MAGNETISM AND THE COMPASS. 671 



which were observed before her sailing, to such an extent, that the standard 

 Compass, which had originally a deviation of 20° to the port side when the 

 ship's head was North, had this deviation reduced to 3° 22' on her return 

 to Liverpool ; while her steering Compass, which originally had been 

 rather over-corrected, having an error of 1° 43', returned with it 22° 25' ; 

 while the companion Compass, having an error of 1° 43', returned with it 

 17° 35'. This change was one which had evidently taken place in the 

 Eetentive Magnetism of the ship. The change shows the complete failure, 

 in such a voyage, for compensation by fixed magnets. 



In 1843, when the Compass Department of the Admiralty was established, 

 the science and the instruments were in an equally bad condition. The 

 labours of Captain E. J. Johnson, E.N., F.E.S., however, soon put a new 

 face on matters, and much was done in his time towards developing the 

 application of the principles enunciated. But the vast increase of iron 

 shipping has caused greater requirements, and the sailor is very largely 

 indebted to Captain (afterwards Sir) Fred. J. Evans, R.N., who produced 

 an excellent series of Variation Charts, and also devoted much talent to 

 the investigation of the problem of Magnetism in Iron Ships. From his 

 Eeport on the Deviation in the Iron Ships of the Royal Navy, April 18fch, 

 1860, we extract the following : — 



On the Nature of the Magnetism in Iron-built Ships. — The magnetic 

 influence of steam machinery having been reviewed, the nature of the 

 Magnetism of iron-built ships can be entered on free, to a certain extent, 

 of conditions arising from this extraneous source of Compass Error, and 

 those examples fairly eliminated where it tends to embarrass the discussion. 



The investigation of the disturbance arising from the horizontal induc- 

 tion of the soft iron in the ship, or the co-eflBcient D, offers several novel 

 and suggestive points of inquiry ; the chief characteristics are : — 



1. That it has invariably a. positive sign, causing an Easterly Deviation 

 in the N.E. and S.W. quadrants, and a Westerly Deviation in the S.E. 

 and N.W. quadrants. 



2. That its amount does not appear to depend on the size or mass of 

 the vessel, or direction when building, or on the iron beams. 



3. That a gradual decrease in amount has occurred, when examined over 

 a number of years, in nearly every vessel that has been reviewed. 



4. That the value remains unchanged in sign and amount, on chano-es 

 of geographic position, confirming theoretical deductions. 



5. That a value for this co-efficient, not exceeding 4°, and ranging be- 

 tween that amount and 2°, may be assumed to represent the average or 

 normal amount in vessels of all sizes. 



The opinion has been long entertained, that the original Magnetism of 

 an iron-built ship, or that acquired in the process of building, undergoes a 

 rapid change after launching, and that from this cause accidents have 

 occurred to recently launched and hastily equipped vessels. The records 

 of ships of the Eoyal Navy do not illustrate this subject. 



I have alluded to the importance of the conclusions to be derived from a 



