GENERAL BEARINGS OF MAGNETIC OBSERVATIONS. Ill 



Naval requirements, however, did not permit of any delay in pub- 

 lishing magnetic charts affecting navigation, for in 1846 the hydrog- 

 rapher of the Admiralty requested Sabine to provide charts of the 

 declination for the Atlantic Ocean from 60° north to 60° south. These 

 were largely used until Evans's chart of the declination for the whole 

 navigable world was issued in 1858. 



The excellent work of Flinders already referred to, of ascertaining 

 from his knowledge of terrestrial magnetism the chief cause of the 

 deviation of the compass in wood-built ships, and providing a corrector 

 for those deviations, had to be followed up on a much larger scale and 

 with more exact methods in the iron-built ship, which, in that period 

 of activity in terrestrial magnetic science — 1835-1845 — was rapidly 

 increasing in numbers on the ocean. 



Thus in 1835 observations were made on board iron ships showing 

 that they acted as a magnet on their compasses, but there was nothing 

 to show in the results what the causes of this condition of the iron ship 

 were, until Poisson in 1838 published his celebrated "Memoir on the 

 deviations of the compass produced by the iron in a ship." This was 

 a rigorous mathematical investigation of the subject, showing that the 

 deviations of the compass were due to induction in the ship by the 

 magnetic force of the earth. 



If the iron ship had simply been built for service in one locality, a 

 limited knowledge of terrestrial magnetism would have sufficed to eluci- 

 date the causes of her magnetic condition ; but she was destined to 

 traverse every navigable sea over large changes of magnetic latitude, 

 hence the necessity for an accurate knowledge of the distribution of 

 magnetism over the great parent magnet, in order to determine the 

 magnetic condition of her comparatively minute offspring the magnet- 

 ized iron ship; and this at all times and in all places in the interests 

 of navigation. Observations of the terrestrial magnetic elements were 

 therefore an absolute necessity if iron-built ships were to be substi- 

 tuted for those of wood. 



The ability to predict the deviation of the compass on change of lati- 

 tude did not, however, satisfy Airy, for after a remarkable mathemati- 

 cal investigation of iron ship's magnetism of a less rigorous character 

 than Poisson's, but sufficiently accurate for his purpose, he in 1839 pro- 

 posed his methods of annulling the deviation of a ship's compass by 

 means of magnets and soft iron, so arranged as to produce equal and 

 opposite magnetic effects to that of the ship. Provided with Airy's 

 admirable and simple directions this method of correction was com- 

 paratively easy in one latitude, but experience at sea, especially in 

 voyages to the Cape of Good Hope, showed that every iron ship required 

 a different application of Airy's correctors. 



To discriminate between the amount that was to be corrected by per- 

 manent magnets, by horizontal soft iron, and by vertical soft iron, an 

 accurate knowledge of the magnetic elements dip and intensity obtained 

 from observations on land and at sea was essential. 



