L1EUT.-C0L0NEL SABINE ON TERRESTRIAL MAGNETISM.

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a ship changes her magnetic latitude, the corresponding change in the magnetism ofthe ship, or more strictly in that portion of it which is derived from induction, fol-lows, but does not always, or altogether, take place instantaneously. It would accordwith this supposition, that the disturbance of the compass should be less in theErebus on her first arrival at Hobarton in 1840, than on her return there in 1841,because in 1840 she had recently passed through the lowest magnetic latitudes, andin 1841 she came immediately from the highest. The observations in 1840 give aless value for a tan 6 than those of 1841, and taking the dip at Hobarton as the valueof 6, to which the induced magnetism of the ship on both occasions should strictlycorrespond, we should have a less value for a in 1840 than in 1841; whereas if withthe same dip we take a mean between the disturbances of the compass on the firstarrival and on the return, by which we may be conceived to neutralize in a greatmeasure the temporary influences which have been supposed, we find the value of ato be almost identical with the result of the former experiments at Gillingham. Fromthis accordance in the value of the constant in dips which differ so greatly as from-f 69° to — 70°, we should infer the probability,—first, that the local attraction of theErebus was due to induced magnetism alone, the influence of any portions of ironwhich, in the strict sense of the term, were permanently magnetic, being insensible ;—and secondly, that no material change affecting the standard compass had takenplace in the distribution of her iron. These inferences are by no means inconsistentwith the supposition above suggested, that some portions of her iron might be of aquality intermediate between that of perfectly soft iron which undergoes instanta-neous change, and that of iron which acquires permanent magnetism, and that suchportions should be liable, in regard to their magnetic condition, to be more or less inarrear of the ship’s magnetic position. I abstain from entering further into thisquestion at present, because a fitter opportunity of doing so will be afforded whenthe whole of the observations of the Expedition shall be collected, including thosewhich have yet to be made at Rio de Janeiro on the return from the high latitudesof the south, and in England after passing through the low magnetic latitudes of theequatorial region. Should it prove that the induced magnetism of a ship due to anyparticular dip requires time for its full development, more or less according to thevarious quality of her iron, the corrections to be applied may possibly in some shipsbe considerably complicated thereby: fortunately in the Erebus the difference in theamount of the disturbance on the two occasions at Hobarton, which gave rise to thisdiscussion, is not of any serious consequence; and we may employ without anymaterial inconvenience for our present purpose the mean of the two series as appli-cable generally between their respective dates, for which interval we specially desirethe corrections.