472 



LIEUT.-GENEEAL SABINE ON TERRESTRIAL MAGNETISM. 



Variation in the Intensity of the Magnetic Force observed by Deflectors. 

 1. In the ^Erebus.' — At sea, where manipulation of the weights causes exposure 

 of the needle, which, in bad weather especially, is liable to occasion injury, the sub- 

 stitution of "deflecting magnets" for "weights" was frequently resorted to. In 

 this case the ratios of intensity in different localities are inversely as the sines of the 

 angles of deflection and directly as the weights equivalent to the deflecting force of the 



deflector on the needle at the respective angles; or (i>'=<p.-. — ,.— , where <p, v, and w 



'^ ° sin V w 



are the intensity, the angle of deflection, and the equivalent weight at a base station, 

 and <p', v', and w corresponding values in another locality. The method of forming a 

 table of the equivalent weights corresponding to the deflecting force of a deflecting 

 magnet at different angles has been explained in No. III. of these Contributions (Philo- 

 sophical Transactions, 1842, Art. II. pp. 9 to 13) ; and a Table of the equivalent weights 

 for the magnet K. F. 4 (used in the ' Erebus ' either as Deflector S. or Deflector N., 

 according as the Deflector was applied to either pole of the needle) is reprinted here 

 (for convenience) from Contribution VI., Philosophical Transactions, 1844, Part 2, Art. 

 VII. p. 114. 



Table XIV. 



Regarding Hobarton- as the base station of the whole of the Survey now under con- 

 sideration, and 13'540 as the absolute value of the magnetic force in British units at 

 Hobarton at the mean epoch of the Survey, the observations of the ' Ei'ebus ' at Hobarton 

 in April 1841 supply the values t'=56° 28'-7 and (from Table XIV.) w=7-38 grs., for 

 Deflector S. ; and v=6B° 02'-7 and w=7-07 grs., for Deflector N ; (for Hobarton). The 

 angles of deflection in other localities furnish the respective values of v', and Table XIV. 

 those of w' corresponding to the angles v'. Hence we have the values of the force in 



other localities; viz. <p'=?-?i^.w' cosec»'=l"53 w'cosec?/. The deeree of accordance 



w '^ 



between the values of <p' obtained by weights in different localities and those obtained 

 by the defle.ctors is shown in the following Table, in which are included aU the stations 

 on land or ice between Hobarton in April 1841 and Port Louis in December 1842. 



