L1EUT.-C0L0NEL SABINE ON TERRESTRIAL MAGNETISM. 
91 
Hence 
at north, — *944«= + *307— *33 1) 
at south, — *944a = — *359 + *331 J ’ 
whence a— +'0*2/5. 
At Port Louis (August 1842) we have 9 = 1*32 ; 0=— 52° 05'; 9 ' at north =1*279, at 
south =1*346; 8 at north = — 52° 50', at south = — 51° 33 ; hence 
at north, — *789a= + *5920 — , 615'| 
at south, — *788a= — *6367 + '615j ’ 
whence «= + *0310. 
The accordance between these values and those deduced from the observations 
with the standard compass is fully sufficient to justify the inference that the effect 
of the ship’s attraction was very nearly the same at the spot where Mr. Fox’s ap- 
paratus was used, as at that at which the standard compass was fixed. 
We may obtain c either by equation (11.), Phil. Trans., 1843, Part II. p. 148, 
c cos £^+d tan d =\/ (cos £ + « tan 0) l -^b L sin tan 8 ; 
or from the observations of inclination and intensity, independently of the values of 
a and b, by the equation 
tpi 
+ sin 8—c cos 0 cos c?sin 0. 
(3 ^ 
Confining ourselves to the north and south points, and to those points on either 
side of N. and S. from which c may be most advantageously derived, the observations 
at Hobarton give the following values to be employed in the equations; 
N. ; £' = 0; £= 0; 8 — — 71° 56' ; 9 ' = 1*812. 
n’nw}; ^ = 22 ° 30 ' ; S= 21° 03'; 8=-7 1°55' ; ^'=1*812. 
j; £'= 45° O'; £= 42° 12'; 8=~7 1°48'; 9'=1*816. 
| ; £'=135° O'; £=131° 17*; 8 — — 69° 56'; <p'=l*847. 
S*S W } 5 ^ =157 ° 30 ' ; £= 155 ° 24'; O'— — 69° 38' ; 9'=1*850. 
S; £'=180° O'; £=180° O'; 0'-69° 14'; 9'= 1*854. 
0= — 70° 39'; 9=1*83. 
Substituting these values in the first of the above equations (11.), we have at 
N. 
N.N.E.) 
N.W. / 
l*000c-2*85r/= — 2*828 ; 
•934c — 2*85^= — 2*832 ; 
N.E. ^ 
N.W. J 
S.E. ) 
S.W. J 
*741 c — 2*85 c?= — 2*841 ; 
*660c- 2*85^=— 2*853 ; 
