We have mentioned her majesty's ships—as an extreme case we now will confine our deductions to the incidents attending their proceeding to sea. It is fair to assume that ships are fitted in still waters, either in harbours or estuaries; whether such ships be new, or have been lying dismantled in ordinary, affects our main scrutiny as to changes in a very immaterial degree. In either ease, whatever changes do occur they are due to what Scoresby has called retentive magnetism. Now guns are formed of a metal which, from partial carbonisation, rapid or unequal cooling, and a tendency to crystallisation, or at least a peculiar arrangement of its particles, has in the ordinary form of east-iron a peculiar aptitude for the absorption of magnetic influence : and, indeed, excellent permanent magnets may be formed of it, inferior only to steel. These guns have moreover to undergo a process of doneussion in firing still further calculated to derange local attraction, by increase or diminution, we cannot tell which : and even the training of these guns may, in their altered position, affect the local attraction of the whole ship.
And again, independent of the evolution of magnetism in the boiler, the very act of combustion of coal has its influence on the condition of an iron steamer. For not only does the combination of carbon with the oxygen of the air (which is decomposed in the furnace in the act of combustion) in the form of carbonic acid gas increase the magnetic intensity of the funnel, but the decomposition of the coal itself is a source from which magnetism is as it were poured into the very mass of the ship herself in abundance, perhaps to saturation, if such can exist. In addition to the electro-magnetic disturbance incidental to all de compositions, coal is composed of carbon and "earthy matter." Now pure carbon is of itself capable of polarity, inasmuch as crystallisation is found in the diamond : but the " earthy matter " is worthy of serious examination. Coal varies greatly in the quantity of earthy matter it contains, and for the following reasons : the lower the specific gravity of the coal used, so much the less probability of magnetic disturbance one might look for.
Earthy matter in coal amounts to, in general, from 1 to 10 or 12 per cent., such being tither pure oxide of iron, or sulphuret of iron with occasional traces of alumina. Now if we, for experiment, burn off the sulphur by the blowpipe from a small portion of sulphuret of iron, or iron pyrites, we shall find the residue strongly magnetic ; and this may be verified by our taking some of the dust of the ash-pit of a furnace, or of a common house-grate, and placing it on a piece of ivritiug-paper ; holding, a magnet under the paper will at once show polarity in the particles, and moving the magnet along the surface and back again will abundantly show that combustion of coal therefore produces constantly a large number of small distinct magnets, the power of which is increased not only by their being in proximity to each other, but by the high temperature at which they are maintained until usually removed from the ash-pit. When we further consider that in
a large steamer some 100 to 200 tons of coal are consumed in a day, it is not too much to say that some tons of magnets are manufactured daily on board these vessels : we say manufactured, for in the form of coal, or even as sulphurct of iron, such is not magnetic.
And again, with all these tendencies to produce a disturbing effect on the known or once ascertained local attraction of the ship, there remains yet the possibility of further inconvenience to the compass arising from heavy pitching or rolling in a sea way : even her "heeling over" is well known to affect the compass to a considerable degree.
Such then is " local attraction " on shipboard ; an influence from which no ship nearing the land would be safe, as the changes re ferred to cannot be anticipated or averted by any process yet known. It remains, therefore, to renew " corrections " as often as possible, using a heavenly body as the sole standard. Under the article COMPASS CORRECTIONS it has already been shown how readily and completely this may be done by means of Saxby's spherograph and Professor Airy's arrangement of soft iron and magnets. Local attraction on ship board is, therefore, a subject for anxious vigilance on the part of every commander. Many ships are found very unsuaceptible of changes ; others, after years of inactivity, suddenly become excited. As a general rule, local attraction is expected to diminish after a ship has been a year or two at sea : but no ship or vessel, especially a steamer, ought to 110 navigated by means of a correcting card, unchecked by frequent celestial observations, for at least the first week or two after leaving port. A prudent commander will never trust to his compass for a single day if azimuths can be taken.
(A aufwal Magazine; Dr. Scoresby, Magnetical Investigations; rto.) LOCI. [Loons.]