DEPARTMENT OF TERRESTRIAL MAGNETISM. 



333 



The relative accuracy of the Galilee-Carnegie results for annual change is 

 different for each determination, since it depends on a different time-interval 

 in each case and also upon a greater or less number of stations more or less 

 differently distributed. If we assume the declination-change with locality to 

 be linear between the stations considered, and the probable error of a single 

 station to be ±0?15, then the probable errors of the tabulated values of the 

 Galilee-Carnegie results would be, in order from top to bottom, respectively, 

 ±?02, ±?02, ±?01, ±?02, ±?01, ±?02, ±?03, ±?01, ±?02, ±?03, ±?02, 

 ±?02. 



Table 3, of chart-corrections for dip and horizontal intensity as determined 

 aboard the Carnegie in the Pacific Ocean in 1912 and 1915 (to May), shows the 

 necessary corrections to be applied to values scaled from the only charts 

 available, the British Admiralty charts (No. 3598 and No. 3603, published 

 in 1906) and the German Admiralty charts (XIV, 2 a and XIV, 2 b, published in 

 1905), to obtain the values given by the Carnegie. The dip is considered posi- 

 tive when the north end of the needle is depressed and the horizontal intensity is 

 always positive. The corrections to the British dip chart are — 5° or over in 

 three separate regions, and even reach — 6?4. The German chart requires 

 maximum corrections of +4?0 and -F4?9, and there are occasional disagree- 

 ments between the two charts of 7° or more. 



The corrections for horizontal intensity amount to nearly two units in the 

 second decimal place c. g. s. for one chart, and over one unit in the same 

 decimal place for the other. 



Table 3. — Chart corrections for dip and horizontal intensity as determined aboard the Carnegie 

 in the Pacific Ocean in 1912 and 1915 {to May). 



The accuracy of geographic positions at sea is dependent on so many factors 

 that it is quite impossible to define it by exact figures based on any one inves- 

 tigation of numerical results. An examination of some of the three-star 

 determinations of the ship's position made on the Galilee indicates that, if the 

 sun or star be favorably situated and the weather and sea conditions fair, the 

 average error to be expected in the determination of the geographic position 

 is less than 2 miles. The error in the control of the ship's run is usually insig- 

 nificant if the controlling astronomic observations are not more than 6 hours 

 apart. This is usually the case in the Galilee and Carnegie observations, 

 except in high latitudes, where fog and clouds prevail. Of course there are 

 exceptional times when no astronomic observations are possible for several 



