A small box or dark room into which the light is admitted through a lens. What is a Stereoscope? It is an instrument exhibiting the effects and advantages of seeing with two eyes. The instrument is so constructed that from a flat picture we may see the solid body in its reality in nature. What is a Kaleidoscope? An instrument invented by Sir David Brewster, consisting of a tube with slips of reflecting glass so arranged in the interior that small beads, bits of colored glass, and similar things are, by revolving the tube, thrown into an endless variety of beautiful shapes. CHAPTER XXI. ELECTRICITY, ELECTRIC CURRENTS, ELECTRIC BATTERY, ELECTROTYPING, STEREOTYPING, TELEGRAPH, OCEAN CABLE, LIGHTNING ROD, THE GULF STREAM, THE MT. CENIS TUNNEL, THE SUEZ CANAL, SUSPENSION BRIDGES, EMINENT AMERICANS. What is the nature of Electricity? A form of energy into which all other forms can readily be converted. What is an Electric current? Electricity manifests itself in a variety of ways, but all may be arranged under two heads, _viz._, 1, as a charge; 2, as a current. By means of friction, many bodies become electrified--that is, have acquired an electrical charge. If this charge is in great quantity we call it high tension. When a body containing an electrical charge is brought in contact with other bodies through which electricity is capable of passing, there ensues a current of electricity. Such bodies are called conductors. What are the sources of currents? There are currents produced by chemical action called voltaic currents; by the action of heat, or thermo-electric currents; by the motion of magnets, or magneto-electric currents. [Illustration: REMOVING THE EARTH FROM THE CANAL BY MEANS OF DROMEDARIES.] [Illustration: OPENING THE SUEZ CANAL--PROCESSION OF SHIPS.] What is positive and what negative electricity? No difference in electricity in itself. When a body has more than its natural amount of electricity, it is said to be charged positively; when it has less than its natural amount it is negatively charged. What is a Cell; what a Battery? If a piece of zinc and copper joined by a wire be dipped in a liquid--generally weak sulphuric acid--which will act chemically on the metals, a current is produced. Such an arrangement is called a couple, or cell. If many cells are connected, then it is called a battery. What is Thermo-electricity? If two bars of any unlike metal--for example, antimony and bismuth--be soldered together at one end, and the other ends be connected by a wire and then the soldered end heated, a current will flow. What effects are produced by currents? They produce heat, light, decomposition and combination in liquid chemical compounds; they melt all metals, excite magnetism, and in the animal body excite movements of the muscles. Can you specify these effects?