LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM.

155

From the observations at E.N.E. and W.N.W.,

$ = - 13° 36' at E.N.E. , and - 22° 06' at W.N.W.;

4 = — 17° 51 ' ; £' = 67° 30', and £ = 63° 15'; whence a = -0261.The mean of the two pairs of observations gives a = + '0263.

Whilst within the Antarctic Circle only a single opportunity occurred of observing

the inclination otherwise than on board, and thus of obtaining a from a tan 6 byhaving an assured value of 6. This was on the 8th of January 1841, in lat. — 68° 30',

long. 176° 35', where the inclination observed on the ice with a needle in which the

observation was complete by the reversal of the poles, was found to be — 83° 35'.The declination was observed on board on the same afternoon and following morn-ing, as nearly as could be in the same geographical position, with the ship’s head onseveral points, from which we may select for a determination of a those nearest tothe east and west points. We have then the following observations :—

]. At W.|N. yJ/ = - 46 02

2. At W. by S. 4*'= - 46 32

3. At W. by S. £S.^'= -47 17

4. At E. | N. y]/= — 20 51

5. At E. by S. i S. 4' = — 19 58

6. At E. by S. f S. yp' = — 20 22

_ 4/ 4- \V

Trom 1. and 4. we have yp = —^—

33 27'5

Mean — 33 32 - 5

33 37'5

„ . . , -f-

rrom 3. and 5. we have y = —^—

Hence

\ = - 1°2 29'5\ — - 12 59-5= — 13 44'5

& 4 = + 12 41'5S 5 = + 13 34'5

\ = + 13 10'5 ;

have those of £' by observation, we obtain

and having thereby the values of as we

Oj = + -0249a 2 = + -0274« 3 = + -0274

— ’0259

o 5 = + ’0275a 6 = -f -0266

Means . . . + -0266

+ -0267

The deviation of the compass observed on board the Erebus during the stay of the

Expedition at Christmas Harbour, Kerguelen Island , in July 1840, when the ship’shead was on the N.E., S.E., N.W., and S.W. points, and at the points on either sideof those points, viz. N.E. by N., N.E. by E., S.E. by S., S.E. by E., &c., will furnish anadditional determination of the value of b :

MDCCCXLIII.

Y