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compass that we are indebted for the ease and safety with whichvessels can be navigated when out of sight of land, it was naturalthat the derangement which it might suffer in an iron vessel shouldbe viewed with alarm; and thus the objections of seamen, whosemovements are so dependant on that delicate instrument the com-pass, were well grounded.

The enquiry with respect to the magnetic attraction of ironvessels is one of extreme interest, and well worthy the attention ofscientific men. Professor Airy, Astronomer Royal, has identifiedhis name with this subject; and others of the highest attainmentshave exerted themselves to investigate the peculiar laws by whichthe electric current is guided, and to discover methods to counteractits effects on compasses, especially those of iron vessels. It wasascertained by Captain Johnson, during some experiments in the“ Garryowen” iron steamer, on the Shannon, that an iron ship be-comes a large permanent magnet, having a strong magnetic currentin the direction of the ship’s length, and causing a serious disturb-ance to the general polar direction in which the needle points. Isuppose, however, that this circumstance is favourable to its cor-rection ; for, as the disturbing force is nearly constant, meansmay be adopted to counteract or so to neutralize the disturbing cur-rent, and allow the needle to point in the same direction as it wouldwere it removed from all local attraction. A deviation is found toexist in the compasses of most wooden vessels, arising from thepieces of iron employed about the deck. In ships of war the gunshave a powerful effect upon the compasses, and many plans havebeen tried for their correction. The cause of disturbance is here,however, probably different from the principal part of that of shipsbuilt entirely of iron,—the latter, as I have before said, being inthemselves permanent magnets ; while, in wooden ships, the effectarises perhaps from the iron bolts, bars, straps, &c., in the neigh-bourhood of the compasses becoming conductors (though feebleones) of magnetic currents which arise from other sources.

An iron vessel is formed of an immense number of plates andbars of different forms, having varying degrees of magnetic force,and the electric currents run in different directions. A curious;question then arises as to the period at which the current begins toassume a uniform course through the vessel, neutralizing or turning