ON BRITISH COLONIAL MAGNETIC OBSERVATORIES. 361 
external inducing cause acting on the magnetism of the earth would be dis¬ 
similar from that which we observe to take place in the iron of a ship. 
When the magnetic heterogeneity of the materials at the surface of the 
earth is considered, the supposition can scarcely be thought an unreasonable 
one, that the change which may be operated in the earth's magnetism by 
the sun’s passage of the equator, supposing the sun to have a direct magnetic 
influence on the earth, may commence with the epoch of the passage, but 
may not be completed until a few days subsequent to it. 
The next instance which I shall bring before you, in which a direct action 
of the sun appears to furnish a better explanation of the phenomena than an 
action operating through the agency of variations of temperature at the par¬ 
ticular stations, is connected with the earth's greater proximity to the sun in 
one part of her aunual orbit than in the opposite part. The orbit being an 
ellipse, the perihelion, or nearest approach of the earth to the sun, is in De¬ 
cember, and the aphelion, or greatest distance from the sun, is in June, the 
amount of difference being about one-thirtieth of the mean distance. Now 
i! there be any portion of the earth's magnetic force which is derived from 
the sun, it would be reasonable to expect that the intensity of that portion of 
die magnetic force should be greater in December than in June, and that 
this effect should be general over the globe, and exhibited of course in both 
hemispheres. It might well have happened, indeed, that the whole effect of 
the sun’s magnetic influence might have been so small, that the difference in 
the effect due to a fluctuation of not more than one-thirtieth of the mean 
distance might have been insensible, or at least too small to be cognizable 
hv our instrumental means. It. docs happen, however, that we do find by the 
observations that there is a small annual variation in the intensity of the 
Magnetic force of the earth, and that it is shown concurrently by the observa¬ 
tions in both hemispheres, viz. at Toronto and Hobarton, and that this varia¬ 
tion coincides with the supposition which has been spoken of, being an excess 
of the force in both hemispheres in the month of December as compared 
"'id) June, and nearly to the same amount iu both hemispheres, viz. about 
two-thousandths of the whole magnetic force of the earth. This amount, 
though small, is not beyond the competency of the instruments or of the 
methods employed to determine it. It is the result at each station of several 
years of observation, but the excess in December is also shown when the 
whole period from which the mean is derived is broken into smaller portions, 
dud this result been obtained in one of the hemispheres only, say, for ex¬ 
ample, in the northern hemisphere, we might have supposed the increase in 
, magnetic force in December to be in some way connected with the 
"niters cold in that hemisphere; or if obtained in the southern hemisphere 
jl e ' a connexion might have been imagined with the summer’s beat; but 
be same effect being found to take pluce at the same period of the year 
•dike in both hemispheres, notwithstanding their opposite climatic circum¬ 
stances, seems to remove it from the category of effects which can be 
ascribed to local variations of temperature, and to assign it a place amongst 
Other concurrent indications of the direct magnetic influence of the sun. 
fiat the most remarkable evidence, and the least intelligible upon any other 
"apposition than that of a direct magnetic connexion between the sun and 
it- earth, is furnished by the knowledge which we have gained through the 
ritish Colonial Observatories*, of the existence of a periodical fluctuation in 
1C disturbances (or magnetic storms, as they are sometimes called), 
* Phil. Trans. 1852, art. viii. pp. 113-124. 
