NEW MAGNETIC CHARTS OF THE INDIAN OCEAN. 



By L. a. BAUER. 

 (Read April 20, 19 12.) 



The charts exhibited embody the results of magnetic observa- 

 tions made during the summer and fall of 191 1 on board the non- 

 magnetic yacht Carnegie operating under my direction as Director 

 of the Department of Terrestrial Magnetism of the Carnegie In- 

 stitution of Washington. 



The necessity of the new charts arose from the exceptionally 

 large errors found in the most recent magnetic charts at present in 

 use by mariners. Thus, for example, the errors in the compass 

 directions for two of the most recent charts approximate respect- 

 ively four degrees and six degrees, though one of them was issued 

 as recently as 1910. With the exception of a few values found by 

 the vessel, the Galilee, used in the Pacific Ocean work, these are 

 the largest errors thus far revealed. In the portions of the Atlantic 

 Ocean thus far covered by the Carnegie the compass chart errors 

 have generally been below two degrees, though running at times 

 up to two and a half degrees. 



The chart errors in the compass directions are usually found to 

 be systematic, that is, in the same direction for large stretches, and 

 are to be ascribed largely to erroneous secular changes allowed for 

 in attempting to bring previously observed values up to date. 



Thus, for example, by comparing the Carnegie values of 191 1 

 with those obtained on board the German Antarctic vessel, the 

 Gauss, in 1903, it is found that the north end of the compass moved 

 to the eastward (hence diminished west declination) at the average 

 rate of about 11' per year off the southeast end of Africa, whereas 

 in the vicinity of the islands of St. Paul and New Amsterdam in 

 the Indian Ocean (lat. 35° 16' S., long. 74° 46' E.) it moved to the 

 westward (increased west declination) at the average rate of about 



240 



