agnetism of Ships._--In 1814 Flinders first showed (see
Flinders's _Voyage_, vol. ii. appx. ii.) that the abnormal values of
the variation observed in the wood-built ships of his day was due to
deviation of the compass caused by the iron in the ship; that the
deviation was zero when the ship's head was near the north and south
points; that it attained its maximum on the east and west points, and
varied as the sine of the azimuth of the ship's head reckoned from the
zero points. He also described a method of correcting deviation by
means of a bar of vertical iron so placed as to correct the deviation
nearly in all latitudes. This bar, now known as a "Flinders bar," is
still in general use. In 1820 Dr T. Young (see Brande's _Quarterly
Journal_, 1820) investigated mathematically the magnetism of ships. In
1824 Professor Peter Barlow (1776-1862) introduced his correcting
plate of _soft_ iron. Trials in certain ships showed that their
magnetism consisted partly of hard iron, and the use of the plate was
abandoned. In 1835 Captain E. J. Johnson, R.N., showed from
experiments in the iron steamship "Garry Owen" that the vessel acted
on an external compass as a magnet. In 1838 Sir G. B. Airy
magnetically examined the iron steamship "Rainbow" at Deptford, and
from his mathematical investigations (see _Phil. Trans._, 1839)
deduced his method of correcting the compass by permanent magnets and
soft iron, giving practical rules for the same in 1840. Airy's and
Flinders's correctors form the basis of all compass correctors to this
day. In 1838 S. D. Poisson published his _Memoir on the Deviations of
the Compass caused by the Iron in a Vessel_. In this he gave equations
resulting from the hypothesis that the magnetism of a ship is partly
due to the permanent magnetism of hard iron and partly to the
transient induced magnetism of soft iron; that the latter is
proportional to the intensity of the inducing force, and that the
length of the needle is infinitesimally small compared to the distance
of the surrounding iron. From Poisson's equations Archibald Smith
deduced the formulae given in the _Admiralty Manual for Deviations of
the Compass_ (1st ed., 1862), a work which has formed the basis of
numerous other manuals since published in Great Britain and other
countries. In view of the serious difficulties connected with the
inclining of every ship, Smith's formulae for as