variations of the Magnetic Forces. 335 
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action will be to diminish the horizontal force; and therefore 
when the number of particles of humid air increases immediately 
above the needle and the barometer falls, the horizontal force 
will diminish. Whether the atmosphere has really any effect, 
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temperature and hiimidity—seems to favor the idea of atmose 
_ pheric magnetic action : but, if it be conceded that, as is generally 
Supposed, electrical currents flow along the auroral columns, we 
magiietic action of the aurora, and have therefore no good ground 
; for Supposing that the atmosphere has a magnetic action in its 
_ hormal state. We may, however, still conjecturally connect the 
_ aurora with the diurnal variations of the horizontal magnetic m- 
_ tensity by imagining that'the daily changes of temperature and 
Ich traverse the atmosphere without producing any percepti- 
tee luminosity, “or perhaps any other perceptible eflects, except 
_ Upon the magnetic needle. = Page 
~The philosophical’ course for the present, however, is to abide 
by the theory which furnishes.the most direct representation po 
sible of the connection subsisting between the barometric and 
_ Magnetic-variations, and at.the same time accords with a general 
_ theory’thaty'satisfies‘the conditions. imposed by the normal state 
figm. Pee A cee 
of the e “th mack i : 
Annual Variations ‘of the Horizontal. Magnetic Intensity. 
a degree * wrobable) that the diurnal variations of the horizontal 
_.  Magitetic intensity ‘are due to changes in the temperature and 
humidity of-the earth’s surface (or atmosphere ), we naturally seek 
| for the explanation of the annual variations of the horizontal 
force in ‘annual changes of, temperature and humidity. 
Moreover seen that the ditirnal variations of the horizontal force 
and barometer are linked together, and due to. the same two gen- 
eral causes. It is therefore to be inferred that a similar connec- 
on must subsist between the annual variations of the horizontal 
oree and barometer, and that the two phenomena must be other 
Parallel effects of changes of temperature and humidity. Now 
if We compare the curve showing the variations of the horizontal 
force om month to month with the corresponding curve for the 
meter, we in fact find an approximate correspondence be- 
dependent upon. the same two meteorological elements, VAZ5- 
ave in these currents apparently a sufficient explanation of the . 
midity of the atmosphere are attended ‘with electrical. currents 
os uhaiag ben established (or at least rendered in the highest » 
wifot 
