in tfu .1 m Physical Laboratory. 



91 



The distance between the floors is L2 feet, from the basement 

 to the first floor 10 feet 



A- it appeared therefore that the distribution of the free 

 magnetism in the pipes, — it' the sole cause of the differences of 



lunation observed, — was such that its variation was not far 

 from symmetrical above and below the plane of observation of 



jjT '■/.V-///W^777 // 



□ no. □ 



1 



mzr-?m m, ■» ^-% *■*■ J 



Fig. 2. Plan of west wing: of Jefferson Physical Laboratory with lines showing dis- 

 turbance of the earth's directive force, produced by steam pipes at A, B, C and F. 



the Horizontal Intensity, I determined as a first approximation 

 to compute the alteration which would be produced in the uni- 

 form field H throughout that plane by the free magnetism indi- 

 cated by the direct observations upon the pipes, i. e. of three 

 north poles five feet above A, B and C and of the strengths 

 above determined. 



