the Jefferson Physical Laboratory. 



4:>T 



It will be remembered that the plane of observation was or- 

 iginally fixed at live feet from the floor to avoid any possible 

 disturbance due to the brick-work of the piers. A few pre- 

 liminary observations had seemed to show such an effect, 

 amounting to about 1-1000 H, in the case of one pier, when 

 the instrument was very near the brick-work ; while observa- 

 tions taken at various points upon one of the piers in room IT 

 showed no apparent magnetic effect comparable with that due 



-+t 



fc 





Fig. 4. Plan of west wing of the Jefferson Physical Laboratory, with lines of 

 equal directive force in plane 5 feet above first floor. Scale 16 feet to the inch. 



tofthe steam-pipes. Recent observations show that this pier 

 somewhat resembles pier 4, hereinafter discussed, and was un- 

 fortunately selected as a test. 



