SURVEY OP THE BRITISH ISLES FOR THE EPOCH JANUARY I, I88G. 263 
characteristics may be reproduced, and a knowledge of tlieir forms is very useful in 
interpreting’ maps on which the true isomagnetic curves are drawn. 
These facts may also be illustrated by means of diagrams of another kind. Thus if, 
in figs. 7 and 8, the slope of the line AD represents the normal rate of increase of the 
Declination with longitude, the effect of an attractive centre below A will be repre¬ 
sented by the curves shown, which correspond to the two cases of a weak and strong 
attraction respectively. If, however, the force is repulsive, the curves assume the 
forms shown in figs. 9 and 10. In this case there may be two maxima and minima, 
following each other in order on opposite sides of the centre of repulsion. The 
isogonals will be drawn together in its neighbourhood, and there may be two systems 
of loops, one on each side of the centre. 
Figs. 7 to 10. 
D 
A centre of attraction will also be indicated by a convergence of the lines of equal 
Horizontal Force and Dip in its neighbourhood. 
The observed will be greater than the calculated Force to the south, and smaller to 
north, of the centre. The Dip, on the other hand, will be less and greater than its 
calculated values at stations to the south and north respectively of a point at which 
the directions of the normal and disturbing Forces coincide. 
The phenomena are more complicated if the effect of the disturbance is to increase 
the value of the element on that side of the centre of attraction on which it would be 
normally the greater, instead of increasing it (as in the case of the Declinations) on 
that side (the east) on which it is normally the less. 
Thus, if we represent a northward movement by progress along the line WE 
(fig. 10) from W to E, the effect of a strong centre of attraction would be to check 
