10 
LIEUT. -COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
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 re- 
sults 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. 
We may form this table in the following manner. For the primary or base station, 
let V be the angle of deflection with a constant weight W, and v the angle of deflec- 
tion produced by the deflector placed at the dip, then is 
w = W sin v cosec V, 
w being the weight equivalent to the deflecting force of the deflector at the angle v. 
If several constant weights were used at the primary station, the value of w may be 
obtained from each separately, and an arithmetical mean taken. Then at another 
station, at which the angles of deflection have been observed both with the deflector 
and with constant weights, the equivalent weight w' to the angle v' produced by the 
deflector may be obtained from 
I being the intensity at the primary station, and I' the intensity derived by the 
method of constant weights at the other station. The values of w 1 , thus computed for 
all the stations where the weights and deflectors were both used, being projected in 
a graphical representation with the corresponding values of v', the former as ordi- 
nates, the latter as abscissre, a line drawn by the eye through the terminations of the 
ordinates will give the values of w' for each degree of v' produced by the deflector. 
In the intensity instrument of the Erebus two deflectors were used, sometimes se- 
parately and sometimes combined : they were designated N. and S, according to the 
pole of the needle to which they were respectively applied. They were contained in 
brass tubes, N. with its north pole, and S. with its south pole towards the end of the 
tube which screwed into the limb of the instrument; consequently “Deflector N.” in 
* This analysis may be made when the needle is in other positions, but Mr. Fox now prefers the vertical 
one, or when the needle stands at 90°, the circle being perpendicular to the plane of the magnetic meridian. 
