380 Royal Society : Lieut.-Col. Sabine on Magnetism. 



2. " Contributions to Terrestrial Magnetism." — No. V. By Lieut- 

 Colonel Edward Sabine, R.A., F.R.S.* 



In this paper the author details and discusses the magnetic obser- 

 vations made on board Her Majesty's ships Erebus and Terror, 

 between October 1840 and April 1841, being the first summer which 

 the expedition under the command of Captain James Clark Ross, 

 R.N., passed within the Antarctic Circle. 



The elimination of the influence of the ship's iron in the calcula- 

 tion of the results of these observations occupies a considerable por- 

 tion of the paper. Formulae for this purpose are derived from M. 

 Poisson's fundamental equations, and the constants in the formulae 

 are computed for each of the two ships, from observations made on 

 board expressly with that object. With these constants, tables of 

 double entry are formed for each of the three magnetic elements, 

 namely, declination, inclination, and intensity, giving the required 

 corrections of each, for all the localities of the voyage. 



These and other corrections being applied, the results are tabu- 

 lated and charts formed from them. The full consideration of the 

 charts is postponed until the whole of the materials collected by the 

 Antarctic Expedition shall be before the Royal Society. Meanwhile 

 the paper concludes with the following general remarks, viz. 



1. The observations of declination, particularly those which point 

 out the course of the lines of and of 10° east, indicate a more 

 westerly position than the one assigned by M. Gauss in the * Atlas 

 des Erdmagnetismus,' for the spot in which all the lines of declina- 

 tion unite. The progression of the lines in the southern hemisphere 

 generally, from secular change, is from east to west ; the difference 

 consequently is in the direction in which a change should be found 

 in comparing earlier with more recent determinations. 



2. The general form of the curves of higher inclination in the 

 southern hemisphere is much more analogous to that in the northern 

 than appears in M. Gauss's maps. For example, the isoclinal line 

 of — 85°, instead of being nearly circular, as represented in the 3 te 

 Abtheilung of Plate XVI. of the ' Atlas des Erdmagnetismus,' is an 

 elongated ellipse, much more nearly resembling in form and dimen- 

 sions the ellipse of 85° of inclination in the northern hemisphere in 

 the same work, Plate XVI. 2 te Abtheilung. The analogy between 

 the two hemispheres in the characteristic feature of the elliptical 

 form of the higher isoclinal lines is the more important to notice, on 

 account of the particular relation which appears to subsist in the 

 northern hemisphere between the change in the geographical direc- 

 tion of the greater axis of the ellipse, and the secular changes of the 

 inclination generally throughout the hemisphere. The present di- 

 rection of the greater axis in the northern hemisphere, is nearly 

 N.N.W. and S.S.E., or that of a great circle passing through the 

 two foci of maximum intensity. In the southern hemisphere, the 

 present direction of the greater axis differs little from E.S.E. and 

 W.N.W. 



3. Captain Ross's observations of the intensity do not appear to 



[* See ante, p. 377.] 



