36 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
§ 6. Observations of Intensity between the Cape of Good Hope and Kerguelen Island. 
On the 6th of April 1840 the Expedition quitted Simon’s Bay, and on the same 
night the Erebus and Terror parted company, and made their passage to Kerguelen 
Island on separate courses. Although the weather was very unfavourable, the prac- 
tice of daily observation with the inagnetical instruments was continued with very few 
exceptions, by the Erebus during the whole passage, and by the Terror from Simon’s 
Bay as far as the meridian of the Crozet Islands, which was the first rendezvous. 
The observations of intensity on board the Erebus were chiefly made with de- 
flector S, the other deflector N having been used only five times during the whole 
passage, whilst the number of observations with S amounts to thirty-six. For the 
values of w', in the case of deflector S, we have the comparisons with the constant 
weights at the Cape and Kerguelen Island, and three good intermediate comparisons 
at sea, viz. on the 11th, 12th, and 18th of April ; a fourth attempt on the 1st of May 
failed from some accidental error in the observation with the constant weight. 
Pursuing the plan of graphical representation already described, we find that the line 
connecting the terminations of the ordinates at the Cape and at Kerguelen Island 
passes either through or extremely near the terminations of the other three ordinates, 
indicating the unchanged magnetism of the deflector ; and we obtain the following 
Table of the values of w' for the degrees of deflection in the Table : 
Values of w 1 , deflector S, Erebus ; Cape of Good Hope 
to Kerguelen Island. 
o S rs - 
O S rs - 
25 - 2-628 
31 = 2-426 
26 = 2-594 
32 = 2-392 
27 = 2-560 
33 = 2-358 
28 = 2-527 
34 = 2-324 
29 = 2-494 
35 = 2-291 
30 - 2-460 
36 = 2-260 
Regarding the Cape as the primary station, and its intensity = 07 15 (London = 1), 
the intensity at the other stations is given by the formula 
I' = *1837 w' cosec v'. 
The observations with deflector N between the Cape and Kerguelen Island being 
few, and the two intermediate comparisons at sea with the constant weights exhibit- 
ing considerable discordances, either from the unfavourable circumstances of weather, 
or possibly in consequence of an actual small change of force in the deflector, I have 
not attempted to deduce results from the observations either with deflector N, or 
with N and S combined. I have also omitted in the mean deductions the results 
of the observations with the constant weight of one grain on the 1st of May and 
29th of June, these observations being obviously affected with some accidental error. 
For the Terror’s deflectors we have only the comparisons with the constant weights 
at the Cape and Kerguelen Island from which to derive the values of w' for the inter- 
mediate degrees of v'. Connecting the values of w' obtained by those comparisons 
