264 STAFF COMMANDER EVANS AND MR. A. SMITH ON THE MAGNETIC 
useful by others engaged in the like investigations, that it has been already translated 
into Russian, French, and German. 
The methods of reduction previously in use, and which are those made use of in the 
paper already referred to, as well as in the valuable Reports of the Liverpool Compass 
Committee, are those deduced from the approximate formula for the deviation, 
S=A+B sin £'+C cos £'+D sin 2£'+E cos 2£', 
as given in the Supplement to the ‘ Practical Rules for ascertaining the Deviations of 
the Compass which are caused by the Ship’s Iron,’ published by the Admiralty in 1855. 
In connexion with this formula use was made of the invaluable graphic method known 
as Napier’s curve. 
At that time observations of horizontal and vertical force did not enter into the usual 
routine of observations made on board ship, although many very valuable observations 
of these forces had been made by the Liverpool Compass Committee ; and no formulae 
had been published for the deduction from such observations of any of the parts of 
the deviation. This will explain why, in the paper of 1860, the discussion was con- 
fined to the coefficients which are derived from observations of deviation only, viz. 
A, B, C, D, E. 
The new modes of construction brought into prominence the diminution of mean 
directive force which a compass-needle suffers in an iron ship, particularly when placed 
between two iron decks. It is well known that in the interior of a thick iron shell the 
effect of the earth’s magnetic force is nearly insensible. This is not caused by the iron 
of the shell intercepting the earth’s magnetism, but by an opposite magnetism being 
induced which nearly neutralizes the earth’s magnetism whatever be the inductive capa- 
city of the shell, and whatever be the thickness of the shell, provided only that the 
thickness bears a considerable proportion to the diameter of the shell. When the shell 
is thin, the diminution of force is still considerable, but it then depends in a very much 
greater degree on the inductive capacity and the thickness of the shell. The destruction 
of force is total in the case of a spherical shell whatever be its thickness, if the inductive 
capacity be infinite. 
An iron ship, as regards a compass-needle between decks, may be compared to a thin 
iron shell. Before the ship is launched, and when every particle of iron in her structure 
has by continued hammering become saturated with magnetism, she may be compared 
to a thin shell of high inductive capacity, and the directive force on a needle in the inte- 
rior is consequently greatly diminished. When the ship is launched and placed succes- 
sively on every azimuth, she may be compared to a thin shell of low inductive capacity. 
The mean directive force on a needle in her interior will be considerably diminished, 
but the diminution will depend much more on the thickness of the surrounding iron. 
This diminution has been found so considerable in the case of iron-built and particu- 
larly iron-plated ships, as to have become a matter of serious consideration in selecting 
a place for the compasses. 
