AND ANNUAL VARIATION OF THE TERRESTRIAL MAGNETIC FORCE. 219 
of geographical longitude apart, and that it might thenceforward be retrograde. 
The observatory at Toronto seems well situated for deciding this question. If the 
progression of the secular change of both systems continues to be the same after 
that epoch is past as it was before, the force at Toronto might be expected to 
sustain on the whole an annual decrease, as it would be more diminished by the 
recession of the neighbouring greater maximum, than increased by the approach of 
the far more distant minor maximum. If, on the other hand, the movement of the 
principal maximum should be retrograde, the force at Toronto might be expected to 
undergo a considerable annual increase. By the observations which have been dis- 
cussed in this paper, it has been shown that the probabilities are considerably in 
favour of the existence of a small annual decrease of force at the present time : and 
as time alone is wanting to convert this probability into certainty, it would seem 
particularly desirable that the series of monthly observations of the horizontal force 
and of the inclination at Toronto should be continued, until the direction and ap- 
proximate value of the secular change of the total force shall be thoroughly de- 
termined. 
2 p 2 
