454 LIEUT.-GENEEAL SABINE ON TEEEESTEIAL MAGNETISM. 



Lieuts. Moore, R.N, and Clerk, R.A. (Contribution No. VIIL, Philosophical Transac- 

 tions, 1846, Art. XVIII.), supply the means for a future revision of M. Gauss's work, 

 by the extension of the numerical values of the three elements to equidistant meridional 

 points on three additional southern parallels, viz., those of 50°, 60°, and 70° of South 

 Latitude. 



In resuming the reduction and coordination of the observations of the Antarctic Mag- 

 netic Survey, it may be permitted to recall to remembrance that the system, in accordance 

 with which the surveying compasses both of the ' Erebus ' and ' Terror ' were employed, 

 was the same which had been originally adopted by the writer of these Contributions, 

 and its practical value exemplified, in H.M. Ship ' Isabella,' in the first of the voyages 

 of Arctic discovery in 1818, as described in the Philosophical Transactions for 1819, 

 Art. VIII, The position of the standard compass, and the methods adopted to pro- 

 vide the data required for the investigation and eventual correction of the deviations 

 occasioned by the disturbing influence of the ship's iron, were the same. In M. Poisson's 

 ' Memoire sur les deviations de la boussole produites par le fer des vaisseaux ' (1838), 

 the applications and verifications of the fundamental equations of his theory were exem- 

 plified and established by M. PoissoN himself (pp. 47-49), by the accordance of their 

 calculated results vdth the facts observed and recorded in the Arctic voyages of 1818, 

 1819, and 1820. The convenient and practical formulae for computing the corrections 

 of the three magnetic elements, which were subsequently derived from Poisson's funda- 

 mental Equations by Mr. Archibald Smith (Philosophical Transactions, 184.3, Art. X.), 

 have since furnished, and still continue to furnish, the means of surmounting, even in 

 an extreme case, such as that of a survey executed in the high magnetic latitudes of the 

 southern hemisphere, and in dips even exceeding —88°, the serious embarrassments 

 which would otherwise have been occasioned by the iron which necessarily formed part 

 of the equipment of the ships. Sir James Ross was one of the junior midshipmen of 

 the 'Isabella' in the voyage of 1818; and thus early commenced that interest in the 

 general subject of Terrestrial Magnetism, and that practical acquaintance with the 

 resources which modern science has introduced, by which in his subsequent career he 

 has earned for himself and for his country so distinguished a place in the history of that 

 great branch of physical geography. 



The disturbance of the needle by the influence of the ship's iron, which the term 

 " Deviation" is now generally employed to designate, was foimd both in the ' Erebus' and 

 ' Terror' to be occasioned chiefly, if not wholly, by the magnetism induced in the iron of 

 their fittings and equipment by the vertical part of the earth's force ; and to be dis- 

 tributed symmetrically on either side of the fore-and-aft vertical section passing through 

 the compass. It manifested itself consequently in the southern magnetic hemisphere, 

 and in the usual place of the Standard Compass, by a repulsion of the north end of the 

 compass needle from the ship's head, increasing with the increase of the earth's vertical 

 magnetic force, and producing a deviation proportional to the tangent of the Inclination, 

 The compass-card being divided into 360°, and counted from 0° at north successively to 



