82 SCIENCE PROGRESS. 



immediate auspices of the Government, made his celebrated 

 voyage over the Atlantic Oceans in a man-of-war, in order 

 that intelligent observation should set at rest much that was 

 doubtful. The results of this voyage, combined with the 

 observations of previous navigators, were embodied in his 

 celebrated chart of lines of equal value of magnetic Varia- 

 tion or Declination, the first of its kind and of so convenient 

 a form that charts of equal values of the three magnetic 

 elements are to this day the most acceptable form for 

 representing the combined results of magnetic observations 

 made over large areas of sea and land, as well as of the 

 special magnetic surveys which in recent years have been 

 made in various countries. 



Here we 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 better has yet been 

 proposed or at any rate accepted ; the result being that 

 while observers on land use the term, seamen adhere firmly 

 to the expression "Variation of the Compass". This is as 

 might be expected when it is remembered that navigators 

 look upon the word Declination as connected with the posi- 

 tion 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 con- 

 sidered 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 considerable elements of error 

 in these charts caused by local magnetic disturbance of the 

 observing compass on land, and from the iron used in con- 

 struction 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. 



