II. Contributions to terrestrial magnetism. — No. III

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 between 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 7½ 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 intensities 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' ( t ' — t ), in which t is the standard and t ' the observed temperature in degrees of Fahrenheit, ⋅00016 a coefficient determined experimentally by Mr. Fox, and I' the observed intensity. 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 recommended 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 I' = I. w' / w ⋅ sin v /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 deflector 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 angles from the dip, each differing one degree from the preceding; the needle is thereby deflected to a smaller angle on the side of the dip opposite to the deflector, and is brought back to the dip by a weight applied to the grooved wheel on the axle; this weight is called the coercing weight corresponding to the angle from the dip at which the deflector was placed. For greater accuracy, the table is formed from results obtained by placing the deflector successively on either side of the needle. Owing to accidental circumstances, no table of this description was prepared for this instrument before the Expedition sailed; the pressure of other duties prevented its being done at St. Helena, the Cape of Good Hope, or at Kerguelen Island; and at Van Diemen Island the end of the axle of the needle being accidentally broken, the needle was returned to England to be repaired, and was thus separated from the instrument and from the deflectors. Under these circumstances we have no other resource for reducing the observations made with the deflectors, than to form a table from the observations of the weights and deflectors (when both methods have been employed at the same station), which shall answer the same purpose as a table of coercing weights. Fortunately the number of such stations is considerable.