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V. Discussion of the Observed Deviations of the Compass in several Ships, IVood-huilt 
and Iron-built : with a General Table for facilitating the examination of Compass- 
Deviations. By G. B. Airy, Esq., Astronomer Royal. 
Received September 14, — Read November 22 , 1855. 
In the year 1839 I communicated to the Royal Society a paper (printed in the 
Philosophical Transactions of that year) containing the results of examination of the 
compass in two iron-built ships, and a general theory of the effect of the transient 
induced magnetism of iron in disturbing the direction of the compass-needle. The 
result of the theory of induced magnetism may be stated as follows. 
First, I must premise, in explanation of the term “polar-magnet-deviation” which 
I shall frequently have occasion to use, the following theorem on the disturbance of 
the compass by a magnetized steel bar, or one which possesses independent polar 
magnetism, in no way referred to the influence of the existing terrestrial magnetism. 
Let the line CA, fig. 1, represent in magnitude and in direction the Fig^ i. 
terrestrial directive force. [In the applications of the theory to 
ships, the terrestrial directive force is diminished in a constant ratio 
differing little from unity; and then it must be understood that CA / 
represents the terrestrial force so diminished.] A is understood to \ 
be the magnetic-north end of the line. And let AB represent, in V,' 
proportional magnitude and in direction, the directive force of the 
magnetized steel bar or “ polar-magnet,” B corresponding to that 
end of the polar-magnet which possesses boreal magnetism. Then 
the directive force which really acts on the compass-needle will be 
represented in proportional magnitude and direction by CB ; and the angle ACB 
will be the angle of deviation of the compass. And if the polar-magnet be turned 
round in azimuth, so that the point B occupies successively different points in the 
circumference of the circle, the angle of deviation will have successively the different 
magnitudes and the different directions (right or left of the line CA) given by this 
construction for these different circumstances. This theorem is very simply founded 
On the ordinary “ composition of forces,” and is abundantly proved by experiment. 
The deviation ACB is what I shall call “ polar-magnet-deviation.” In some cases 
it is convenient to refer the azimuth of the polar-magnet to the true magnetic meri- 
dian or CA, and then the polar-magnet-deviation is given by this formula: 
. Afli.sinBAD 
tQU 13 — 7TT J A T> TJ A 
CA + AB.cos BAD 
