12 



Land Magnetic Observations, 1914-20 



On the other hand, the successful and somewhat extended use of the Department's 

 design of field earth-inductor in several difficult expeditions has shown it to be an in- 

 strument of relatively high, absolute precision. It is noteworthy that numerous inter- 

 comparisons, covering extreme ranges in inclination and involving various types of in- 

 ductor, show the corrections for inductors on standard to be practically constant for 

 every value of inclination, and certainly well within the limit of accuracy of observation 

 possible with vertical circles of the sizes used. An inspection of the corrections on 

 standard for various earth-inductors and comparison with those for various dip-circles, 

 as given in Table 4, points forcibly to the desirability of replacing the dip circle by the in- 

 ductor wherever possible, both in the field and at observatories. 



The inclination corrections adopted for the various instruments, used in the ob- 

 servations contained in this volume, are given in Table 4; these corrections are to be 

 applied algebraically, regarding inclination, north end of needle down as positive, and 

 south end of needle down as negative. 



Table 4 also gives the corrections for the compass attachments of the dip circles; 

 these corrections are to be applied algebraically to observed results, regarding east 

 declination as positive and west declination as negative. 



1 For explanation of types, see p. 6. 



