532 Royal Society :— 
The observations made in the iron-built ships extend over periods 
varying between thirteen and five years; and having been made with 
the same description of compass—the Admiralty standard—and under 
similar conditions of arrangement and situation, in accordance with 
the system carried out in Her Majesty’s Navy, details of which are 
given, the general results are strictly comparable. 
In the analysis of the Tables, amounting to nearly 250 in num- 
ber, of deviations observed in various parts of both hemispheres, the 
formula deduced from Poisson’s General Equations by Mr. Archi- 
bald Smith, given in the Philosophical Transactions for 1846, p. 348, 
has been employed. 
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 :— 
Deviation (6)=A-+B sin Z'+C cos Z/+D sin 22'+ E cos 22’, 
é' being the azimuth (by compass) of the ship’s head, reckoned 
from the magnetic north towards the east ; 
A, D, E being constant coefficients depending only on the amount, 
quality, and arrangement or position of the iron in the ship: B and 
C, coeflicients depending on these, and also on the magnetic dip and 
horizontal intensity, are each consisting of two parts; one caused by 
the permanent magnetism of the hard iron, the deviation produced 
by which varies inversely as the horizontal force at the place ; and the 
other, caused by the vertical part of the earth’s force inducing the 
soft iron in the ship, the deviation produced by which varies as the 
tangent of the dip: B representing that part of the combined attrac- 
tion acting in a fore-and-aft direction, C that acting in a transverse, 
or athwart-ship direction. 
Ay 
From the equation tan aoe the direction of the ship’s force, and 
V 6*+C?, the total magnetic force of the ship in proportion to the 
‘horizontal force at the place of observation is obtained: for con- 
venience, 1000 has been adopted to represent the value of the earth’s 
horizontal force at the English ports of observation, in order, by an 
easy comparison, to note the changes on foreign stations. 
By comparison of the coefficients of the several descriptions of 
ships, it is observed that in wood-built steam-vessels, the coefficients 
B and C vary nearly as the tangent of the dip; from whence it may 
be inferred, as a general rule, that in steam machinery permanent 
‘magnetism bears but a small proportion to induced; but in ion- 
built ships, B and C generally vary more nearly as the inverse hori- 
zontal force, showing that they depend more on the permanent 
magnetism of the iron of the ship, and thus confirming the view of 
the Astronomer Royal, given in his earliest deductions (Phil. Trans. 
1839), that the effect of transient induced magnetism is in these 
ships small comparatively. Numerous examples are given in detail 
of this permanency of magnetism, as also of the gradual diminution 
of the ship’s force resulting from time. 
- An investigation of the coefficient D, which is caused entirely by 
the horizontal induction of the soft iron in the ship, and which is 
