312 On the Polar Magnetism of H.M.S. 'Northumberland: \}Uv. 26, 



The 'Northumberland' was the subject of a singular attempt to 

 " depolarize" her, by the Patentee of a process "for an improved method 

 of correcting the deviation of compasses in iron ships"*. 



The attempt was made — first, on the 4th August 1866, in Victoria Docks, 

 by moving electromagnets over the external plates of the ship, but without 

 effect; next at Sheerness, in January 186/ — a similar process without effect ; 

 then by applying electromagnets to the beams of the poop -deck, in imme- 

 diate proximity to the poop- and two steering-compasses, and with consider- 

 able effect, as a powerful north pole of a subpermanent quality was de- 

 veloped in the centre of those beams (about 5 feet abaft the compasses), 

 producing a repulsive force on the poop- and steering-compasses amounting 

 nearly to two-thirds of the earth's force. 



By this the semicircular deviation of the poop and port steering-com- 

 passes was reduced to 5 of its original amount, returning, however, in 

 the course of a year (eleven months), as determined by observations made 

 in June, August, and December 1867, to | of its original amount. The 

 deviation of the starboard steering-compass was altered slightly in amount, 

 and largely in direction ; but is now, in common with that of the two com- 

 passes just named, gradually returning to its original state. 



The deviation of the standard compass was not affected by the operations 

 in the slightest degree. 



The "heeling" deviation of the poop-compass was affected in nearly 

 the same way as its semicircular deviation: the "heeling" deviation of 

 the starboard steering- compass was increased ; but the increase, like the 

 decrease of the poop-compass, was fast disappearing in December 1867. 



The correction by the " depolarizing " or " demagnetizing " process 

 was therefore both imperfect and transient, and productive of more 

 injury than benefit ; in consequence of which the author has submitted to 

 the Admiralty that no so-called "depolarization" should be allowed 

 within 20 feet of any compass placed for the navigation of the ship. 



* A full account of this "depolarizing" process, with the general views of the 

 patentee, will be found embodied in two papers read before the Royal United Service 

 Institution, 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, &c.," read May 6th, 1867, — both papers by Evan 

 Hopkins, Esq., C.E., F.G.S, 



