ATMOSPHERIC MAGNETISM — GENERAL DISTRIBUTION OF MAGNETIC FORCE. 53 
general or average arrangement of the air temperature. For instance, the diversity 
of sea and land causes variations of temperature differently in different times of the 
year, and the extent to which this goes may be learned from the beautiful isothermal 
charts of Dove, now fortunately to be had in this country*. These variations may be 
expected to give, not merely differences in the regularity, direction and degree of 
magnetic variation ; but because of vicinity differences so large as to be manifold 
greater than the mean difference for a given short period, and they may also cause 
irregularities in the times of their occurrence. 
2881. On considering the probable results of the magnetic action of the atmo- 
sphere, it appears to me that if the terrestrial magnetic force could be freed from all 
periodical and small perturbations, and its disposition ascertained for any given time, 
it might still include certain effects constituting a part of atmospheric magnetism. For 
instance, there is more air, by weight, over a given portion of the surface of the earth 
at latitudes from 24° to 34°, than there is either at higher latitudes or at the equator ; 
and that should cause a difference from the disposition of the lines of force which 
would exist if there were equality in that respect, or if the atmosphere were away. 
Again, the temperature of the air is greater at the equatorial parts than in latitudes 
north or south of it ; and as elevation of temperature diminishes the conducting power 
for magnetism, so the proportion of force passing through these parts ought to be 
less, and that passing through the colder parts greater, than if the temperature of the 
air were at the same mean degree over the whole surface of the globe, or than if the air 
were away. Again, there is a greater difference in range of temperature of the air at 
the equator as we rise upwards than in other parts, and hence the lower part is not 
so good a conductor proportionately to the upper part, or to space, as elsewhere, 
where the difference is not so great; the magnetic power, therefore, should be in 
some degree weakened there, the lines of force being diverted, more or less, from the 
v/arm air and thrown into other parts, as the cooler atmosphere and space above, or 
the earth beneath, according to the principles before explained (2808. 2821. 2877-)- 
2882. The result of annual variation that may be expected from the magnetic con- 
stitution and condition of the atmosphere seems to me to be of the following kind. 
Assuming that the axis of rotation of the earth was perpendicular to the plane of its 
orbit round the sun, and dismissing for the present other causes of magnetic varia- 
tion than those due to the atmosphere, the two hemispheres of the earth, and the 
portions of air covering them, would be affected and warmed alike by the sun, or at 
least would come into a constant relative state, dependent upon the arrangement of 
land and water ; and the lines of magnetic force having taken up their position under 
the influence of the great dominant causes, whatever they may be, would not be 
altered by any annual change due to the atmosphere, since the daily mean of the 
* Report of the British Association, 1848, Reports, p. 85. 
