CHAf. 8J 



MAGNETIC METHOD 



369 



15" = 60 min. of At). For some countries formulas are available giving 

 diurnal variation as a function of latitude and longitude. With an in- 

 crease in distance of the observatory from the area, these calculations 

 decrease in accuracy, since the correction applies only to periodic and not 

 to nonperiodic variations. This has been shown by Soske^^ who found 

 differences between records taken near Pasadena and observatory records 

 of Tucson (AX = 7°) amounting to IO-2O7 on days of magnetic activity. 

 To what extent local meteorologic and geologic conditions affect the 



Mft. 6 12 18 Mn. 6 J2 18 Mn. 6 IB 18 Mn. 6 12 J8 Mn. 



N.ComponefitX B. Component Y Vert. Component Z Dip I 



Fig. 8-41. Diurnal variation curves for X, Y, and Z (after Bartels). 



magnetic variations has been the object of some discussion. H. Aurand,*^ 

 in an analysis of survey data accumulated between 1928 and 1931 by the 

 Midwest Refining Co., obtained conclusive evidence of the influence of 

 meteorological factors (up to 5O7). Whether variable static charges are 

 produced on magnetic instruments by atmospheric electric fluctuations or 

 whether magnetic fields originating from electrically charged particles in 

 motion are responsible is difficult to decide. Inasmuch as there appears 



" J. L. Soske, Terr. Mag., 38(2), 109-116 (June, 1933). 

 ^^ Personal communication to the author. 



