th> Jefferson Physical Laboratory. 



463 



general course for that and for each of the other piers. The 

 curves corresponding to the values in columns 4 and 5, ob- 

 served nearly over the eastern and western ed«res of the brick- 

 work, show maxima and minima at the same points but of 

 about half the values reached on the center lines, the values 

 being very nearly the same for each edge. 



























































































































































































































































































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as 



Fig. 6. Variations of the Horizontal Intensity in a line seven inches above piers. 



The general form of the curve is accounted for by the as- 

 sumption that free south magnetism is distributed nearly 

 uniformly over the upper surface of each of the piers, 

 strengthening the field on the south side and weakening it on 

 the north as before described, while over the center the Hori- 

 zontal Force has nearly its normal value. 



The fact that the values of the maxima over the edges are 

 about half those in the central line strengthens this assumption 

 of a uniform distribution of the magnetism over the surface, 

 since, if the pier be supposed divided by a vertical plane passed 

 meridionally through the center, the half of the pier farthest 

 removed contributes but little to the disturbance over the edge, 

 while if transferred parallel to itself so that the two edges 

 coincide, each half would be similarly situated with respect to 

 the points observed, and the effect would be doubled and 

 should obviously give the same value of the disturbance as 

 that actually observed on the central line. 



The double maximum observed in the case of pier 6 appears 

 to be owing to an irregularity in the distribution of the mag- 

 netism, perhaps due to a single brick or bricks near the sur- 

 face of the pier possessing permanent magnetism, not pro- 

 duced by the induction of the earth upon it in its present 

 position. 



Such permanent polarity has been observed in varying de- 

 gree in many bricks and portions of bricks of the same manu- 



