron with a hollow disk is welded, and placed to the opening, or mouth of the pot for re-heating; when sufficiently soft, it is blown into a muff, as hereafter described, and illustrated, under the head of the Cylindrical Lamp Glass: the end furthest from the blowing-iron is cut off, the cylinder is flattened into pieces or plates of fourteen inches long, ten inches wide, and of about half an inch thick, and