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LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM.

Royal Society must regard it with the greatest satisfaction, its best praise will un-doubtedly be found in the record of its performances ; and I hasten therefore to enteron that portion of them which I am now enabled to present to the Society.

The peculiar feature in the magnetic survey of the portion of the southern hemi-sphere now under notice is, that it was conducted almost exclusively on board ship,the observations being subject to the disturbance occasioned by the ship’s iron, in apart of the globe where the effect of this influence becomes excessive. The first con-sideration, therefore, must be to investigate the corrections which it is necessary toemploy in compensation. The analysis of the effects produced by the iron of a vessel,and the theory of their corrections, have been given by the late M. Poisson, in a me-moir read in 1838, and published in the Transactions of the Acadfimie des Sciences,entitled “ Memoire sur les deviations de la Boussole produites par le fer des Vaisseaux.”In cases in which the disturbance is due, partly to the magnetism induced by theearth’s influence in the soft iron of the vessel, and partly to permanent magnetismacquired and retained by harder portions of her iron, the complexity of the sourcefrom whence the disturbance originates renders its correction very difficult. But inwood-bililt ships, when proper precautions are taken in regard to the place in the shipin which the instrument is used in observation, the disturbing influence is generallyfound to be that of induced magnetism alone: and in this case the correction maybe obtained with tolerable facility*. The disturbance produced by the iron of theErebus and Terror appearing to be of the latter class, I requested my friend Mr. Archibald Smith , Fellow of Trinity College, Cambridge, who in his academic courseobtained the highest distinction conferred by the University , to draw out fromM. Poisson’s fundamental equations, applicable to induced magnetism, the most con-venient and practical formulce for computing the corrections of the three magnetic

* Since this communication was read to the Royal Society , Mr. Airy has favoured me with the following

note :—“ M. Poisson’s deductions are founded on the assumption, that the phenomena of magnetism depend

on the action of two fluids which attract each other, but which each repel other portions of fluid of the same

kind: and that induction is caused by an alteration in the arrangement of these fluids among the particles of

iron, produced by the attraction and repulsion of the earth’s magnetic fluids. His fundamental equations in

common language may be stated as follows :—

Horizontal force towards the 1 _ f Undisturbed horizontal \

ship’s head, as disturbed, J l force to ship’s head /

Horizontal force towards the | ( ^ f Undisturbed horizontal I

ship’s head, as disturbed, J ”” 1 force to ship’s side /

Tr , ,. , , _ f Undisturbed horizontal l

Vertical force, as disturbed ... = Gx , \

l force to ship’s head J

+ C X undisturbed vertical force.

+ K 7 x undisturbed vertical force.

“ These equations are the same as those obtained by Mr. Airy in the Philosophical Transactions , 1839, the firstand second being the same as the two equations in page 184, and the third being the same as the last of thegroup of three equations in page 181. Mr. Airy’s expressions however imply that G is equal to C. Thecalculations in the sequel of this paper seem to show that in the Erebus G is greater than C. Mr. Airy’sdeductions are founded on the assumption that each particle of iron is converted, by the earth’s magneticaction, into a magnet with its length parallel to the direction of terrestrial magnetism.”