MAGNETISM AND THE COMPASS. 669 



ships, was but little understood, and even now is not established on a 

 firm basis in all its particulars. 



This important discussion, upon the Magnetism of Iron Ships, began by 

 Professor Airy's experiments, recorded in the "Philosophical Transactions " 

 for 1839. His conclusions were somewhat opposed by Dr. Scoresby, in 

 1854, which was replied to in the " Mercantile Marine Magazine," in 

 1854 and 1855. To further investigate the still obscured principles in- 

 volved in this very important topic. Dr. Scoresby undertook his well-known 

 voyage to Australia and back, in the Boyal Charter, between February and 

 August in 1856. ' 



The attention of the shipping interest was strongly called to this question 

 at the meeting of the British Association at Liverpool, in 1854, and the 

 result was the formation of the Liverpool Compass Committee, who re- 

 ported to the Government, in 1855 and 1856, and the valuable series of 

 observations they collected were discussed by their able secretary, Mr. 

 Kundell, and by Mr. Archibald Smith, Mr. Towson, and others, in 1857. 

 One important conclusion arrived at is stated by Mr. Archibald Smith, as 

 follows : — 



" Whatever differences of opinion may be entertained as to applying 

 corrections to the steering Compasses of iron ships, it can hardly admit of 

 question that every iron ship should have at least one Compass removed 

 as much as possible from the influence of iron and not corrected by magnets, 

 and should be swung at the beginning and end of every voyage of any 

 length, and the Deviation of the uncorrected and^ corrected Compasses (if 

 any) observed. No man is competent to command an iron ship who is 

 not competent to make these observations."* 



More refined calculations, for determining the relation between a ship 

 and her Compass, were given in a pamphlet by Archibald Smith, Esq., 

 " Instructions for the Computation of a Table of the Deviations of a Ship's 

 Compass," 1848, as a supplement to the " Practical Kules for ascertaining 

 the Deviations, &c.," 1854. 



Dr. Scoresby gave the following summary of leading deductions on the 

 Character, Distribution, and liability to Change, of the Magnetism of Iron 

 Ships : — 



(1.) As to the Sources of the Intense Magnetism of Iron Ships. — Ships 

 built of iron must not only be strongly magnetic, because of the vast body 

 of this metal which is subjected to the action of terrestrial induction ; but 

 by reason of the elaborate system of hammering, as well as from the bend- 

 ing of the plates and bars during the progress of construction, there must 

 be an extremely high development of the quality of Retentive Magiutism. 



(2.) Effect of the Position of a Ship when Building. — Each iron ship 

 must have a special individuality of the magnetic distribution, depending 

 essentially on the position of the keel and head whilst building, such dis- 

 tribution having, in each individual case, a Polar axis and Equatorial plane 

 conformable to those of the earth at the place where the ship is built. 



• From the Introduction to Dr. Scoresby's " Journal of a Voyage to Australia," by 

 Archibald Smith, Esq., M.A., page 48. 



