LIEUT. -GENERAL SABINE ON TERRESTRIAL MAGNETISM. 
375 
the ship’s iron so far prevailed over the directive influence of the earth (upon the hori- 
zontal needles), that it was obvious that we had nearly attained the limit within which 
the compass, as it had been previously employed, could be available in navigation. In 
a paper presented to the Boyal Society on the return of the expedition to England, and 
printed in the Philosophical Transactions for 1819, Art. XVI., I have related these par- 
ticulars, together with the partial remedies which suggested themselves at the time and 
on the spot, and which were so far practically successful that, both in that voyage and 
in the next (viz. in 1819 and 1820, when Barrow’s Straits were passed and still higher 
magnetic inclinations were encountered), we were still enabled to use the ship’s com- 
passes in navigation, and in some degree also as aids in fixing geographical positions by 
compass bearings, until the inclination became so great that the directive force of the 
earth on the horizontal needle ceased to be appreciable, and the compass pointed uni- 
formly to the general resultant of the ship’s attraction, whatever might be the direction 
of the ship’s head at the time. 
In 1824 M. Poisson communicated two Memoirs to the French Institute, in the 
first of which he propounded a mathematical theory of transient induced magnetism 
founded on the physical theory of Coulomb, that by induction each particle of soft iron 
becomes a magnet, having an intensity proportional to that of all the forces which act 
on it, including the force of the magnetism developed by induction in all the other 
particles of the mass. In a subsequent Memoir, published in 1839, “ Sur les Deviations 
de la Boussole produites par le fer des vaisseaux,” M. Poisson adapted his formulae to 
observations made on shipboard in the particular case of the soft iron being symmetri- 
cally distributed on either side of the principal section of the ship. The memoir was 
accompanied by a practical application and verification of the theory, showing the 
accordance of the calculated results with the facts recorded in the Arctic voyages of 
1818, 1819, and 1820, spoken of above. A careful examination of the disturbances of 
the needle in the ‘ Erebus ’ and ‘ Terror ’ in their passage from England to the Cape of 
Good Hope in 1839, showed that in both ships they were occasioned chiefly, if not 
wholly, by the magnetism induced in the iron of the fittings and equipment by the 
vertical part of the earth’s force, and which was distributed symmetrically on either side 
of the fore-and-aft vertical section passing through the compass. In the case of these 
ships, therefore, M. Poisson’s method of analysis was strictly applicable ; and its practical 
application was greatly facilitated by a memorandum drawn up by Mr. Archibald Smith, 
F.B.S., with which he obligingly supplied me, and which was printed in the provisional 
discussion of the observations of Sir Jambs Boss’s expedition in the first year of the 
Survey, in the V.th Number of these Contributions (Philosophical Transactions, 1843, 
Art. VIII.), — and by a supplementary memorandum, printed in the VIII. th Number of 
the Contributions (Philosophical Transactions, 1846, Art. XVIII.). Mr. Smith has now 
furnished me with a third memorandum, which I subjoin. 
