26 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
mutually afford each other ; the Erebus making the intensity on shore at Christmas 
Harbour, in Kerguelen Island, to be as 1*068 to 0*715 at Simon’s Bay, and the Terror 
as 1*0675 to 0*715. At sea, where in consequence of the motion of the ship, the inferior 
size of the four-inch instrument cannot be compensated by additional time given to 
the reading, or by other arrangments conducing to minute accuracy, the probable 
error of a single result appears, as might be expected, to be somewhat greater with 
the four inch-than with the 7^-inch needle ; but, with this reservation, the observations 
made at sea with the two instruments, when the ships were in company, are highly 
confirmatory each of the other. 
The table which has been received from the Terror contains almost daily obser- 
vations from the 1st of January 1840 until her arrival at the Cape of Good Hope in 
the following March. The intensities were observed both by deflectors and by con- 
stant weights, which latter, in the four-inch instrument, were *5 and *3 of a grain. 
St. Helena is the only land station observed at in the passage ; the intensities ob- 
served at sea have therefore been computed relatively to the observations at James 
Town St. Helena, taken as a base station ; and the value of the intensity at James 
Town has been taken as determined by the Erebus, viz. as 0*61 1, to unity in London, 
or as 0*838 to 1*372 in London. The values of w' for the Terror’s deflectors N. and S, 
required, instead of a table of coercing weights, in computing the intensities when 
the deflectors were used, have been derived by the method already explained. Tables 
of these values for each degree of deflection are subjoined : neither on a careful ex- 
amination of the observations, nor in the process of forming these tables, does there 
appear any reason to suppose that the deflectors or the needle sustained any change 
in magnetic condition during the period embraced by the observations under notice. 
Values of w 1 , Terror’s Deflectors. 
N. 
S. 
O gr- 
o S r - 
38 = -837 
33 = -732 
39 = -828 
34 = -726 
40 = -819 
35 = -721 
41 - -811 
36 = -716 
42 — -803 
37 — -711 
43 = -797 
38 = -707 
44 — -7925 
39 = -703 
45 = -791 
40 = -700 
46 = -7895 
47 = -7885 
With the values of w 1 taken from this Table, the intensities at sea are obtained rela- 
tively to 0*611 at St. Helena (London = 1), by the formula 
I' = *5393 w 1 cosec v 1 . 
