thi Jefferson Physical Laboratory, 459 



effect due to the general magnetization of the bricks, soon to 

 be recorded, should have escaped even a very superficial 

 examination. 



The result of the general survey undertaken to determine 

 what change in the field may have been produced by the lay- 

 ing of the water-pipe mentioned above is exhibited in figs. 4r 

 and .">. 



Fig. 4: gives the lines of equal horizontal intensity recently 

 determined and is to be compared with fig. 1, p. 88. These 

 lines connect points at which the horizontal intensity is the 

 same, the values at points in adjacent lines differing by '0025 

 H. It is evident that there is no change in the general char- 

 acter of the field except that due to the removal of the stove 

 above G. This will be more evident on consulting fig. 5, which 

 gives the results obtained by correcting the observed values 

 for the disturbance due to the steam-pipes at A, B, C and F 

 !>v the method explained on page 91 (fig. 5 may be compared 

 with fig. 3, p. 92). 



The disturbance in room 13 being unchanged although the 

 iron table top to which it was ascribed has been removed, some 

 other cause must be sought for this disturbance. 



The observations upon which the system of lines depend 

 were made at points distant five feet from each other an'd only 

 the general features of the system can be thus determined. 

 In order to settle definitely the cause of the disturbance in 

 room 13, observations have recently been made at distances of 

 one foot apart in the eastern part of the room, and the lines of 

 equal directive force thus much more accurately drawn. As 

 the object was simply to determine the position of the disturb- 

 ing body, the plane five feet from the floor was abandoned in 

 these observations, and the arrangement adopted which had 

 been used in the examination of the piers, which greatly les- 

 sens the labor and increases the accuracy of observations made 

 at points very near together. The results of this examination 

 pointed unmistakably to a soap-stone sink, shown at K, fig. 5, 

 as the main cause of the disturbance. (This sink showed a 

 considerable free south magnetism on its upper surface, tend- 

 ing to decrease H at points north and above ; some further 

 account of this will be given later.) 



The only disturbances remaining unaccounted for are that 

 in room 15-16 and that in room 14 ; which I now believe to 

 be almost entirely due to magnetic action of the bricks of the 

 piers and walls of the building itself. If this is the case, the 

 whole system of lines of figs. 4 and 5 must be looked upon as 

 giving only the most prominent characteristics* of the field, 



* Compare with fig. 10, which gives a portion of the field more in detail, but for 

 a different plane. 



Am. Jour. Sci.— Third Series, Yol. XXXIX, No. 234.— June, 1890. 

 30 



