104: Prof. Challis on a Theory of Magnetic Force. 
resulting from changes of their positions relative to the sun ; and 
it is therefore not surprising to find that one of them (that of 
111 years) differs little from the periodic time of Jupiter, the 
influence of this planet bemg likely to be predominant on 
account of his large size and rapid rotatory motion. The 
periodicity of the magnetic action of any planet must clearly 
depend in part on the position of the plane of its equator, the 
gyratory motion being at a maximum in this plane. 
38. If the preceding account of the periodicity of the maxima 
of solar spots be true, it will be seen to be a necessary conse- 
quence that the planetary gyrations produce disturbances of 
terrestrial magnetism, having in the long run a reguiar and 
periodic character. The researches of General Sabine, applied to 
magnetic observations taken at various positions on the earth’s 
surface, have in fact conducted him to the result that magnetic 
storms are pericdic, and led him to assign to them a period of 
ten years. Considering that the same planetary magnetic 
streams must operate on the earth as on the sun, and that the 
sun’s position is central with respect to the earth’s orbit, it 
might have been anticipated, on theoretical grounds, that a like 
periodicity of effects would be detected, and that the periods in 
the two cases would not greatly differ. Possibly the period of 
the magnetic storms may eventually be found to be in some 
degree variable, and the mean period inferred from observations 
extended over a longer interval, to be somewhat different from 
ten years. 
39. The theory, by giving to extraordinary magnetical disturb- 
ances a planetary origin, accounts for their occurrmg simul- 
taneously at places on the earth’s surface widely distant. But 
the amounts of disturbance at the same instant may differ 
ereatly at different places, owing to difference of latitude, and 
difference of position relative to the magnetic poles. Consider- 
ing, however, the changes of the configurations of the planets, 
the amounts of disturbance, regarded as functions of the time 
of day, may be expected in the long run to follow the same law 
at different places. This is found to be the case, although, as 
might have been anticipated, the Iceal hours of the maximum 
values are different. | 
40. The fact of there being local hours of maximum disturbance, 
in the case of the planetary as in that of the solar disturbances, 
seems to be referable to the circumstance that the disturbances 
of the superior planets will be greatest when they are in opposi- 
tion, or about midnight, they being then least distant from the 
earth, and the disturbances of the inferior planets will for the same 
reason be greatest when they are in inferior conjunction, or about 
mid-day. If this explanation ke true, we should expect to find 
