54 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVI.) 
atmospheric effect in a given place would at all parts of the year be alike. Under 
such circumstances the intensity and direction of the magnetic forces might be con- 
sidered constant, presuming no sensible change to take place by the difference in 
distance from the sun which would occur in different parts of the orbit ; and, as 
regards the two magnetic hemispheres, each would be the equivalent of and equal to 
the other, and they may for the time be considered in their mean or normal state. 
2883. But as the axis of the earth’s rotation is inclined 23° 28' to the plane of the 
ecliptic, the two hemispheres will become alternately warmer and colder than each 
other, and then a variation in the magnetic condition may arise. The air of the 
cooled hemisphere will conduct magnetic influence more freely than if in the mean 
state, and the lines of force passing through it will increase in amount, whilst in the 
other hemisphere the warmed air will conduct with less readiness than before, and 
the intensity will diminish. In addition to this effect of temperature, there ought to 
be another due to the increase of the ponderable portion of the air in the cooled 
hemisphere, consequent upon its contraction and the coincident expansion of the air 
in the warmer half, both of which circumstances tend to increase the variation in 
power of the two hemispheres from the normal state. Then as the earth rolls on in 
its annual journey, that which at one time was the cooler becomes the warmer hemi- 
sphere, and consequently in its turn sinks as far below the average magnetic inten- 
sity as it before had stood above it, whilst the other hemisphere changes its magnetic 
condition from less to more intense. 
2884. As the sum of the magnetic forces which crop out from the earth wherever 
there is dip on one side of the magnetic equator must correspond to the sum of like 
force on the other side (2809.), so they would not become more intense in one hemi- 
sphere, or more feeble in the other, without a corresponding contraction on the one 
hand, and enlargement on the other. The line of no dip round the globe may there- 
fore be expected to move alternately north and south every year, or some effect 
equivalent to that take place. The condition of the two hemispheres under this view 
may be conceived by supposing an annual undulation of the force to and fro between 
them, during which, though neither the character nor the general disposition of the 
power be altered, there is in our winter a concentration and increase of intensity in 
the northern parts coincident with a diffused and diminished intensity in the soutii, 
and in summer the reverse. 
2885. In respect of direction, alterations may also be anticipated. In the first place, 
and assuming that the magnetic poles and the poles of the earth coincide, the dip 
would increase in the cooling hemisphere towards the middle and polar parts ; but it 
ought to diminish towards the magnetic equator, to accord witli the concentration 
of the hemisphere of stronger power and enlargement of the weaker one ; whilst on 
the other hand the dip ought to diminish at the polar and middle parts of the warm^ 
ing hemisphere and increase towards the magnetic equator. The magnetic equator 
would shift a little north and south of its mean place during each year, simultaneously 
