LIEUT. -COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
11 
the Table signifies that the deflector having its north pole towards the screw was 
placed opposite that division of the circle which the north end of the needle had pre- 
viously indicated as the dip ; and the angle of deflection v' is a mean of the deflec- 
tions of the needle, first on the one side and then on the other side of the deflector. 
In the case of this deflector we have the angle v observed in London 22° 57' ; and 
the value of w, derived from the angles with the four constant weights of 1, 2, 3, and 
4 grains, = 2T14 grs. Regarding London as the primary station, and the intensity 
= 1, the values of w' at the several stations where both weights and deflectors were 
used are found by 
w' — 5*422 I' sin v 1 . 
The table of observations furnishes seventy-four occasions between England and the 
Cape of Good Hope, in which this deflector was used in comparison with the con- 
stant weights : we have consequently so many values of w' from which to form a table 
for each degree of deflection. The angles v' produced by this deflector increased 
from 22° 57' in London to above 34° where the intensity was weakest, and again de- 
creased to 29° 53' at the Cape ; consequently the ordinates corresponding to the 
smaller angles are derived partly from the earlier and partly from the later observa- 
tions of the series. The line drawn freely through the points forming the termina- 
tions of the ordinates shows by its continuity that the force of the deflector remained 
unchanged during the whole of the series ; it exhibits no discordances with any of 
the values of w', but such as may well be attributed to the unavoidable discrepancies 
of single observations. By means of this graphical representation the subjoined 
Table has been formed of the values of w' for each degree of v', permitting the inten- 
sities I' to be computed, relative to the force unity in London, by the formula 
I' = *1845 w 1 cosec v' . 
Values of w', 
Deflector N. 
o grs- 
o S rs - 
23 - 2-113 
30 = 1-929 
24 = 2-085 
31 = 1-904 
25 = 2-058 
32 = 1-880 
26 = 2-031 
33 = 1-857 
27 = 2-005 
34 = 1-834 
28 = 1-979 
35 = 1-810 
29 = 1-954 
In the case of deflector S, the table of observations furnishes 109 occasions be- 
tween London and the Cape of Good Hope in which the angle v' was observed in 
comparison with the angles produced by the constant weights ; consequently we have 
109 values of w' to be combined in a graphical representation. The line freely drawn 
through the terminations of the ordinates is continuous from August 1 839 to the 
noon-observation of February 12, 1840, when the continuity becomes interrupted, 
and a second line, corresponding to a diminished force in the deflector, commences, 
and continues unbroken to the Cape of Good Hope. The loss of force in the de- 
c 2 
