and coalesce with each other over the equatorial, as would be the
case in a globe having one or two short magnets adjusted in relation to
its axis, and it is probable that the lines of force in their circuitous
course may extend through space to tens of thousands of miles. The lines
proceed through space with a certain degree of facility, but there may
be variations in space, _e.g._, variations in its temperature which
affect its power of transmitting the magnetic influence.

Between the earth and space, however, is interposed the atmosphere, and
at the bottom of the atmosphere we live. The atmosphere consists of four
volumes of nitrogen and one of oxygen uniformly mixed and acting
magnetically as a single medium. The _nitrogen_ of the air is, as
regards the magnetic force, neither paramagnetic nor diamagnetic,
whether dense or rare, or at high or low temperatures.

The _oxygen_ of the air, on the other hand, is highly paramagnetic,
being, bulk for bulk, equivalent to a solution of protosulphate of iron,
containing of the crystallised salt seventeen times the weight of the
oxygen. It becomes less paramagnetic, volume for volume, as it is
rarefied, and apparently in the simple proportion of its rarefaction,
the temperature remaining the same. When its temperature is raised--the
expansion consequent thereon being permitted--it loses very greatly its
paramagnetic force, and there is sufficient reason to conclude that when
its temperature is lowered its paramagnetic condition is exalted. These
characters oxygen preserves even when mingled with the nitrogen in the
air.

Hence the atmosphere is a highly magnetic medium, and this medium is
changed in its magnetic relations by every change in its density and
temperature, and must affect both the intensity and direction of the
magnetic force emanating from the earth, and may account for the
variations which we find in terrestrial magnetic power.

We may expect as the sun leaves us on the west some magnetic effect
correspondent to that of the approach of a body of cold air from the
east. Again, the innumerable circumstances that break up more or less
any average arrangement of the air temperatures 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 many times greater than the mean difference for a given short period,
and they may also cause irregularities in the