346. Lines of magnetic force.—This eminent philosopher has also shown that if you place a magnetized bar of iron on a smooth surface, and sift fine iron filings down upon it, these filings will arrange themselves in curved lines as in Fig. 1; or, if the bar he broken, they will arrange themselves as in Fig. 2. The earth itself, or the atmospheric envelope by which it is surrounded, is a most powerful magnet, and the lines of force which proceed whether from its interior, its solid shell, or vaporous covering, are held to be just such lines as those are which surround artificial magnets; proceed whence they may, they are supposed to extend through the atmosphere, and to reach even to the planetary spaces.
Many eminent men and profound thinkers, Sir David Brewster among them, suspect that the atmosphere itself is the seat of terrestrial magnetism. All admit that many of those agents, both thermal and electrical, which play highly important parts in the meteorology of our planet, exercise a marked influence upon the magnetic condition of the atmosphere also.
347. The magnetic influences of the oxygen of the air and of the spots on the sun. —Now, when, referring to Dr. Faraday's discovery (§ 345), and the magnetic lines offers as shown by the iron filings (§ 346), we compare the particles of oxygen gas to
