338 
]ME. EYAIs'S OX THE DEYIATIOX OF THE CO:\IPASS 
the cards and needles, are uniform throughout; the intensities of the latter being 
remarkably so, and also permanent over long periods of time : the observations made 
with this compass are therefore strictly comparable, and it will be understood that the 
results given in this Report, except when specified to the contrary, and the inferences 
arising therefrom, must be considered as appl}ing to a compass in the various details of 
position and structure just described. 
The analysis of the deviations of the compass in iron-built vessels, embraces forty-two 
ships of the Royal Navy ; and including the wood-built screw and paddle-wheel steam- 
vessels, as also the deviations observed in various parts of the steam ship ‘ Great Eastern’ 
during her experimental trip to Portland in the autumn of 1859, has involved a com- 
putation of nearly two hundred and fifty Tables. 
In order to render the analysis complete as a standard of reference, there is appended 
certain statistical information relating to each vessel, including tonnage, and exact posi- 
tion and height of standard compass : these details will be found necessaiy in a critical 
examination of the magnetic characteristics of ships in general. 
In the analysis of the Deviation Tables, I have employed Mr. Aechibald Smith’s 
formula for computing the magnetic coefiicients, as deduced from Poissox's General 
Equations given in the Supplement to the “Practical Rules for ascertaining and 
applying the Deviations of the Compass caused by the Iron in a Ship,” furnished to Her 
Majesty’s Ships; and also in the Philosophical Transactions for 1846. 
In this formula the deviation (§) of the compass on board ship, reckoned positive 
when the north point of the needle deviates to the east, is given by the following 
expression : — 
A+B sin ^'+C cos D sin 2^'-f E cos 2 ^, 
A, D, E being constants depending only on the amount, quality, and arrangement or 
position of the iron in the ship ; B and C depending on these, and also on the magnetic 
dip and horizontal intensity; and the azimuth (by disturbed compass) of the ship’s 
head, reckoned from the magnetic north to the east. 
This formula is only approximate ; but is sufficiently correct when the delation does 
not exceed about 20°. The accurate formula involves calculations too laborious to be 
applied to ordinary cases. 
Without entering on the nature and action of the magnetic forces which cause the 
deviation of the compass on board ship, — to be found in the works to which I have 
alluded, as also in the introduction to Dr. Scokesbt’s late Voyage round the IVorld 
for magnetical research, edited by Mr. Aechibald Smith, — further than to attach a 
physical meaning to the coefficients for practical elucidation, it is necessary to premise 
that the investigations of the Astronomer Royal* are in accord with the general terms 
* “ Account of Experiments on Iron-built ships, instituted for the purpose of discoveriug a coiTection for 
the deviation of the Compass produced by the Irou of the ships,” Philosophical Transactions, Part 1. 1S39. 
“Discussion of the observed Deviations of the Comj^ass in several ships, AYood-built and Ii’on-built: with 
a G-eneral Table for facilitating the examination of Compass Deviations,” by G-. 13. Aiur, Esq., Astronomer 
Eoyal, Philosophical Transactions for 1856. 
