THE BEARINGS OF MAGNETIC OBSERVATIONS. 87 



period of activity in terrestrial magnetic science — 1835-45 — 

 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 com- 

 passes, 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 elucidate the causes of her magnetic con- 

 dition ; 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 magnetised 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 

 substituted for those of wood. 



The ability to predict the deviation of the compass on 

 change of latitude did not, however, satisfy Airy, for after 

 a remarkable mathematical investigation of iron ship's 

 magnetism of a less rigorous character than Poisson's, but 

 sufficiently accurate for his purpose, he in 1839 proposed 

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

 by means of magnets and soft iron, so arranged as to pro- 

 duce equal and opposite magnetic effects to that of the 

 ship. Provided with Airy's admirable and simple directions 

 this method of correction was comparatively 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 



