1868.] On the Polar Magnetism of H.M.S, ' Northumberland^ 311 



March 26, 1868. 



Lieut.-General SABINE, President, in the Chair. 



Pursuant to notice given at the last Meeting, Theodor Ludwig Wilhelm 

 BischofF of Munich, Rudolph J. E. Clausius of Wiirzburg, Samuel Hein- 

 rich Schwabe of Dessau, and Hugo von Mohl of Tubingen, were balloted 

 for, and elected Foreign Members of the Society. 



The following communication was read : — 



On the Amount and Changes of the Polar Magnetism at certain 

 positions in Her Majesty^s Iron-built and Armour-plated 

 Ship ' Northumberland/^" By Frederick J ohn Evans, F.H.S., 

 Staff-Captain B.N., in charge of the Admiralty Magnetic De- 

 partment. Communicated with the sanction of the Lords 

 Commissioners of the Admiralty. Received March 5, 1868. 



(Abstract.) 



The 'Northumberland' is a ship of 6621 tons, built at Millwall, River 

 Thames ; head N. 39|° E. magnetic, and completed with the armour-plates 

 in the same direction : the launch was effected on l/th April 1866 ; she 

 then lay for eight months in the Victoria Docks, head S. 22° W. magnetic, 

 or in a direction nearly opposite to that occupied in building. 



From January to March 1867 she lay at Sheerness swinging to wind and 

 tide : the ship was then removed to Devonport and placed in dry dock, 

 head S. 84° E. magnetic, where she has remained till the present time. Ob- 

 servations of deviation and horizontal and vertical force were made at the 

 standard compass (elevated 8| feet from the iron deck, and 172 feet 

 distant from the stern), the poop-, and two steering-compasses (starboard 

 and port), the latter being on the quarter-deck below the poop-compass, — 

 the group being placed near the steering-wheel, 52 feet from the stern, 

 and each compass 4 feet above its own deck. 



The polar force at each compass was originally directed to the part of 

 the ship whicli was south in building ; it diminished in the Victoria Docks, 

 showed a tendency to return to its original value at Sheerness ; and finally 

 a large force to starboard was developed by the position of the ship in 

 dock for the lengthened period at Devonport. 



Tables are appended giving the magnetic coefficients for each compass 

 from time to time, extending from 17th April 1866 to 10th December 

 1867. 



The results show that the greater part of the polar magnetism was 

 caused by the subpermanent magnetism of the whole mass of the ship, 

 due to her position in building and afterwards : this polar force was so 

 great as to require correction by magnets in addition to the usual tabular 

 corrections. 



