92 



i?. W. Willson — Magnetic Field, etc. 



Fig. 2 gives the results of this computation, the lines con- 

 necting points at which the effect of the pipes as computed on 

 the above assumption is to increase the Horizontal Intensity by 

 equal amounts. From this plotting the value of the disturb- 

 ance at each of the various points of observation due to the 



^;//;/,js;s i '/.-,;»>„,/% ', 



W//?//- ■y/A<v//<'/////V///wJ& 



m 



m 



% 



w^>/;-///M wmm >. 



I 



Wm. 



Fig-. 3. Plan of west wing of Jefferson Physical Laboratory with lines showing 

 disturbances of the earth's directive force, due to causes other than presence of 

 steam pipes shown ou Fig. 2. 



G-, point below iron stove on second floor. 



K, point below iron base plate ou second floor. 



presence of the pipes was determined and subtracted from the 

 corresponding observed values as shown on fig. 1. By the use 

 of the numbers thus obtained a new system of lines of equal 

 directive force was found which probably fairly represents the 



