communicated this design to his Portland correspondent, the conception was immediately acted on, with fortunate success, and despatches were transmitted for two hours in that manner more effectually than could then have been done with the customary batteries.
A like extraordinary application was made of the auroral current, on the same day, on the Fall River and South Braintree Line.
To a correct apprehension of this strange occurrence it is necessary to remember that the direction of the poles of the several batteries on a line is immaterial, provided it be uniform, otherwise the currents would neutralise each other. When the aurora supervenes on a line, following in successive and differently polarised waves, the ordinary voltaic current is alternately neutralised and intensified beyond control. In the above cases—the batteries having been detached—the abnormal positive current would not increase, or the negative one decrease, the availability of the wires. The waves were observed to endure about fifteen seconds, intensifying with the time, to be succeeded by one of the reverse polarity. The singular phenomena indicating disturbances of the equilibrium of the earth’s magnetic forces have been collectively classed by Humboldt as magnetic storms. They are marked, as we have seen, by cirrous disposition of the clouds, perturbations of the needle, obsession of telegraph-wires, and the aurora. The evolution of light in the latter invariably terminates the movement, as in a thunderstorm lightning re-establishes the equilibrium of the atmospheric electric forces.
After these illustrations of the phenomena attendant on the aurora, some attention may be directed to an inquiry into its causes.
Whatever may be its origin, that the auroral action takes place within the limits of the atmosphere, scarcely higher than the region of cirri, and that it participates” in the movement of the earth, appears from the fact that the diurnal rotation, at the rate of a thousand miles an hour, effects no perceptible change in its aspect. Its absolute height has been variously estimated: by Euler at thousands of miles, by others as within the cloud region. It has been erroneously conceived that the height might be determined by observation of the corona, which is only an effect of perspective, owing to the convergence of parallel rays; each individual seeing his own aurora, as his own rainbow, from his particular point of view. As the centre of the arc is always in the magnetic meridian, simultaneous observation from two stations on the same meridian, with an interval sufficient to constitute a reliable base, might however effect the desired object.
The accepted theory with scientific men is, that the aurora is an electrical phenomenon occurring in the atmosphere, consisting in the production of a luminous ring with divergent rays, having for its centre the magnetic pole, and its production is supposed to be thus accounted for. The atmosphere and the earth are in opposed electric conditions, the neutralisation of which is effected through the moisture wherewith the lower air is charged. In the Polar regions, whereto the great tropical currents are constantly bearing aqueous vapour, which the cold condenses in the form of haze, this catalysis would most frequently occur. When the positively electric vapour is brought into contact with the negatively electric earth, equilibrium would be effected by a discharge, accompanied in certain states of the atmosphere by the auroral light. This is assumed to be contingent on the presence in the atmosphere of minute icy particles, constituting a haze, which becomes luminous by the electric discharge. Aeronauts have found the atmosphere at great heights, while serene and cloudless, to be pervaded by this transparent haze of which cirri are conceived to consist.
In confirmation of this hypothesis, it has been experimentally shown that when the union of the two electricities is effected in rarified air near the pole of a magnet, a luminous ring is produced which has a rotary motion according to the direction of the discharge. Thus then, when electrical discharges occur in the polar regions between the positive electricity of the atmosphere and the negative electricity of the earth, the magnetic poles of the earth would exercise a similar influence on the icy haze which is conceived essential to the evolution of the auroral light. Thus the arc seen by the observer would be that portion of the luminous ring above his horizon, varying with the distance from the pole. Only when it reaches his zenith could he be in immediate contact with the auroral haze, and then only would the asserted crepitation become audible, which is assumed to be identical in nature with that produced by an electrical machine. The sulphurous odour would be due to the generation of ozone from the oxygen of the air.
Now, though this theory would intelligibly explain the mode of phenomenal manifestation, it may reasonably be objected that, in hypothetising a continuous electric action in the atmosphere, it does not sufficiently account for the ascertained periodicity of auroras by assuming that their visibility and the variation in their intensity are consequent on the condition of the atmosphere. It is discreetly silent as to the mode of induction of this special atmospheric condition; and therefore—assuming their invariable coincidence and connection—as to the efficient cause of auroras.
We humbly conceive that the cause must be sought beyond the atmosphere in the fluctuations of that great solar force, to which is primarily attributable the induction of telluric magnetism, and which must enter as a prime motive in all atmospheric phenomena.
The irregularities of solar action have an intelligible exponent in the phenomenal changes observable on the disk of the sun. Its spots are subject to remarkable variations in form and size, contracting or dilating in unison with the variable vivacity of its constitutional force, and the period of these variations—secular, annual, and diurnal—have been approximately determined.
The direct relation between these oscillations of the solar atmosphere and the intensity or direction of the magnetic forces, as indicated by the needle, long inferred, are now satisfactorily established. From late observations made at Christiania, in Norway, by Hansteen, it has been