ence in every direction to which that extends. Soft iron, and both horizontal and vertical induction, are T's characteristics.
In all cases of iron which becomes magnetic through the mild inductive influence of terrestrial magnetism, it should be remembered that this influence may be variously modified, if, indeed, not in some instances entirely superseded, by the inductive action of a powerful surrounding field of permanent magnetism in the hull itself.
According to the location of the bulk of each class of iron—the hard and the soft, the vertical, longitudinal, transverse, and unsymmetrical—its resultant or representative, which we may designate as a rod or a bar, will occupy a position relative to the compass, either forward or abaft, to starboard or to port; only one such position for each is shown in Fig. 16; there are, however, two possible positions for every rod, and four for some.
The problem has now been stated, so we will pursue it no further, as the vein of solution would introduce trigonometrical formulæ.
By swinging a ship at compass-buoys, or steaming in a circle on the open sea, the magnetic effect of the ship—that is, of the three imaginary magnets in the axis of X, Y, Z—is brought to bear at every point on the needle, causing it to deflect from the magnetic meridian by different angles at different points. These various deflections, being serially arranged, constitute what is known as "a table of deviations." Upon analyzing this, the numerical strength of each imaginary magnet is obtained, and further disintegration exposes to view their individual component parts. And thus it is that from effect we seek backward to an intelligent comprehension of the cause.
But as a ship sails the ocean she passes through ever-varying fields of terrestrial magnetism; also, her own magnetism is undergoing constant change, due to the wrenching and straining, the shock of waves, and the vibrations set up by firing her battery; from this mutability of cause naturally results a variety in the effect—the deviations. They are never the same.
Let a ship proceed to Havana, and she will find them different from the series determined at New York; at Hong-Kong they differ from those at Rio de Janeiro; in tropical seas they are moderate, in polar regions enormous; when a ship is upright, they have one value; when she heels, they have another. Their varying phases are a manifestation of the strife and successive domination of the three magnets whose intimate relationship has been pointed out; now it is the ship, as when she steers a certain course for many days and thus strengthens her forces; again, it is the earth, when the compass ventures into her frozen strongholds, where it but wavers sluggishly and totters about every course; and finally
