466 



R. W. Willson — Magnetic Field in 



The length of the pier being about 6 meters, this corres- 

 ponds to a magnetic moment of 6500 C. Gr. S. units or rather 

 more than that which may be readily given to a hard steel 

 magnet of 1 sq. cm. section and 20 cm. long. 



Piers 1, 2, 3, 5, 7, 8, 9, 11, 12, figs. 6, 7 and 9 show variations 

 nearly as described for a typical pier ; while 4 and 13 show the 

 same irregularity as 6* I may here note that the pier in 

 room 17, which was superficially examined in 1887 for the 

 purpose of detecting local influences was of this class, the var- 

 iation over a large part of its surface being very small. 



The nature of this deviation from this normal type is made 

 more striking by comparison of the adjacent piers 4 and 5, the 

 former presenting the case of least and the latter of greatest 

 local effect of piers. The variations of H from its value at 

 the center are given in the preceding tables, and fig. 8 shows 

 the lines of equal horizontal force in a plane seven inches 

 above each pier. The field above 4 although the extreme vari- 

 ation amounts to but "0025 H, indicates a distribution of mag- 

 netism far from uniform. The existence near the northeast 

 corner of free north magnetism would account for the ob- 

 served variations. In the case of pier 5 the effect of the pier, 

 though large, is quite symmetrical, indicating a very uniform 

 distribution. 



nor 



th 



north 



^ ] 



\W, 





•r^\ 



- . - -, 







s^ 





1 



l 







\-t.ooos__ 

 'r*-OQio __ 





TSs^^ **. 



1 





!' ~^\ 



S~ 



W.oois 











+J)OZO 





; 



Pier 5. Pier 4. 



Pig. 8. Lines of equal directive force in a plane seven inches above piers 4 and 5. 



The magnitude of the disturbance is noteworthy. Above 

 pier 5, for instance, the Horizontal Force at the two edges 

 varies by one per cent, while a change of one tenth of one per 

 cent is caused by passing over two inches near the center, so 



* Pier No. 10 has been omitted being covered with a heavy stone. Piers in 

 rooms 11 and 17 are not included for reasons before given. 



