LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
99 
satisfactory as could be wished or expected ; and I have accordingly taken the above 
stated values of b, c, and d, for the whole period under notice. 
On a general review of the examination to which the observations in the Erebus 
and Terror in this and the preceding voyage have been subjected, in reference to the 
magnetic influence of their iron, we find reason to conclude from the consistent ex- 
perience of both voyages, that the disturbance in them was altogether such as would 
be occasioned by the magnetism induced in the soft iron of the ship by the magnetism 
of the earth, — if we permit ourselves to include as possessing the quality of softness, 
certain portions of iron which, though not permanently magnetic, do still retain 
polarity, and require some time to conform to the changes in magnetical relations 
induced by changes of geographical position. It is not improbable that this may be 
a general case in sailing vessels similar to the Erebus and Terror ; but we should 
by no means be warranted in deriving a corresponding inference in regard to ships 
which contain steam machinery, and still less in the case of iron vessels. These 
may possibly possess permanent magnetism strictly so called ; in addition to in- 
duced magnetism, and temporarily-abiding polarity. It is very desirable that we 
should have some means of judging of what may be expected in vessels of these 
two classes. The knowledge would be valuable were it only for the compass cor- 
rections necessary for the ordinary purposes of navigation ; and it appears indispen- 
sable before a correct judgment can be formed of the confidence to which methods 
may be entitled, which have been already, or may hereafter be devised, to supersede 
these corrections by the employment of compensating forces. It is not necessary that 
steam or iron-built ships should perform voyages like those of the Erebus and Terror 
to procure this knowledge ; a voyage from the British Channel to the Tropics would 
be sufficient ; the ship should be swung before her departure from these islands, and 
immediately on her arrival in the Tropics, and at intervals of three or six months 
during her continuance there ; the experiment should also be repeated on her return 
to England before any material alteration is made in the distribution of her iron. 
Index Correction. 
Index Correction of R. F. 5 for the Observations of the Inclination in the Erebus . — 
The observations at sea with this needle having been made in the one position of the 
instrument only, viz. with the face of the circle towards the east, and the marked side 
of the needle towards the observer, — we have to obtain the index correction, by com- 
paring the inclinations observed in the same manner on shore, or on the ice, with the 
results given at the same places by needles of which the poles were reversed and the 
needle and circle used in the eight ordinary positions. 
The stations which furnish this comparison are Hobarton, Sydney, New Zealand, 
the Falkland Islands, and two stations on the ice in the latitudes of —63° 23' and 
— 65° 49'. The results of the observations at Hobarton with needles with which the 
complete process for determining the inclination was gone through, were given in 
No. V. of these Contributions*. Those at the other five stations are as follows : — 
* Philosophical Transactions, 1843, Part II. p. 165. 
