wheels, and weights by that mysterious agency we call magnetism, and the oscillations of the magnet, when drawn out of parallelism to the lines of force, will be entirely similar. Such are the efforts of the compass to regain its normal direction when disturbed; and the test of a good compass is the sensitive quickness with which it will turn aside from the magnetic meridian when another magnet is brought near, and the celerity of its return thereto when the intruder is removed.
Fig. 8—Mechanical Illustration of the Magnetic Force that gives the Compass-Needle Direction. The dotted parallel lines represent the earth's magnetic lines of force.
The Iron Ship a Magnet.—It is a characteristic of every mass and particle of iron on the earth's surface to acquire in varied degree the terrestrial magnetism that surrounds it; and this agency enters naturally, without effort or force: it is gently induced in the material so congenial to it, by the mere fact of the material quietly lying in its midst—the magnetic field, which pervades all space. And the word iron is not here used in a specific sense, but as a general term to include wrought-iron, cast-iron, and steel, which are all susceptible to magnetism.
The steel rails that afford transit from seaboard to interior, the trestle-work upon which the elevated trains traverse the metropolis, the heavy castings in a foundry, the massive forgings in a machine-shop, even the little scraps upon a neglected heap, have one and all magnetic features that distinguish them from other metals, and point out the common kindred among themselves. And these features are entirely analogous to those of the steel magnet already described—two poles, one at each end of the mass, with a neutral belt between.
Let us conceive a metallically pure cylinder of wrought or cast iron that has not been hammered, and let us further conceive it entirely free from magnetism: hold it vertically, and instantly the upper end becomes a south, and the lower a north pole (in this latitude). Reverse it as quickly as we may, and the magnet-
