Magnetic Survey of the United States 



95 



trie car lines, whose powerful influence 

 may affect magnetic instruments five 

 miles away. Similar stations have also 

 been established at Baldwin, near Law- 

 rence, Kansas; at Sitka, Alaska, and 

 near Honolulu, in the Hawaiian Islands, 

 in order to assist in the magnetic survey 

 of those regions. All of these observa- 

 tories are furthermore taking part in the 

 international magnetic work conducted 

 in cooperation with the present Arctic 

 and Antarctic Expeditions. 



The practical application of magnetic 

 data is, however, not entirely limited to 

 a knowledge of the direction of the com- 

 pass needle. The mariner, with the 

 modern iron ship now in use, carries 

 with him a continuous source of disturb- 

 ance, so that his uncompensated com- 

 pass will fail to give even the true mag- 

 netic direction for the ship's position. 

 It is therefore necessary to apply such 

 counteracting or correcting devices 

 which will annul to a large extent the 

 ship's magnetic influence. These me- 

 chanical devices are not, however, en- 



tirely compensatory for all the places a 

 ship is likely to be in, owing to the 

 changing character of the ship's own 

 magnetism, and so the mariner must 

 determine a table of correction (the so- 

 called deviation table) for the different 

 positions and directions of the ship's 

 head. For this purpose a knowledge of 

 the dip of the magnetic needle (the 

 angle by which a magnetic needle 

 mounted in the vertical plane passing 

 through the magnetic meridian is pulled 

 down by the earth's magnetic force ) and 

 the intensit}' of the magnetic force are 

 essential. The electrician, the geolo- 

 gist, and the physicist likewise desire a 

 knowledge of these quantities. They 

 are furthermore essential in ascertaining 

 the precise laws underlying the varia- 

 tions of the earth's magnetism. 



A complete magnetic survey, there- 

 fore, embraces the determinations of the 

 three magnetic elements, magnetic dec- 

 lination (variation of the compass) , dip, 

 and intensity, and their changes from 

 time to time. 



Lines of Equal Magnetic Declination (Variation of the Compass) and Equal 

 Magnetic Dip for the Year 1900 



