301. Explain the effect of moisture upon the charged jar. State the experiment represented in Fig. 274. 302. What is the electrical battery? What is said of the light produced by electricity? 303. To what is the report of electricity owing? What is said of mechanical injuries caused by electricity? What of the heat caused by it? What effects may be produced by this heat? 304. What was the discovery of Franklin, and how did he make it? 305. Relate the accident which occurred at St. Petersburg. What is said of lightning-rods? 306. What was Sulzer's experiment? What were Galvani's observations? 307. What is said of the pile of Volta? What of his cup battery and of other batteries? 308. What difference is there between frictional and voltaic electricity? CHAPTER XVI. 308. What are loadstones? Where do they abound? What is said of discoveries in magnetism? Whence come the terms magnetism and magnet? 309. What is said of the attraction of magnetism? What law is there in regard to it? What is said of the poles of a magnet? What of magnetism by induction? 310. What is said of attraction and repulsion in magnets? Explain the formation of the curves of iron filings in the experiment represented in Fig. 282. 311. How may artificial magnets be made? What is said of the horseshoe magnet and its armature? 312. What is said of the magnetic needle and the mariner's compass? 313. What is the declination of the needle? When was it first observed? What is said of observations after this? What is said of the dip of the needle? 314. What is said of the earth as a magnet? What of it as a magnetizer? 315. What is said of fixing magnetism? What of impairing it? In what other substances besides iron does magnetism exist? In what is magnetism like electricity? In what is it unlike it? 316. What relation has magnetism to electricity? What were the original observations of Oersted in regard to it? 317. Describe the manner of making the most powerful electro-magnets. Describe the experiment represented in Fig. 287. 318. Show the application of electro-magnetism in the electric telegraph. 319. What is the contrivance called the signal key? How is the alphabet of Morse's Telegraph constructed? 320. What is said of the communication through the earth in telegraphing? INDEX. [The numbers refer to the paragraphs] Action and reaction equal, 182 Air, compressibility of, 152 Air, elasticity of, 161 Air, density of, dependent on pressure, 158 Air, pressure of, as affecting the boiling point, 171, 327 Air, currents in, from heat, 281 Air, solution of water in, 287 Air attracted by the earth, 148 Air in interstices of substances, 160 Air a non-conductor of heat, 302, 304 Air-pump, 155 Air-guns, 164 Animals, coverings suited to climate, 305 Archimedes, his lever, 229 Archimedes, his discovery in the bath, 138 Archimedes and the crown, 143 Artesian wells, 120 Aqueducts, 119