488 STAFF CAPTAIN EVANS, R.N., ON THE CHANGES OF POLAR MAGNETISM 
Admiralty, some results as to the amount and changes of the magnetic forces at several 
positions in that ship. 
These results may be thought to have some special interest from the circumstance 
that the c Northumberland’ was made the subject of an attempt to “ depolarize ” her, 
which created some interest and expectation not only in the general public, but even in 
the Naval profession* *. 
The ‘ Northumberland’ is an iron-built ship of 6621 tons, 400 feet long, 59^ broad, 
26^ feet draught of water, 1350 horse-power, screw engines, armour-plated completely 
round, with plates of an average of 6 inches in thickness. She has further three com- 
plete iron decks supported by iron beams and iron uprights, and a poop-deck of wood, 
but supported by iron beams and iron uprights. The five lower masts are also of iron. 
The weight of iron employed in the construction of the hull was about 4250 tons, and 
in the armour-plating about 1550 tons. 
The ship was built at Millwall on the river Thames ; the direction of her head in 
building being N. 39-§-° E. magnetic. Contrary to the usual practice with ships built 
on a slip, the armour-plating was completed previous to launching ; the latter operation 
it may be recollected was performed with great difficulty and occupied several days. 
The launch was completed on the 17th April, 1866, when the ship was anchored in the 
river, and allowed to swing with the tide. On the 18th April she was towed to the 
Victoria Docks, where, on the recommendation of the Compass department, she was 
placed in a direction as nearly opposite to that of building as could be conveniently 
arranged (viz. S. 22° W. magnetic). 
Unfortunately two large iron-plated ships (one iron-built) lay close alongside, and no 
doubt affected considerably the magnetic phenomena observed in the ‘ Northumberland.’ 
This circumstance prevents the observations made during her stay in the Victoria 
Docks being strictly comparable with those made before and afterwards; and this 
must be borne in mind in looking at the Tables. It has, as far as possible, been 
allowed for in the assumed value of X. Some irregularities were also caused by the 
introduction and movement of the large masses of iron constituting the steam-boilers 
and engines. 
have also to acknowledge my obligations to Mr. Archibald Smith, F.R.S., for the assistance which he has given 
me in the discussion of the observations, especially in their mathematical and graphical treatment. 
* I would refer especially to two papers read before the Royal United Service Institution, London, and the 
discussions thereon, as published in the Journal of the Institution, — the first paper on “ Terrestrial Magnetism 
with reference to the Compasses of Iron Ships ; their Deviation and Remedies,” read January 29th, 1866 ; 
the second paper on “ The Demagnetization of Iron Ships, and of the Iron beams, &c. of Wooden vessels, to 
prevent the deviation of the Compasses, experimentally shown by means of a model,” read May 6th, 1867, both 
papers by Evan Hopkins, C.E., F.G.S. The latter paper was also read at the Salle des Conferences, Champ 
de Mars, Paris (in connexion with the International Exhibition), on the 22nd June 1867, by Captain F. A. B. 
Crah-furd, R.N. 
