What is Gum Arabic? The juice of a small tree of the Acacia tribe, growing in Egypt, Arabia Petraea, Palestine, and in different parts of America. Are there other plants or trees which produce Gum, besides those already mentioned? A great number, though not all commonly in use. The leaves of rhubarb, the common plum, and even the sloe and the laurel, produce a clear, tasteless gum; there are also a number of different gums, brought from foreign countries, of great use in medicine and the arts. Most of the Acacias produce gums, though the quality of all is not equally good. What is Rhubarb? A valuable root growing in China, Turkey, and Russian Tartary. Quantities of it are imported from other parts of the world: that from Turkey is esteemed the best. Rhubarb is also cultivated in our gardens, and the stalks of the leaves are often used in tarts; but the root, from the difference of climate, does not possess any medicinal virtue. CHAPTER XII. SPECTACLES, MARINER'S COMPASS, BAROMETER, THERMOMETER, WATCHES, CLOCKS, TELESCOPE, MICROSCOPE, GUNPOWDER, STEAM ENGINE, AND ELECTRO-MAGNETIC TELEGRAPH. When were Spectacles invented, and who was their inventor? It is supposed that they were first known about the thirteenth century, and invented by a monk of Pisa, in Italy, named Alexander de Spina. Spectacles are composed of two circular pieces of glass set in a frame. What are these glasses called? Lenses. They are either convex or concave, according to the kind of sight requiring them. Old people, and those who can only see things at a distance, from the flatness of the eye, which prevents the rays of light converging so as to meet in the centre, require convex lenses. People who can only distinguish objects when viewed closely, from the eye being too convex, require concave lenses to counteract it by spreading the rays, and thus rendering vision distinct. _Convex_, rising outwardly in a circular form; opposite to concave. _Concave_, hollow; round, but hollow, as the inner curve of an arch, &c. _Converging_, tending to one point from different parts. _Vision_, the faculty of seeing. What is the Mariner's Compass? A most useful and important instrument, by the aid of which the navigator guides his ship on the sea, and steers his way to the place of his destination. The inventor of the Mariner's Compass is not known, nor the exact time of its introduction; it was employed in Europe in navigation about the middle of the thirteenth century, and has been in use more than five hundred years. The Chinese are said to have been acquainted with it much earlier, but no reliance can be placed on their dates. The power of the loadstone to attract iron was known to the ancient Egyptians, but it was not applied to any practical purpose. _Navigator_, one who guides a ship. _Steer_, to direct or guide a vessel in its course. _Destination_, the place to which a person is bound.