Tables 728 and 729 (charts) r-77 



TERRESTRIAL MAGNETISM 



TABLE 728. — World Isodynamic Lines, Epoch 1930 (Lines of Equal Horizontal 



Intensity (//)) 



The annual changes in position of isomagnetic lines shown in Tables 726-728 are most 

 conveniently represented by the isoporic charts, Tables 729-731. To these are added also 

 similar charts showing lines of equal annual change in vertical force (Z) and total force 

 (F), Tables 735 and 736. The difficulty of securing data suitably distributed over the 

 entire surface of the earth is such that only approximate positions of isopors are known, 

 although there is sufficient evidence to show that these positions change with relative 

 rapidity especially near the foci of change. The rates of change of the magnetic elements 

 and the accelerations in those rates can be derived for regions where there are magnetic 

 observatories by reference to Table 737, " Mean annual values of magnetic elements at 

 observatories." 



TABLE 729. — World Isoporic Lines for D (Lines of Equal Annual Change) 

 Approximate Epoch 1920-1925 



Smithsonian Tables 



