TERRESTRIAL MAGNETISM. 23 
to be obtained by extended magnetic surveys of the surrounding 
country. It has also been shown that the lands hitherto visited 
South of 40° 8. are subject to like disturbance. Extended land 
surveys being probably out of the question for this Expedition, the 
normal values which can be obtained on board the ship which will 
traverse large tracts of sea and clear of the land are of great value. 
As observation alone will show how far the instruments on board 
the ship are free from disturbance by the ship’s iron, it is considered 
necessary to provide methods of computing the corrections for all 
observations in case of such disturbance. 
1. Declination or Variation. 
To obtain the correction for the deviation (6) of the compass 
from the magnetic meridian in all latitudes, we require to know the 
value of the coefficients A, B, C, D, E, at a number of widely 
distributed positions on the globe. These can be obtained whenever 
the ship is swung to ascertain simultaneously the deviations of the 
Standard and Fox compasses. (See * Admiralty Manual for Deviations 
of Compass,’ 7th ed., 1901, pp. 50-63.) A, D, EK, do not change by 
change of magnetic latitude. C changes inversely as the horizontal 
force. The changes in B may be computed by the following formule, 
B having been observed in two widely different magnetic latitudes :— 
Ee! eo Sa Be HG 
nr 7 (1) 
Pp P 
~ + < H, tan @, = sin By . Hy 
in which P is due to hard iron, and varies inversely as the horizontal 
force; ¢ due to vertical induction in vertical soft iron and changing 
as the tangent of the dip. 6, and @, the undisturbed dip at the two 
stations. H, and H, the horizontal force at the two stations 
expressed in terms of the horizontal force at Greenwich = 1:0. 
The coeflicients being known, 6 for any direction of the ship’s 
head can be computed by the equation 
6=A+4+Bsin f+ Ceosf'+D sin 2 6'+ Ecos 2 & 
¢' denoting the azimuth of the ship’s head by the disturbed compass. 
Forms are provided to facilitate this computation. 
It should be noted that whenever the ship is completely swung, 
any disturbance from the ship’s iron is eliminated and valuable 
determinations of the Variations obtained. - 
The values of X, » and g (sce ‘ Adm. Man, Dev. Compass,’ pp. 65-74) 
