II. Contributions to Terrestrial Magnetism. — No. III. 
By Lieut. -Colonel Edward Sabine, R.A., F.R.S. 
Received December 16, 1841, — Read January 20, 1842. 
In the present number of these Contributions, I propose to give an account of the 
observations on the magnetic intensity made at sea by the officers of Her Majesty’s 
ships Erebus and Terror, on their passage from England to Kerguelen Island, the 
unreduced observations, transmitted to the Admiralty by the Commanders, Captains 
Ross and Crozier, having been placed in my hands for that purpose. 
They will be divided for convenience into two sections, viz. 
§ 5. Observations betiveen England and the Cape of Good Hope. § 6. Observations 
between the Cape of Good Hope and Kerguelen Island. 
§ 5. Observations between England and the Cape of Good Hope. 
The observations in the Erebus were made by the statical method devised by 
Mr. Fox, with one of his instruments of inches diameter. The intensities were 
measured by the angles of deflection produced, in different localities, by a constant 
weight applied to a grooved wheel on the axle of the needle ; and the ratio of the inten- 
sities is inversely as the sines of the angles of deflection, subject to a correction for 
differences of temperature of the needle, computed by the formula '00016 I' if — t), 
in which t is the standard and f! the observed temperature in degrees of Fahrenheit, 
•00016 a coefficient determined experimentally by Mr. Fox, and I' the observed inten- 
sity. At sea, where the manipulation of the weights causes an exposure of the needle, 
which, in bad weather particularly, is liable to occasion injury, the plan recom- 
mended by Mr. Fox, of using deflecting magnets instead of weights, was frequently 
resorted to. In this case the ratio of the intensity in different localities is inversely 
as the sines of the angles of deflection, and directly as the weights equivalent to the 
deflecting force of the deflector on the needle at the respective angles ; or 
x , _ w' sin v 
V — I . T 
w sin v 
where I, v, and w are the intensity, angle of deflection, and equivalent weight at a 
base station ; and I', v', and w' corresponding values at another station. A table is 
usually formed for each instrument experimentally, under Mr. Fox’s own direction, 
♦ 
of the equivalent, or as they are termed by him, the coercing weights, for each de- 
flector on each of the needles at the different angles which are likely to occur in 
the course of the observations. This is done by placing the deflector successively at 
MDCCCXLII. C 
