[footnote] *Arago, in the 'Annuaire', 1836, p. 284, and 1840, p. 330-338.
This fourth curve, which might be called the 'curve of no motion', or,
rather, 'the line of no variation of horary declination', has not yet been
discovered.
The term 'magnetic poles' has been applied to those points of the Earth's
surface where the horizontal power disappears, and more importance has been
attached to these points than properly appertains to them;* and in like
manner, the curve, where the inclination of the needle is null, has been
termed the 'magnetic equator'.
[footnote] *Gauss, 'Allg. Theorie des Erdmagnet.', 31.
The position of this line and its secular change of configuration have been
made an object of careful investigation in modern times. According to the
admirable work of Duperrey,* who crossed the magnetic equator six times
between 1822 and 1825, the nodes of the two equators, that is to say, the
two points at which the line without inclination intersects the terrestrial
equator, and consequently passes from one henisphere into the other, are so
unequally placed, that in 1825 the node near the island of St. Thomas, on
the western
p 184
coast of Africa, was 188 1/2 degrees distant from the node in the South Sea,
close to the little islands of Gilbert, nearly in the meridian of the Viti
group.
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