108 GENERAL BEARINGS OE MAGNETIC OBSERVATIONS. 



Here ve may pause to consider the word declination as applied to the 

 angle which the direction of the horizontal magnetic needle makes with 

 the true meridian. Many magneticians object to the word, but no bet- 

 ter has yet been proposed, or at any rate accepted; the result being that 

 while observers on laud use the term, seamen adhere firmly to tbe expres- 

 sion "variation of the compass." This is as might be expected, when 

 it is remembered that navigators look upon the word declination as con- 

 nected with the position of the sun and other heavenly bodies, and 

 would find it most inconvenient to have the same word in daily use, 

 meaning two totally different things. 



During the eighteenth century charts of the magnetic declination 

 were published by Mountaine and Dodson, Bellin, and Churchman, and 

 for their time may be considered as fair approximations to the truth. 

 Churchman's design was not only to give values of the declination but 

 to furnish the seaman with a means of ascertaining the longitude — an 

 ambitious project, especially as we now know there were probably con- 

 siderable elements of error in these charts, caused by local magnetic 

 disturbance of the observing compass on land, and from the iron used 

 in construction disturbing the compass on board the ships. 



This latter source of error was only beginning to be viewed in its 

 true light at the close of the eighteenth century. 



In the years 1801-1802 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 tbe observed decliuation 

 agreed very nearly, but when she lay with her head east or west, it dif- 

 fered largely. Moreover these errors on the east and west points of the 

 compass had the opposite sign to those observed in England. 



Flinders, however, had supplemented the existing scanty knowledge 

 of the distribution of the dip over navigable waters by several observa- 

 tions of his own in northern and southern latitudes, and from these he 

 drew the conclusion that tbe errors in the declination observed on board 

 ship were caused by magnetism induced by tbe 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 knowledge 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 tbe 

 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 1814 Flinders induced the Admiralty to have experiments made 

 on board men-of-war at Portsmouth, Sheerness, and Hevonport, to 

 ascertain tbe amount of the magnetic disturbance of the compass 

 caused by the iron in each ship. Tbe chief reason for making these 

 experiments was to show the necessity for ascertaining and applying 



