THE BEARINGS OF MAGNETIC OBSERVATIONS. 83 



In the years 1801-2 Commander Flinders of H.M.S. 

 Investigator, then surveying the southern coasts of Australia, 

 found that when his vessel's head was north or south by 

 compass the observed Declination agreed very nearly, but 

 when she lay with her head east or west, it differed largely. 

 Moreover these errors on the east and west points of the 

 compass had the opposite sign to those observed in Eng- 

 land. 



Flinders, however, had supplemented the existing 

 scanty knowledge of the distribution of the Dip over 

 navigable waters by several observations of his own in 

 northern and southern latitudes, and from these he drew 

 the conclusion that the errors in the Declination observed 

 on board ship were caused by magnetism induced by the 

 earth in the vertical iron of the ship, and changed in value 

 proportionally to change of Dip. Here Flinders was wrong, 

 as the errors were really proportional to the tangent of the 

 Dip. 



In spite of this mistake he was enabled from his know- 

 ledge of the Dip to conceive the idea of so placing vertical 

 bars of iron that they produced an equal and opposite effect 

 on the compass to that of the ship in all latitudes, and thus 

 invented what is now called the Flinders bar, one of the 

 most important correctors of compass disturbance in the 

 iron and steel ships of the present day. 



In 1 8 14 Flinders induced the Admiralty to have ex- 

 periments made on board men-of-war at Portsmouth, 

 Sheerness, and Devonport, to ascertain the amount of the 

 magnetic disturbance of the compass caused by the iron in 

 each ship. The chief reason for making these experiments 

 was to show the necessity for ascertaining and applying 

 these errors to ensure the safe navigation of the ships, but 

 it had also a direct bearing in enabling observers to elimi- 

 nate the hitherto inexplicable divergencies in the values of 

 the Declination observed in different ships in the same 

 geographical position. The results of these experiments 

 bore no immediate fruit, for with the death of Flinders the 

 subject was temporarily neglected. 



In 1 8 19, Hansteen published his Magnetismus der Erde 



