MAGNETISM AT HALIFAX, AUGUST, 1904. — DIXOX. 249 



Isogonic Charts. 



For practical purposes the secular changes in declination have 

 long been studied. Owing to the use of the compass made in 

 navigation the ' variation charts ' must be always up-to-date, as 

 in cloudy weather the seaman depends altogether on them and 

 on his compass. On land the surveyor should be able to find the 

 declination and then running his course by geographical bear- 

 ings, avoid the trouble such as has already been caused in this 

 province by using the compass without recoi'ding ' the variation. 

 Variation charts are made by drawing on them lines passing 

 through places where the declination is the same — isogonic lines. 

 Halley, the celebrated astronomer, was the first to publish these 

 charts, though this method of mapping lines of magnetic declina- 

 tion seems to have been used by Christoforo Borri. Dr. Bauer 

 in 1895 discovered in the British Museum a copy of Halley 's 

 first chart, published in 1701, on which were marked the results 

 of Halley 's observations taken in the Atlantic on the Paramour 

 Fink, a vessel equipped by the British government for the first 

 systematic survey, 1698-1700. In 1702 Halley published 

 a second chart, on which wei'e marked the isogonic lines for the 

 Indian and Pacific oceans. The map on plate 24 gives the 

 isogonic lines as determined by Halley, and on plate 25 are given 

 the isogonic lines for 1905 as computed by the British Admiralty. 

 On tracing these lines in either chart, we notice the two lines 

 of no declination, or agonic lines, which separate places having 

 east from tho.se having west declination. The very irregular 

 distribution of equal values of declination is easily seen, the 

 chart for 1905 showing the remarkable circular agonic line in 

 China. 



But except on land we have little real knowledge of these 

 isogonic lines. " No magnetic data have been obtained on the 

 ocean areas since the advent of iron ships, except from occasional 

 expeditions. Our present lines of equal magnetic declination 

 over these waters depend almost entirel}^ upon data acquired in 



