DEPARTMENT OF TERRESTRIAL MAGNETISM* 



L. A. Bauer, Director. 

 GENERAL SUMMARY. 



It is now somewhat over two centuries ago that the first attempt at an 

 ocean magnetic survey was made, the originator and leader of the expedition 

 being the famous astronomer, Edmund Halley, of whom so much has been 

 written recently in connection with the comet named after him. While a 

 great deal has been said of Halley 's various attainments and achievements, 

 but little mention has been made regarding his contributions to the advance- 

 ment of the art of navigation and to the science of terrestrial magnetism by 

 his "Chart of the lines of equal magnetic variation" — the first of its kind. 



The subject of the earth's magnetism had early attracted Halley, and espe- 

 cially the cause of the mysterious change ever going on in the magnetic state 

 of our planet, by reason of which the compass changes its direction, even in so 

 short a period as 5 to 10 years, by an amount sufficient to affect the purely 

 practical interests of surveying and navigation. In 1692 he proposed a 

 unique theory to the Royal Society of London to account for the secular 

 change of the earth's magnetism, and in the year 1698 he set out from Eng- 

 land as captain of the ship Paramour Pink, under the auspices of the English 

 government, to determine the compass direction in various parts of the 

 oceans. After devoting two years to this task, in which he encountered 

 various difficulties, he laid down the results of his labors on a chart giving 

 the "Lines of equal variation." The first edition of this chartf was pub- 

 lished probably in the year 1701, and covered the Atlantic Ocean from about 

 60° north to nearly 60° south, the lines being in no case extended over land 

 areas. In the following year most likely he published his "World chart," 

 the lines of equal variation covering the Atlantic Ocean, the Indian Ocean, 

 and the extreme western part of the Pacific Ocean. 



Since Halley's time the "Charts of the lines of equal magnetic variation," 

 upon which the mariners rely in navigating their vessels, have been repeat- 

 edly reconstructed and revised with the aid of newer data. With the advent 

 of iron-built vessels, the acquirement of compass data of sufficient reliability 

 became increasingly difficult ; those whose duty it was to keep magnetic 

 charts up to date have thus been frequently unable to gage the precise value 

 of data from modern vessels even when acquired by the most experienced 



* Address : The Ontario, Washington, District of Columbia. Grant No. 608. $76,920 

 for investigation and maintenance during 1910. (For previous reports see Year Books 

 Nos. 3-B.) 



t A copy was found by the writer in the British Museum and republished in the Janu- 

 ary, 1896, issue of the journal Terrestrial Magnetism and Atmospheric Electricity. 



195 



