it-ease of entry and ease of exit: an analogous condition of iron for magnetism is called its Permeability; in any particular mass of the metal, it is the ratio of its magnetization to the magnetizing force-a numerical quantity, since both the cause and its effect can be expressed by numbers. Magnetization, however, does not increase in a constant ratio to field intensity: when this is feeble, the metal acquires but little magnetism; when strong, the magnetization is great and rapid; while a further increase of field adds but little to the amount already acquiredthe tendency is toward a limit. This point is explained in Art. 191.

The name Iron is used here in the generic sense, and therefore includes all its varieties-wrought and castpure and mixed with carbon to form steel of varied hard

ness.

The mixture of certain ingredients with iron lessens its susceptibility to magnetism: it is said that 12 per cent. of manganese, and even a less quantity of antimony, combined with steel, will make this metal non-magnetic; and that arsenic produces the same effect on nickel. Besides iron, nickel and cobalt are sufficiently receptive of magnetism to be injurious in close proximity to the compass.

Every mass and particle of iron on the Earth's surface acquires in some degree the intensity of the natural field surrounding it; and this without violence of any kind to the metal, but quietly, by induction alone by the mere fact that the metal is in the midst of the Earth's 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-features

entirely analogous to those of a steel magnet, and acquired from the gentle, steady impress of terrestrial magnetism alone.

Mechanical work upon iron-such as hammering, bending, twisting, boring, drawing into wire, heating and suddenly chilling, tensile stress and compression-will affect the degree of magnetization: after any of these processes, the magnetism possessed will differ from what it would have been before them; and in the hull, armament, and equipment of a ship of war, there are masses of iron that have undergone one or other of these processes the iron of her structure will be differently affected by the Earth's magnetism.

Soft iron affords a standard of magnetic permeability, and should therefore be defined: pure Swedish iron is "made soft by soaking in a blood-red fire for some hours, and then cooling very slowly by burying in the hot ashes, or allowing the fire to go out." It is susceptible of the highest magnetization; but this is only transient.

Let us consider a metallically pure cylinder of soft iron that has not been hammered, and conceive it free of magnetism: hold it in the line of Dip, and instantly the upper end becomes a south-pole and the lower a north-pole—that is, the FORMER (a blue pole) will attract the north point of a compass-card, and the LATTER (a red pole) will repel it; this is what occurs north of the Magnetic Equator-the converse will be the case south of it; while on that line, neither end, with the cylinder vertical, will exhibit these characteristics, because no vertical component of terrestrial magnetism exists there to produce them.

Reverse the cylinder as quickly as we may, and the magnetism also reverses, so that the upper and lower poles are the same as before.

Hold it horizontally in the meridian, and the end toward the north becomes a red pole, while that toward the south

a blue pole. Revolve it slowly or rapidly in azimuth, and the foci of magnetic polarity also move with the fidelity of a shadow, until, when the cylinder points east and west, all the side facing the north is pervaded by red magnetism, and that facing the south by blue magnetism. Of course, the colors are here used to express the oppositeness of polarity, not an actual fact.

Now, let us conceive the hull of a ship to be like the cylinder, of metallically pure soft iron and as susceptible to magnetic induction in her ever-changing course as the cylinder is when turned round. Then as the ship heads north (in north latitude), the bow will become the center of red polarity, and the stern that of blue. As she gradually changes course to the eastward, so will the red focus shift to the port bow, the blue focus to the starboard quarter, and the neutral line dividing them (which, while the ship headed north, was athwartships) will now become a diagonal from starboard bow to port quarter. When the ship heads east, all the starboard side is pervaded with blue polarity, the port with red, and the neutral line takes a general fore-and-aft direction. Continuing to change course to the southward, the poles and neutral line continue their motion in the opposite direction, until at south the conditions at north are repeated, but this time it is the stern that is red and the bow blue.

At west the conditions at east prevail, only that it is the starboard side that has red polarity, and the port side blue. In south magnetic latitude, the reverse of all the preceding would occur.

And this transitory induction in both the cylinder and the ideal ship is solely the effect of the Earth's magnetic field.

But, to consider this in connection with an actual ship: the iron of no vessel, its armament, or equipment is metallically pure; nor has it acquired shape without much hammering; moreover, it cannot be made an abstraction from

a magnetic state. By the varied processes of building, the whole structure has become as permanent a magnet as a steel bar, with the poles and neutral line located according to the magnetic direction in which the ship lay on the stocks, in conformity to the places they occupied in the ideal vessel just described. Therefore, she is not now as susceptible to the mild induction of the Earth as the softiron cylinder and ideal hull are, although the straining while on a passage and the buffeting of the waves do assist the inducing tendency; besides, once that the magnetic impress has been made-as in building-it does not move with the facility it did in the soft-iron cylinder, so that the transitory induction finds a tenacious occupant of the vessel, and must adapt itself accordingly: it is the resultant of both the temporary and permanent magnetism we always find, and not the individuality of either.

Time is an important element in the acquisition and intensity of this transient magnetism; for the longer a ship steers on a given course, or swings at anchor in the same general direction, the greater will be its amount: it is of prime importance in navigation; the characteristic magnetic features of a ship are not very mobile, but those that acquire temporary lodgement are treacherous and changeable to a degree that necessitates constant vigilance to prevent disaster.

Instead of attributing the loss of vessels to improbable influences upon the compass, it were more reasonable to ascribe it to the changed condition of her magnetism by temporary induction during the passage, and which has not been discovered or kept account of by frequent azimuths previous to closing in with the land. Suddenly, a course the Captain thought perfectly safe, carries the ship upon a shoal or rock, and the fault is laid upon the compasses, whereas they but obeyed the magnetic influences that became altered during a long passage.

260. Experimental proof of the ship being a magnet.Occasional reference has been made in this Treatise to the SCORESBY-Fig. 428. In 1883, when the Compass Office was under the Bureau of Navigation, I received authority from Rear-Admiral J. G. Walker, U.S.N., then chief of the Bureau, to have this little vessel made at the Washington Navy Yard: it was designed to illustrate experimentally the mathematical theory of the deviations, the magnetism of ships, and other related matters. It is still in use for these purposes, and also for showing officers ordered as navigators of ships the practical methods of compensating compasses: indeed it is the final experimental arbiter of all questions in this branch of enquiry. For convenience of reference, it was given a name-that of the able investigator who accomplished so much in this field of research.

A few changes have been made in it since 1883, so that the description here given applies to the original design, with which the experiments described in this Part of the Treatise were made.

Dimensions of the SCORESBY: length, 6 ft. 9 in.; beam, 3 ft., 2 in.; height of upper deck from floor, 3 ft. 2 in. The stem, keel, and stern-post form one piece-a stout bronze casting. From strong bronze cross-pieces attached to the keel near the bow and stern, four heavy bronze screws rose and supported the three wooden decks; the middle and lower decks could be raised and lowered at will. The vessel was pivoted at the stern in a wooden socket screwed to the floor; this socket was in the center of a large brass circle (also fixed to the floor) and upon which a bronze wheel at the bow turned, affording motion. in azimuth.

There was a contrivance for heeling the vessel to any angle up to 45°, and also for giving it a deep and rapid. rolling motion.

Thus the SCORESBY, like a ship at compass-buoys,