174 
LIEUT. -COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
of deflection with deflector S down nearly to 20°. The weather was too unsettled 
to admit of any comparison being- made with the weights at sea, and an accident 
which befel the needle on or before the arrival at Hobarton prevented the comparison 
which otherwise would have been made there. The values of w' corresponding to 
the angles of deflection from 25° to 20° have, therefore, been supplied, by continuing 
the rate of progression at which the deflecting force of the magnet S had been found 
by experiment, in angles from 36° to 25°, to increase as the angle diminished, viz. 
0'033 gr. for each degree ; we have thus the following values : — 
gr. 
v' = 26 ; iv' = 2"594 
v' = 25 ; w' = 2-628 
v' = 24; w' = 2-661 
v' = 23 ; w' = 2-694 
v' = 22 ; w' — 2-727 
v 1 = 21; w' — 2*760 
v 1 = 20 ; vo' — 2-793. 
At Kerguelen Island we have the angle of deflection with the magnet S = 
26°2l'-3 = v; the equivalent weight = 2’58 = w ; and (§6.) I = T465 (London 
= T372) ; whence in other localities 
,, T w } sin v , , 
I = I — : — 1 = -252 1 w ' cosec v , 
w sin v ' 5 
v' being furnished by the observation and w' taken from the preceding Table. 
The last observation recorded to have been made with needle F was on the 11th of 
August, 1840, in lat. — 44° 16', long. 142° 38'; when the angle of deflection being 
21° 06'*5, 
I' = T929 uncorrected for the ship’s attraction, or (the course being E. by N. \ N.), 
I' = T934 corrected. 
On the return of the Expedition from the Antarctic Circle in the following year, the 
ships regained their former track, and on the 5th of April, 1841, Captain Ross re- 
peated the observations with different instruments within a few miles of the spot on 
which he had observed on the 11th of August, 1840: these observations gave 
I' = T927 in lat. — 45° 02', long. 143° 10'. If we examine the map in which the inten- 
sity observations are inserted, we perceive that the direction of the two geographical 
positions in relation to each other is very nearly that of the isodynamic lines in that 
quarter; and if we refer generally to the Tables, we see that the difference in the re- 
sulting intensity on the two occasions is within the limits of the differences of partial 
determinations with the same instrument at one spot. As far as circumstances permit 
us to judge, therefore, we may view the observations of the two voyages as forming 
parts of one connected series. 
As the ship’s head during the run under consideration varied but little on any 
occasion from her direct course, and as that course is one on which the corrections, 
both of inclination and intensity, are small, I have taken them from the Table com- 
puted by means of the constants deduced in the preceding section. 
