MAGNETISM OF IKON SHIPS. 175 



magnetism. In regard to subpermanent magnetism, 

 the first experiments on iron bars, &c., were made by 

 Mr Scoresby, about 1821 ; and the first virtual observa- 

 tion in ships was made by General Sabine in discussing 

 the compass-deviations in Sir James Boss's voyage, 

 1839 to 1843, in which he remarked that the pecu- 

 liarities in the disturbances of the compass lasted for 

 a short time after the ship had left the region in which 

 the terrestrial forces were such as would tend to ex- 

 plain the disturbances. These observations were made 

 in wood-built ships having many accidental masses of 

 iron. 



The first explanation of the character of the com- 

 pass-disturbance produced by iron ships was given by 

 the writer of this Treatise in the Phil. Trans., 1839, 

 as resulting principally from examination of the iron 

 steamer, Rainbow, in 1838. The disturbing forces on 

 the steering-compass of that ship were so great that 

 in one position of the ship the north end of the needle 

 was deflected more than 50 to the east, and in another 

 position it was more than 50 to the west. The first 

 light that was thrown upon the causes of these de- 

 viations was obtained by placing the ship with her 

 head exactly north (which can be done in various 

 ways, one of the most convenient being to use an 

 azimuth-compass on shore, and to adjust the ship 

 by signal till her masts, as seen by the shore-compass, 

 are all in the magnetic meridian), then observing the 

 deviation of the compass, and, replacing the compass 

 by a vibrating needle whose time of vibration on shore 



