ON “Ql'IET” DAYS DUL’INCI T[TE ELEVEN YEARS 1.^90 'I’O 1900, ETC’. 
34;i 
arc and tangent may be neglected, so that e(|nal lengths on the paper represent epual 
changes of direction in the magnet. The curve, it will be noticed, does not depart 
very far from a straight line. 
§ 9. The above graphical method throws no direct light on the absolute magnitudes 
of the changes of the components of force. It is, however, of some interest to know 
Fig. 1. 
the direction and amplitude of the disturlnng force to wliicli the secular change may 
be attributed. To find tliis we may proceed as follows :— 
Let N, W, V represent the values of the mndherly, westerly, and vertical com¬ 
ponents of force at the beginning of a period, at the end of winch the values have 
become N + 8N, W + 8W, V d- 5V. Tlie total force meantime has changed from 
T to T -f ST. 
SN, 8W, 8V represent the components of a force 8R (totally different usually from 
8T) which may be regarded as producing the secular change. If secular change data 
for individual years were sufficiently reliable, one woidd naturally take the 8R 
answering to a.single year’s change as measuring in direction and magnitude the force 
to which the secular change may be attributed. 
In general, however, it will be best to calculate a SR for a considerable number of 
years interval, and to regard its line of action as indicating the general direction of 
the secular change force throughoid the period. 
