4S0 LIEUT. -GENEBAL SABINE ON TEEEESTBIAL MAGNETISM. 



deviations observed on the sixteen principal points of the compass at Port Louis on 

 August 15th, 1842, and at Simon's Bay on April 20th, 1843. The observations at Port 

 Louis have been already printed in the Philosophical Transactions forl844, Part XL p. 1 95. 

 Those at Simon's Bay are now given in page 461 of the present communication. The 

 results obtained with Needle F.C.B. by the direct observation have been employed for 

 this purpose in preference to a mean between them and those obtained by deflector N., 

 in consequence of a small uncertainty in the index-correction of the results with the 

 deflector (Philosophical Transactions, 1844, Part II. Art. VII. p. 106). At Port Louis 

 the arithmetical mean of the Inclinations observed on the sixteen points (—51° 31') has 

 been taken as the standard, by comparison with which the deviations on the several 

 points have been assigned. The same needle, when observed on shore on the 25th of 

 July, 1842, with the face of the circle towards the east, and the needle direct, gave 



Philosophical Transactions, 1844, Part 11. p. 194. 



At Simon's Bay the arithmetical mean of the Inclinations observed with needle F.C.B. 

 used " direct," and with the face of the circle towards the east, on the sixteen points, 

 viz., —52° 40', has been in like manner taken as the standard of comparison for the 

 results on the several points. The same needle when observed with on shore on the 

 6th of April, face east and needle " direct," gave 



page 531 of the present communication. 



In the subjoined Table (Table IV.) column 2 contains the dips observed at Port Louis 

 on the points of the compass specified in column 1, and column 3 the deviations on each 

 point from the mean placed at the foot of column 2. Columns 4 and 5 exhibit the 

 same particulars at Simon's Bay. In column 6 is placed the mean deviation at the two 

 stations, and in column 7 the mean deviation applied to —53°, taken as an approximate 

 mean dip at both stations. In columns 8 and 9 are shown the values of ^' and ^ on each 

 point, viz., 5'', the azimuth of the ship's head shown by the standard compass and con- 

 sequently aff'ected by the deviation, and ^ the true direction computed from the observed 

 declinations and shown in Table II. Column 10 exhibits the dips on the several points 

 computed by the coefficients ; 



