272 POPULAR LECTURES AND ADDRESSES. 



to bring to light for their successors, not for 

 themselves, that great marvel of nature, the secular 

 variation of terrestrial magnetism. Borough, Con- 

 troller of the Navy of Queen Elizabeth, seems to 

 have been the first to determine by accurate ob- 

 servation the variation of the compass in England. 

 He found it to be 11 15' to the east of north at 

 London in 1580. It was then imagined to be 

 essentially constant, and Gilbert obviously had not 

 learned that it had changed when, in 1600, he 

 reckoned its amount as about " half a point " (or 

 5|). Twenty or thirty years after Gilbert's death 

 observers began to notice that the variation had 

 diminished considerably from the amount found for 

 it by Borough. An accurate observation in 1633 

 made the variation 4 5', so that it seemed to have 

 diminished by 6 10' in the preceding fifty-three 

 years. 



In 1659 the needle pointed due north in London ; 

 in 1700 it pointed ioj to the west of north. 

 From 1700 to 1818 the westerly variation continued 

 increasing, but more and more slowly, till 1820, 

 when at an extreme westerly variation of 24^ it 



