Dr. Scoresby, in his patient experiments and indefatigable attention to the subject through many years, has shown that some portion of a ship's local attraction may be traced to her very cradle—that the very hammering and rolling, and bending, and riveting of her plates engen lers a mischievous influence variable in its intensity as In its duration, and is thus coexistent with the origin of her frame-work itself. That even the direction of the line of keel while building, as measured by its angle with the magnetic meridian, eatablishes fora time a aystematio ilerangement of the action of terrestrial forces in which the effects of terrestrial dip and obliquity have their united influence, which may or may not effect the future navigation of the ship by means of a mariner's compass.
Were the efforts of Dr. Scoresby, and Mr. J. II. Napicr (the eminent iron shipbuilder of Glasgow), and the approach to principles developed by them, not countervailed by the changes incident to every iron ship on leaving her birth-place, little would remain to the sea captain but to receive from the hands of the builder, a diagram of the ships magnetic condition, as produced by the causes already referred to; and indeed as palpably evident and accurate as her load and light water lines, &c., would be the correcting card so handed over ; for it is not difficult to discover a neutral point in every iron ship, at which a standard compass would feel the effects of terrestrial magnetism only. But no sooner, unfortunately, in the process of launching does the ship, if built of metal, plunge into a denser medium, such as water, than the sudden change of temperature affecting the submerged portions of the masa, communicating itself to the upper works, challenges at once the stability of her magnetic condition. Nor is this, at this period, the only enemy to magnetic permanency, for it has been shown by Dr. Scoresby, that the very twist and strain in the framework of the ship consequent upon her being, in launching, raised gradually by the buoyancy of the water, is sufficient to affect materially that which other influences had matured.
The iron ship being once afloat, we shall find still other perplexing disturbances at work, and especizlly if she be intended for a steamer, with iron masts and yards.
If the latter, the increased hammering in the fitting of the engines adds its quota to the general confusion of conflicting and counteracting forces. The lighting of the fires, the heating of the boilers, the pro duction of steam—each of these is a fertile source of further electric, and therefore magnetic, disturbance. It would seem that the con sideration of thermo-electricity, as operating upon the mass of metal in an iron ship, has been somewhat neglected. The enormous change of a single drop of water, which, on its conversion into steam, is expanded into 1700 times its bulk, has been proved years since to generate, or rather to release, a vast amount of electric or magnetic power i • and as we cannot circulate electric currents without magnetising Iron, when in certain favourable positions with regard to it, the com plication of disturbances is so great in an irou steamer that it is not surprising if the wisest heads fail in providing a permanent remedy for local attraction. Ships arc "swung" generally at this period, and the
question of compass error is supposed by many to have been satisfied if the amount of then existing deviation is recorded. But what follows f If we test an iron pillar or bar of even a tradesman's shop.front, wo shall find that in time, and from its vertical, position, it becomes magnetic, and exhibits polarity : and this law affects every vertical bar in the ship. It is an error to suppose that the weight of an iron mass has reference to its magnetic intensity ; for, as magnetism seems to collect on surfaces of metal, it follows that a G-inch hollow iron pipe or funnel, having two surfaces, an inner and an outer, would exhibit more power of disturbing a compass than if it were of solid iron : therefore stove-pipes have been often found dangerous neighbours to a standard or binnacle compass—much more so should we expect an iron mast to be. Hence the very parts used in the construction and fitting of an iron ship are liable to the change to which we have adverted, and this change may be either an increase or diminution of magnetic in fluence, we know not which ; as one piece of hennaed in the construction of the ship may and does counteract another, partially or wholly ; the change moreover may be either rapid or protracted, until some other new source of disturbance comes into controlling operation.
In process of preparation for sea, cargo of various materials, such as hardware, machinery, or iron bars themselves, loads the ship, increasing the strain upon her plates and knees, &e. With regard to iron bars as a cargo, let any one walk round a heap of bars lying upon a wharf, with a pocket-compass, and remark their effect on the needle (whether lying eta of or in the magnetic meridian should be noted). It must be remembered that these are to be placed, we will suppose, in an already magnetised iron ship, and subject, especially lu an iron steamer, to another serious source of change, a change in temperature. Now it is known from experiment that the Intensity of a magnet is nearly doubled by being raised to a "cherry-heat :" the change of temperature in a ship's hold may, even in the tropics, be considered comparatively Insignificant, but the fact remains that disturbance of temperature is disturbance of electricity, and as no electric current can circulate without its attendant magnetism acting at right angles to the direction of such current, it follows that totally unexpected and most intricate and untraceable influences may be set in operation, even by lighting the furnace fires for a few hours in order to take still further precautions in the usual " trial trip" to test both engines and compass. This is another stage at which it is deemed by the majority prudent to finally adjust the "correcting card" for the voyage. This is especially the period of "swinging" her majesty's ships, with what certainty of per manent advantage may be still further illustrated.