OF TIIE MAGNETIC FORCE. 



55 



A graphical representation of the above tabular numbers is given in diagram H, 

 based upon the three variables : the hour of the day, the month, and the difference 

 of vertical force from the normal monthly value. The contour lines of the mag- 

 netic surface differ 0.001 of vertical force (in absolute measure). Full lines indi- 

 cate greater value, lines of dashes less value than the normal, dotted lines represent 

 the normal value. 



