52 
LIEUT.-COLONEL SABINE ON THE DIURNAL VARIATION OF 
nieme aiguille eprouvera tous les matins un mouvement vers 1’orient. II est impos- 
sible que ce passage du mouvement occidental au mouvement oriental se fasse d’une 
maniere brusque : il y a necessairement entre !a zone ou s’observe le premier de 
ces mouvements, et celle oh s’opere le second, une ligne oh, Je matin, 1’aiguille ne 
marche ni a l’orient ni a I’occident, c’est-a-dire reste stationnaire. 
“ Une semblable ligne ne peut pas manquer d’exister ; mais ou la trouver ? Est- 
elle l’equateur magnetique, I’equateur terrestre, ou bien quelque courbe degale 
intensite?” 
In the recent work of the Baron von Humboldt (Cosmos, vol. i.), this question is 
also adverted to, and the problem is stated in nearly similar terms : after noticing 
the contrariety of movement in the two hemispheres, Baron von Humboldt remarks 
that attention has been justly called to the belief “ that there must be a region of the 
earth, probably between the terrestrial and magnetical equators, in which no horary 
variation of the declination is sensible. This fourth curve, which might be called the 
curve of no motion, or rather the line of no horary variation of the declination, has 
not yet been found.” 
In the choice of the stations at which the magnetical observatories established by 
the British Government and by the East India Company in 1840 were placed, the 
solution of this problem was not overlooked. Singapore is situated close to the 
terrestrial equator; it was therefore well-suited to meet the suggestion of M. Arago 
in that respect: it is also not far distant from the line of no dip, and might be ex- 
pected therefore to exhibit, in some degree at least, the peculiarities which might 
appertain to stations so circumstanced. In this country, a somewhat different mode 
of viewing the magnetic system of the globe from that which prevailed generally in 
France, caused an opinion to be entertained, that a different line from any of those 
suggested by M. Arago might not improbably prove the dividing line of the two mag- 
netic hemispheres in this respect ; and that the phenomena of the diurnal variation, 
whatever they may be, which should characterise the dividing line, might be most 
advantageously studied at a station chosen in its vicinity. The line here referred 
to passes round the globe, crossing the several meridian lines at points where the 
magnetic intensity in each is a minimum : or, it may be more precisely defined, as 
the locus of the points of minimum intensity of all lines on the surface of the earth 
drawn at right angles to itself. Its position has been traced through all the meridians 
of the globe with considerable approximation*. It is not by its definition necessarily 
an isodynamic line, or a line of equal magnetic force ; and in fact, it is far from 
being so, the intensity of the force ranging in different parts of the line from 6'4 to 
7* 6 in absolute measure. It happens that Singapore, which, as already stated, is 
situated close to the terrestrial equator, and near the line of no dip, is in a part of the 
globe where the lines of least force and of no dip approach each other most nearly; 
consequently the observations at Singapore might be expected to exemplify the 
* Reports of the British Association, 1837. Philosophical Magazine, vol. xiv. p. 81. 
