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V. On the Magnetic Character of the Armour-plated Ships of the Royal Navy , and on 
the Effect on the Compass of particular arrangements of Iron in a Ship. By 
Frederick John Evans, Esq., Staff Commander R.N., F.R.S. , Superintendent of the 
Compass Department of Her Majesty's Navy; and Archibald Smith, Esq., M.A., 
F.R.S., late Fellow of Trinity College , Cambridge, Corresponding Member of the 
Scientific Committee of the Imperial Russian Navy. 
Received March 9, — Read March 16, 1865. 
The present paper may be considered as a sequel to a paper published in the Philo- 
sophical Transactions for 1860, page 337, under the title “ Reduction and Discussion of 
the Deviations of the Compass observed on board of all the Iron-built Ships, and a selec- 
tion of the Wood-built Steam-ships in Her Majesty’s Navy, and the Iron Steam-ship 
‘ Great Eastern’; being a Report to the Hydrographer of the Admiralty. By F. J. Evans, 
Master R.N.” Like the former, the present paper is presented to the Royal Society, 
with the sanction of the Lords Commissioners of the Admiralty. 
In the brief interval which has elapsed since the publication of that paper, changes 
of the greatest importance have taken place in the construction of vessels of war, which 
have been accompanied by corresponding changes in the magnetic disturbance of their 
compasses. Not only has there been a great increase in the surface and mass of iron 
used in the construction of those parts of the ship in which iron was formerly used, 
but iron has been adopted for many purposes for which it was not then used, and much 
of the iron thus added far exceeds in thickness any that was formerly in use. Among 
the masses thus added we may specially mention iron masts and yards, armour-plating, 
and gun-turrets. 
These changes have materially affected the problem of the correction of the deviation 
of the compass. They have not only greatly increased those errors which were formerly 
taken into account, but they have given importance to errors and causes of error which 
it was formerly considered might be safely neglected. These changes led to, if they did 
not necessitate, a complete revision of the mathematical theory of the deviations of the 
compass, and of the practical methods of ascertaining and applying the deviation. 
This revision was undertaken by us at the request of the Admiralty, and the results 
are contained in the ‘ Admiralty Manual for ascertaining and applying the Deviations 
of the Compass caused by the Iron in a Ship,’ published by the order of the Lords 
Commissioners of the Admiralty. London: Potter, 1862. Second edition, 1863. It 
is gratifying to us to be able to state, as an indication that this work has been found 
mdccclxv. 2 o 
