IN THE IRON AND ARMOUR-PLATED SHIP NORTHUMBERLAND. 
491 
polarity acquired while the ship is building, by passing electro-currents through the 
hull.” In the final specification, dated 23rd July 1866, it is described as “moving an 
electro-magnet from end to end over, and in contact with the main plates of the ship.” 
There is some obscurity in both specifications, from the patentee not distinguishing 
between electric currents and lines of magnetic force ; but whatever may be the meaning 
of the method described in the provisional specification, it is certain that that method 
would be utterly inadequate to produce any sensible effect in a large ship. 
The method described in the final specification would no doubt produce, wherever 
applied, some local effect ; but the effect produced by the local application of magnetic 
force of high and rapidly varying intensity must necessarily be wholly different from that 
which has arisen from the general application of a force of low and uniform intensity, 
and the former cannot possibly produce any general destruction of the latter. The pro- 
cess is in fact not one of general demagnetization, but of partial counter magnetization. 
The result will be an irregular distribution of magnetism of very variable intensity, 
necessarily very unstable, and producing, wherever effective, a rapidly varying field of 
force. The justice of these remarks will, I think, be shown in the sequel. 
In April 1866 Mr. Hopkins applied to the Admiralty for permission to experiment on 
the ‘ Northumberland,’ the largest and most heavily armoured ship in the Royal Navy. 
The application was in the usual course referred to the Magnetic department, and on a 
report that no injury was to be apprehended, the required permission was granted*. 
The first trials were made on the 4th August 1866, and are thus described by 
Mr. Hopkins in a report dated 10th August, and received at the Admiralty a few 
days afterwards. “ After having ascertained the actual magnetic condition of the ship, 
I applied two of Grove’s batteries of five cells each, with the electro-magnets to the 
main plates at the stern and bow, and in a few hours the polarity of the hull was 
destroyed ” f. 
The results of the observations shown in the General Table, I think entitle me to say 
that there is no foundation whatever for the statement that the polarity of the hull was 
destroyed ; there is in fact no evidence of its having been affected in even the slightest 
degree. To facilitate the examination I subjoin the values of the semicircular deviation 
of the two compasses which were most continuously examined in the period comprising 
the 4th August, viz. the poop and starboard steering compasses ; the position of the 
Standard compass being at that time occupied by machinery. 
* That report, for which I am responsible, was made on the supposition that the process was one to be applied 
to the hull of the ship generally, according to the specification of the patent. Had I understood that the iron 
in the immediate vicinity of any compass was to be magnetized to a high degree of intensity, I should certainly 
have reported differently ; and it will be seen in the sequel that from the results of Mr. Hopkins’s experiments, 
I was obliged to submit that no such experiments for the future he permitted within 20 feet of any compass 
placed for the navigation of the ship. 
f This statement was subsequently repeated at a Meeting of the British Association for the Advancement of 
Science held at Nottingham in 1866, in a paper “ On the Depolarization of Iron Ships, to prevent the Devia- 
tion of the Compass,” by Mr. Evan Hopkins, C.E. See Athenaeum of September 8th, 1866. 
