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?