rife up in a round cylindrical form like the trunks of trees, and divide and subdivide, so as to reprefent branches and boughs; and has a truly wonderful and beautiful appearance. And every different kind of falt forms a different figure. There are feveral other objects, which I must recommend to the attention of the young obferver : : as, 1. The common Fly, the vibration of whofe wings is repeated feveral hundred times in a second of tine. The great quantity of eyes with which this animal is favoured renders it worthy of notice, each having a diftinct optic nerve. 2. The Loufe. 3. Mites in cheese. 4. The cuticular Pores in the human skin, fo clofe and numerous, that a fingle grain of fand will cover hundreds of them. 5. The construction of the Scales of Fishes. 6. The Animalcules in several sorts of infufions and liquids. 7. The construction of common Feathers. 8. Hair. 9. The Sting of a Bee, with the form of its barbs. Configuration of Wood. 11. The common Mildew, which displays a numerous group of vegetable substances. 12. Small vegetable Seeds. And feveral other articles. 10. The I fhall close this chapter with a few words concerning the method of making fmall fpherule lenfes for microfcopes, as recommended by the late celebrated Mr. George Adams. And alfo the method of mixing the metals for the great fpeculum in the reflecting telescope. To make the finall fpherule lenfes, a piece of window.glafs is to be cut into flips, about an eighth of an inch in breadth. Then, holding one of thefe flips of glafs at each end in the flame of a lamp, as the glass begins to melt, it is to be drawn by each hand into a fine thread, and at length it will break. Then one end of this thread being held in the flame of the lamp, it will run up into a fmall globule, which is to be taken off; and is a small fpherule lens. Several of thefe lenses are to be made, and examined; and those that are the best, are to be preferved for ufe. For fome will always prove faulty, though every precaution be taken. In In this procefs, the lamp is to be supplied with spirits of wine inftead of oil, and the flame is to be blown in an horizontal direction by a blow-pipe, or a pair of bellows for that purpose; and the glass held in the whiteft part of the flame, left the fmoke fully the glass. The metal for the fpeculum of the reflecting telescope is generally formed of copper and grain tin; and in the proportion of two pounds of Swedish copper, to fourteen ounces and an half of grain tin; and this mixture is to be melted twice over, before it be caft into the mould. When the metal is caft into a concave mirror, it is ground upon, what is called, the rough grinder, or even a common grind-stone, of the same radius as the concavity of the metal, to take off the rough face. Then it is ground on a brafs convex grinder, to give a true spherical figure; and laftly, upon a convex bed of hones, which is to perfe& that figure, and give the metal a fine fmooth face. Then the concave face of the metal is to be polished by a convex tool, covered with pitch. And, laftly, it is to be brought into the parabolic form by a merely mechanical method of grinding it on the polisher in a different direction. But the beft metal for fpeculums is that propofed by the Rev. Mr. Edwards, and which was proved by Dr. Mafkelyne to excel, in brightness and diftin&tnefs of the image, every other metal then known. It confifts of thirty-two ounces of copper, with fifteen or fixteen ounces of grain tin (according to the purity of the copper), with one ounce of brass, one ounce of filver, and one ounce of arfenic. I once was prefent at the cafting of a speculum of this metal; and, when finished, it reflected more light than any fpeculum I have ever feen. END OF THE FIRST VOLUME. S. GOSNELL, Printer, |