464 



R. W. Willson — Magnetic Field in 



facture. With this exception the magnetism shown by the 

 piers is exactly what might be expected to result from the 

 effect of the earth's nearly vertical induction upon a mass con- 

 taining a relatively small amount of magnetic matter in the 

 form of isolated particles, disseminated through a non-magnetic 

 shaft nearly 20 feet high and of relatively small diameter. 



The nearly uniform distribution of the magnetism is further 

 confirmed by observations of the other piers, the results of 

 which are shown in figs. 7 and 9. 



"s 



-f 



-3 



+ 3 



-3 



-3 



+ 3 



-3 



+ 3 



+ 3 



•s 



Fig. 7. Variations of the horizontal intensity in a line seven inches above piers. 



In fig. 7 the variations of the horizontal force along the cen- 

 tral line are shown for piers 1, 3, 4, 9, 11, 13. The abscissas 

 give the distance in feet from the center of the pier in each 

 case, positive toward the North, and the ordinates in the same 

 manner as before give the variations of H from its normal 

 value, which is here assumed to be the mean of observations 

 North and South at a distance of 3 feet from the center. 



Let 21 be the length in centimeters of the side of a square 

 pier, a the density per sq. cm. of the magnetism uniformly dis- 

 tributed over its upper surface. Then a unit pole h cm. above 

 the pier, and in its meridian plane at a distance x cm. from the 

 axis, is urged horizontally toward the axis by a force in C. Gr. S. 

 units 



