the Jefferson Physical Laboratory* 469 



not over the edge of the brick-work as in the case of the piers, 

 owing to the different form of the surface, and in the case of 



the door, to the greater distance of the plane of observation.) 

 The absence of local disturbance in the north doorway of the 

 tower is due to the fact that there is on that side a correspond- 

 ing doorway in the basement, and the opening in the brick- 

 work extends to the ground level. 



The amount of irregular disturbance arising from nearness 

 to walls is shown in fig. 10, which gives the lines of equal 

 directive force in room 15 in a plane 7 inches above the piers, 

 plotted from a very careful survey in which observations were 

 made at points one foot apart over the whole room. 



This shows the same general features as the former survey 5 

 feet above the tloor. 



This detailed observation, however, enabling a much more 

 accurate determination of the lines brings out very clearly the 

 connection of the piers with these disturbances. 



The effect of the individual bricks is so great when the 

 points are within 6 inches of the wall that I have only con- 

 tinued the lines within one foot of the bricks, although many 

 observations were made within that limit to determine more 

 accurately the position of the lines. It is worthy of notice 

 that no considerable disturbance is produced by the small soap- 

 stone sink near the chimney at L. 



The normal value assumed is the mean of the values ob- 

 served at 96 points taken in a parallelogram 15 feet X 5 feet 

 about the arrow, fig. 10, no point being less than three feet 

 removed from the nearest bricks. 



Within this space the deviations though less than *001 H 

 from the mean are plainly discriminated by the variometer, and 

 it may be fairly said that there is no point in the room where 

 the magnetic effects of the brick-work is not an easily measur- 

 able quantity. 



It is hardly worth while to multiply observations or to treat 

 of such points as the effect of change of temperature, since the 

 magnetic effect of the bricks is so great. A few facts are, how- 

 ever, of interest. Bricks similar to those used in the Lab- 

 oratory Building have been found to be in various degrees 

 permanently magnetic, and to show the alterations in their 

 temporary magnetism produced by change of position with 

 respect to the earth's field, two or three specimens of brick 

 from other sources show magnetic effects comparable with 

 these. 



Portions chipped from the piers and crushed to coarse 

 powder delivered to a 2 oz. magnet many magnetic particles, 

 which when examined under a microscope were seen to contain 

 among their number octahedra of magnetite. Magnetic par- 



