LIEUT. -GENERAL SABINE ON TEEEESTEIAL MAGNETISM. 



461 



merived from the deviations of the Declination. 

 ^=l-2D= + -990lJ 



"*" derived from the deviations of the Dip in column 7. 



<Z= + 09950J ^ 



And, finally, in column 11 are shown the differences between the observed and the com- 

 puted dips. The values of c and d have been derived by equation (0) (Philosophical 

 Transactions, 1843, p. 148) on all the points from N.N.W. to S.S.W. and from N.N.E. to 

 S.S.E. ; and on the north and south points by equation (10): the computed dips 

 by equation (13) on all points excepting east and west, and on those points by 

 equation (12). 



The Table for the correction of the deviations of the dip in the third year's survey 

 has been formed by computing, by means of the coefficients above stated, the deviations in 

 dips successively of —53°, —59°, and —65°, and interpolating the intermediate values. 



Table IV. 



Variation in the Intensity of the IVIagnetic Force. 



The Magnetic Survey which is here discussed was carried on chiefly by observations 

 made at sea, and (in reference to the variations of the Magnetic Force especially) by 

 instruments and methods either wholly novel or very nearly so. Some little repetition 

 of what may have been already said some years ago must therefore be hazarded (and it 

 is hoped will be pai-doned), in the endeavour to convey an intelligible description of the 

 methods by which the objects of the Survey were sought to be accomplished, and of the 



MDCCCLXVL 3 T 



