the Jpffi rsoti Physical Laboratory, 



407 



that the position of an instrument must be determined with 

 considerable precision to ensure even ordinary accuracy. 

 Moreover all of the points observed are distant at least 11 

 inches from the nearest brick-work. It is plain then that the 

 observations made 5 feet from the floor and 2 feet 8 inches 

 above the brick-work of the piers are by no means unaffected 

 by the presence of tlie latter. 



' The effect of the walls too may extend to points at some 

 distance. The magnetism due to the earth's induction pro- 



■OOS 



-OOS 



IS 



25 



+.00 S 



.000 



-.005 



35 U0 1,5 50 55 6» 



Fig. 9. Variations of the Horizontal Intensity on a line seven inches above piers 2 and 12, 



duces disturbances in the neighborhood of openings in the 

 brick-work, such as doors and windows, where free magnetism 

 appears, and the local effect of permanent magnetism in the 

 single bricks can be in many places detected. The first point 

 appears in the observations represented in fig. 9 which shows 

 the variations along a line passing above the piers 2 and 12 

 and through the center of the wing. 



The dotted curve gives in the same manner as before the 

 variations of H from its normal value at the center of the 

 wing, the position of the walls, piers and door-ways being 

 indicated as on fig. 6. The line of observation passes 11 

 inches above the brick-work of the piers, 12 inches above that 

 of the window sills, and 3£ feet above the lower brick-work 

 of the door opening in the tower wall. The door openings 

 are 10 feet high and the instrument is therefore much nearer 

 to the bricks below than to those above. The general increase 

 of H toward the north shown in the dotted curve is due to the 

 influence of the iron steam-pipes. Correcting the observed 

 values for this effect the full line curve is obtained. Both 

 curves show south magnetism in the vicinity of the walls and 

 piers, the earth's induction producing free south magnetism in 

 the lower surfaces of the window and door openings through 

 which the line of observation passes. This results in a 

 strengthening of the field on the south in the immediate 



