146 
L1EUT.-C0L0NEL 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 enter 
on 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 a 
part 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 to 
employ 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 Academie des Sciences, 
entitled “ Me moire 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 the 
earth’s influence in the soft iron of the vessel, and partly to permanent magnetism 
acquired and retained by harder portions of her iron, the complexity of the source 
from whence the disturbance originates renders its correction very difficult. But in 
wood-built ships, when proper precautions are taken in regard to the place in the ship 
in which the instrument is used in observation, the disturbing influence is generally 
found to be that of induced magnetism alone : and in this case the correction may 
be obtained with tolerable facility*. The disturbance produced by the iron of the 
Erebus 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 course 
obtained the highest distinction conferred by the University, to draw out from 
M. Poisson’s fundamental equations, applicable to induced magnetism, the most con- 
venient and practical formulae 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 : — 
f Undisturbed horizontal 1 
1 force to ship’s head J 
{ Undisturbed horizontal "1 
force to ship’s side / 
f Undisturbed horizontal") 
Horizontal force towards the 1 
/ — X 
ship’s head, as disturbed, J 
Horizontal force towards the | 
+ C X undisturbed vertical force. 
ship’s head, as disturbed 
Vertical force, as disturbed 
E' X 
. = G X 
1 force to ship’s head J 
+ K' X undisturbed vertical force. 
“ These equations are the same as those obtained by Mr. Airy in the Philosophical Transactions, 1839, the first 
and second being the same as the two equations in page 184, and the third being the same as the last of the 
group of three equations in page 181, Mr. Airy’s expressions however imply that G is equal to C. The 
calculations in the sequel of this paper seem to show that in the Erebus G is greater than C. Mr. Airy’s 
deductions are founded on the assumption that each particle of iron is converted, by the earth’s magnetic 
action, into a magnet with its length parallel to the direction of terrestrial magnetism.” 
