'The Transactions of the Bombay Geographical Society. From January 1863 to December 1864. (Edited by the Secretary.) Volume XVII.' [‎19] (182/524)

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DESCRIPTION OF A MAGNETIMETER. 19
proportion the magnetism in the bar from terrestrial induction under-
goes a change, so that if a proper proportioned needle once set at a
distance from the bar so as to give a deflection equal to the
proper amount of dip, according to the foregoing laws, the dip
will be in accordance with the change of position all over the globe.
I was at first led to experimentalize on the magnetic change in the
bar of soft iron, from the discussion between the Astronomer Royal
and Dr. Scoresby. For a more correct determination, I constructed
the instrument about to be described ; the result is, that the condition
of soft iron changes exactly with the magnetic dip, and the change
agreed so well with the Admiralty Dip Chart lately published, that
I conceived the idea, that it may be employed for determining the latitude
in many parallels with the aid of the longitude, and the longitudes in
other parts of the globe with the help of the latitude ; and another
experiment of no little interest was, a bar of iron of ordinary hardness,
hammered while in a vertical position in the southern hemisphere, gave
a deflection from sub-permanent magnetism of 13° upper end of bar,
attracting south pole of needle while we were on the magnetic equator
13° South Atlantic Ocean : the condition of this bar, kept in the same
position, did not become neutral until we arrived at 45° N. latitude,
having undergone a geographical change of 48° latitude in a space of
forty days.
Description of Instrument *
No. I.—A wooden frame hung in gimbals to support a very soft
iron bar.
No. II.—A turntable, moving smoothly round an entire circle,
having the bar for its centre.
No. III.—-A plumb line, to ascertain that the bar is vertical before
observation.
No. IY.—A pedestal, supporting a fine magnetic needle exactly
over the centre of the iron bar, sufficiently high to be out of the iron’s
influence, and the box containing needle revolves on pedestal, and
is fitted with sight vanes, by which means the needle is moved free
from local deviation by the common means of correction (turning it
by the bearings of a distant object).
No. Y. is a highly magnetized needle, with a graduated arc to
note its deflection, and is capable of being moved to or from the iron
bar, until the deflection agrees with the magnetic dip of the geo-