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VI. On the Diurnal Variation of the Magnetic Declination at St. Helena. 
By Lieut. -Colonel Edward Sabine, R.A., For. Sec. R.S. 
Received January 21, — Read February 18, 1847. 
It has long been known that in Europe the north end of a magnet suspended 
horizontally (meaning by the north end the end which is directed towards the north), 
moves to the East from the night until between 7 and 8 o’clock in the morning, when 
an opposite movement commences, and the north end of the magnet moves to the 
West. Recent observations have shown that a similar movement takes place at the 
same hours of local time in North America, and that it is general in the middle 
latitudes of the northern hemisphere. 
It has also been known for some years past, and has been confirmed by recent 
observations, that in the middle latitudes of the southern hemisphere, the north end 
of the magnet moves in a contrary direction to that which has been described as 
taking place in the northern hemisphere, viz. that it moves to the west until 8 o’clock 
in the morning, or thereabouts, and then returns towards the east. 
From the contrariety of the movement which is thus found to take place in the 
same meridians at the same hours in the opposite hemispheres, it seemed a not un- 
reasonable conjecture, that at some intermediate point in each meridian, the causes, 
whatever they might be, which occasion these movements might counterbalance each 
other, and that the diurnal variation might consequently disappear : and questions 
were raised whether the line connecting these points in the different meridians would 
be found to coincide with the terrestrial equator, or with the line of no dip, or with 
one of the isodynamic lines. 
The problem, which observation was thus called upon to resolve, has been stated 
with so much perspicuity by M. Arago in the Annuaire for 1836, that I may be 
permitted to reproduce it in his own words : — 
“ Dans l’hemisphere nord, la pointe d’une aiguille horizontale aimantee, qui se 
tourne vers le nord, marche, de l’est a l’ouest, depuis 8P 1 du mat. jusqu’a l^ h apres 
midi ; de l’ouest a 1’est, depuis l^ h apr&s midi jusqu’au lendemain matin. Dans l’h6- 
rnisphere sud, la pointe tournee vers le nord marche, de 1’ouest a l’est, depuis 8^ h du 
matin jusqu’a apres midi; c’est precisement 1’oppose du mouvement qu’effectue, 
aux memes heures, dans notre hemisphere, la meme pointe nord. 
“ Supposons qu’un observateur partant de Paris s’avance vers l’equateur. Tant qu’il 
sera dans notre hemisphere, la pointe nord de son aiguille effectuera tous les matins 
un mouvement vers 1’occident ; dans l’hemisphere oppose, la pointe nord de cette 
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