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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



from the single measurements this would introduce a complication 

 if the instrument were not always turned over so that the strips 

 were parallel to the earth's surface. When this precaution is ob- 

 served, we may regard the influence of the dimensions of the strips 

 as negligible. 



If a and |3 are not large, so that higher powers than the fourth may be 

 neglected, the integration gives : 



r=7ra 2 (l— a 2 — 2(3 2 )dr (1) 





Fig. 9. 



Now we proceed to consider the case, where the hole radiates to a strip 

 of negligible width ds and of the length 2 m. The line is symmetrical in 

 regard to the perpendicular from the central point of the hole. For the 

 central point of the line we put : l — n. Then we have : 

 dT=dm' ds 



9 _ r- _ m' 2 +n 2 

 /3_— i? 2 — R2 



