OF THE HORIZONTAL FORCE. 



57 



Diagram G exhibits the changes in the horizontal force (in absolute measure) 

 from the monthly normal value for each hour of the day and for each month of the 

 year. The three variables are: the hour of the day, the month of the year, and 

 the difference of the horizontal force from the normal. The contour lines of the 

 magnetic surface differ 0.0005 of horizontal force in absolute m'easure. Full lines 

 indicate greater value, lines of dashes less value than the mean; dotted lines repre- 

 sent the normal value. 



