DEPARTMENT OF TERRESTRIAL MAGNETISM * 



L. A. Bauer, Director. 

 GENERAL SUMMARY. 



This report is written on board the Carnegie, cruising in the Indian Ocean, 

 during the months of July and August, between Colombo, Ceylon, and the 

 island of Mauritius. The necessary additions have been made as later re- 

 turns have been received from the various field parties, so as to give a com- 

 plete account of all the operations of the Department to the end of the fiscal 

 year (October 31). 



Speaking of the first object in mind, attention may be called to the large 

 corrections on present mariner's compass charts we are finding in the Indian 

 Ocean, and those, too, over traversed routes. For two of the most extensively 

 used charts of the "lines of equal magnetic variation" these errors approxi- 

 mate, respectively, 4° and 6°, though one of the charts was issued only a 

 year ago. With the exception of a few values found by the Galilee in the 

 Pacific Ocean, these are the largest errors thus far disclosed. In the portions 

 of the North Atlantic covered by the Carnegie during her present cruise, the 

 compass-chart errors have generally been below 2°, though running at times 

 up to 2.5°. In the South Atlantic the errors, in general, have not gone over 

 1°. As heretofore, the information obtained is supplied promptly to the chief 

 hydrographic establishments issuing magnetic charts, in order to enable them 

 to revise their charts from time to time. 



The chart errors are usually systematic, i. e., in the same direction for 

 large stretches, and are often found to be largely due 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 ofit" 

 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 13' per year. The charts would give secular changes of only about 

 one-fourth of these amounts, so that the error of reduction in but ten years 

 amounts almost to 2°. It is doubtless due to these large secular changes dis- 

 closed in the Indian Ocean, and especially their rapid variation with geo- 

 graphic position, that the large errors mentioned have crept into the charts. 

 These interesting facts madr it desirable to alter our projected track from 



Colombo to Mauritius sufficiently to get data for locating the region where 



* Address : The Ontario, Washington, District of Columbia. Grant No. 683. $94,510 

 for investigation and maintenance during 191 1. (For previous reports see Year Books 

 Nos. 3-9.) 

 198 



