t of the
aurora, have been satisfactorily ascertained. The aurora of April 7th,
1847, was a favorable one for observation. It was carefully and
attentively watched by Professor Olmsted, Mr. Herrick, Dr. Ellsworth, and
others, and they are intelligent and skillful observers.[9] But the nature
of the aurora forbids reliance on parallax, or measurements founded on the
time when, any portion of the bow or arch rises in range of a particular
star. The bow or arch moves southwardly, but the same rays or currents do
not. The wave of magnetic _activity_ moves south, and each successive
current, as it is reached by the _impulse_, becomes luminous. Hence the
observers, when distant, do not see, at the same time, or at different
times, the same rays. The phenomenon is unquestionably magneto-electric.
Electricity becomes luminous in a vacuum, and De la Rive, by combining the
electric currents with those of magnetism, produced all the peculiarities
of the aurora. The magnetic currents, passing from the earth, have
associated electric ones in connection, and these, in the upper attenuated
atmosphere, become luminous. Whether, as De La Rive supposes, by combining
with the positive electricity existing there, or because the associated
electric currents are _then_ in excess, not being intercepted by
atmospheric vapor and returned to the earth in rain, we can not know, nor
is it very important we should.

Having thus taken a general view of the nature of magnetism and its
associated electricities, and their connection with the general and
obvious peculiarities of climate, let us approach more nearly the varied
atmospheric phenomena, resulting from variations of pressure, temperature,
condensation, and wind, and give them a closer consideration. They all
have regularity and periodicity--they all occur in degree, and in
connection with magnetism and electricity, during the twenty-four hours of
every serene and normal summer's day. Grouped together, in comparison with
the changes in the activity and force of the magnetic elements, their
connection is clearly discernible.

The day may be said, with truth, to commence, in some portion of the
summer, at 4 A.M. The atmospheric does at all seasons. At that hour the
barometer is at its morning minimum. It has, as we have said, a
perceptible diurnal variation of two maxima and two minima. Its periods of
depression are at 4 A.M., and 4 P.M., and of elevation at 10 A.M., and 10
P.M. The difference