'The Transactions of the Bombay Geographical Society. From January 1863 to December 1864. (Edited by the Secretary.) Volume XVII.' [‎18] (179/524)

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Art. III.— A Description of a Magnetimeter, by which the Magnetic
Dip is determined from the Horizontal Deflection of a Magnetic
Needle, produced by terrestrial induction in a bar of soft iron. p
Lieutenant F. Wherland, R.N.R., Commanding Ship “ Tudor ”
[Read before the Society, May 21st, 1863.]
I call this model a Magnetimeter, as the late Dr. Scoreshy called
his invention by the same term. With his, the dip of the Magnetic
Needle is determined by the position in which a bar of soft iron is when
it ceases to be magnetic from terrestrial induction ; the principle of
the above is, a bar of iron kept in the vertical deflects a horizontal
magnetic needle when at a proper distance at a given place to give
an angle equal to the dip, gives the dip afterwards for every part of
the globe, with an accuracy that may be made use of in checking dead
reckoning when astronomical observations cannot be had.
Before going into a description, it will be as well to state, that there
are errors attending this instrument, such as induced magnetism from
the magnetic needle employed in the bar of soft iron, and the length
of the needle, in proportion to the distance from the deflecting force,
the remote end of the needle s repulsive force diminishing in a much
greater ratio than the augmentation of attractive force of the nearer
end.. The first can be very much decreased, by employing a large
bar in propoition to the needle, and the second may be corrected, if
requisite, by a very simple calculation; but for practical purposes,
I believe it (as far as my experience goes) to be sufficiently correct
when got up on a larger scale and a properly graduated arc.
According to well known laws, a bar of very soft iron becomes
magnetic when held in a direction with the dip of the magnetic needle,
and as this bar . departs from this direction, the cosine of the angle
which it makes is equal to the dip : for example, we will say the dip
at Bombay is 20 , the bar of iron hanging vertically is 70° from
dip,.therefore the cosine 20°, is dip. In England the dip is 70°, the
vertical bar is only 20°, giving the cosine 70°; and just in the same