250 



CARNEGIE INSTITUTION OF WASHINGTON. 



improvement in the instrumental appliances and observational 

 methods. Magnetic observations on the Carnegie, as shown in table 1, 

 are being obtained practically daily and at an average distance apart 

 of 91 to 138 nautical miles. 



Table 1. — Summary of ocean magnetic work of the Galilee and the Carnegie 1905-1917 (March). 



In accordance with the general plan of work, every opportunity is 

 embraced, while the general magnetic survey of the Earth is in progress, 

 to secure at the same time data regarding the annual changes of the 

 magnetic elements. Such opportunities arise in the ocean work at 

 intersections of the tracks of the Carnegie, for example, with her pre- 

 vious tracks or with those of the Galilee. Owing to the accuracy 

 attained in the ocean magnetic work with the appliances and methods 

 used in our work, it appears to be possible to determine the annual 

 changes of the magnetic elements v.dth a fair degree of certainty from 

 the intersection of tracks about 4 or 5 years apart in time. Tables 3, 

 4, and 5, on pages 271-272, show the results thus obtained at sea by 

 the Department of Terrestrial Magnetism up to the present year (1917) . 



It was found that the systematic errors in the magnetic charts, as 

 reported upon in previous annual reports, were to be explained in large 

 measure, at least for the magnetic-declination charts (''Unes of equal 

 magnetic variation"), as being due to lack of accurate information of 

 the secular magnetic changes over the ocean areas. Usually con- 

 structors of magnetic charts have had to rely upon data obtained from 

 more or less disturbed land observations along the coasts and on 

 islands, or upon more or less uncertain sea observations, separated 

 often by rather long intervals of time. Hence the value of the data 

 determined from the observations of the Galilee and the Carnegie. . 



