Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/108

Rh &quot;3. In the perturbations of declination which I have observed for 28 vears, I have been unable to recognise any general law.

&quot;4.&quot; The perturbations of horizontal intensity commence in general by an increase of that force, and finish always by a diminution, which lasts for two or three days.

&quot;5. In all perturbations there is a constant relation between changes of inclination and the simultaneous changes of horizontal intensity, such that an augmentation of intensity of $$\tfrac{1}{10000}$$ corresponds to a diminution of inclination of 8°28 (for Munich).

&quot;6. In telegraphic wires we cannot observe the existence of a constant terrestrial current, since the conductivity of the soil is infinitely greater than that of the telegraphic wire, and it is only sudden changes that manifest themselves. In consequence, during magnetic perturbations in the galvanometer of a telegraphic wire, we only see irregular deflections to right or left, succeeding each other at intervals of a few minutes.

&quot;In 1850 and 1851 we made electrical observations from hour to hour, from 7 to 6, without being able to see any connection between the atmospheric electricity and the magnetic perturbations. Later I abandoned these observations, because the indications of the electrometers depended too much on local and accidental circumstances.&quot;

It should be noted here that the horizontal component of magnetic force varies with the inclination as well as with the intensity of the total force, and the ratio noted above is almost exactly that which would be produced by a change in the inclination alone; and it would appear as if the actual horizontal force, independent of the inclination, was subject to comparatively little variation. This is not improbable, since variations in the horizontal force could correspond only to electro-magnetic easterly or westerly currents, while changes in declination, inclination, and vertical force might correspond to currents from the magnetic north and south, which there is reason to believe are most frequent in auroral displays.

To give some idea of the extent of magnetic perturbations, we may mention that during the aurora of 13th May 1869, the declination at Greenwich varied 1°25′, while the vertical force experienced four successive maxima, and the greatest oscillation amounted to 0.04 of its total mean value. The horizontal force at the same time only varied 0.014 of its mean value. During the aurora of the 15th April of the same year the declination at Stonyhurst varied 2°23′14″&quot; in nine minutes.

The electric currents produced at such times in telegraph wires, though transient, are often very powerful. Loomis (Sill. Jour., vol. xxxii.) mentions cases where wires had been ignited, brilliant flashes produced, and combustible materials kindled by their discharge. It often happens that the ordinary signals are completely interrupted during their continuance.

In addition to the resemblance between the auroral character phenomena and those of electric discharges in rarefied of aurora, gases which we have already mentioned, we have seen that auroral displays are accompanied by marked disturbances both in the direction and force of terrestrial magnetism. This fact is in itself almost proof of their electrical character, and, taken in conjunction with the strong &quot;earth-currents&quot; which are at such times produced in lines of telegraph, and with the manifest polarisation of the arches and rays with regard to the magnetic meridian, may be considered as conclusive that the aurora is some sort of electric discharge. There are still some points with regard to the origin of this electricity which are unexplained, and it is uncertain whether the magnetic disturbance causes the electrical phenomena, or vice versa. It has been shown by Prof. Plücker that when an electric discharge takes place through rarefied gas in the field of a magnet, it is concentrated in the magnetic curves, which are the only paths in which it can move without being disturbed by the magnet. This is well shown in De la Rive's well-known experiment, in which an electro-magnet is enclosed in an electric egg. As soon as the magnet is set in action, the discharge, which had before filled the egg, is concentrated into a defined band of light, which rotates steadily round the magnet,&mdash;the direction of its rotation being changed by reversal either of the current or of the polarity of the magnet. If we suppose that the aurora is an electric discharge passing from one magnetic pole to the other, and following the terrestrial magnetic curves, we shall find that the theory agrees with observed facts even in its lesser details. In these latitudes the magnetic curves are sensibly straight and parallel, and are inclined S.E. at an angle of about 70° from the perpendicular, and, by the well-known laws of perspective, will appear to converge towards this point, as, in fact, the auroral streamers do. The streamers should move from east to west, or frcm west to east, according as the discharge is from north to south, or vice versa, and, in fact, they are in constant motion. Professor Loomis (Sill. Jour, of Sc., xxxiv. 45) gives a catalogue of forty-six cases of such movement, of which thirty-one were from E. to W. and only fifteen in the opposite direction; and as part of these apparent motions are due to a real motion from N. to S., he concludes that the actual motion of the streamers is from about N.N.E. to S.S.W. This would make the north pole the negative electrode, which is most likely usually the case. Prof. Loomis has shown that during auroral displays electrical currents traverse the earth s surface in the same general direction, though subject to great variation in intensity and even to reversal. Waves of magnetic disturbance are also propagated in the same direction (ibid., xxxii. 318).

With regard to the arches it is evident that they are generally circles concentric to the magnetic pole, and it is very probable that they are analogous to the striæ often seen in discharges in rarefied gases. Gassiot, quoted by B. V. Marsh (Sill. Jour., xxxi. 316, and Roy. Soc. Proc., vol. x. Nos. 38 and 39), describes an experiment with his great Grove's battery of 400 cells, in which the exhausted receiver was placed between the poles of the large electro-magnet of the Royal Institution: &quot; On now exciting the magnet with a battery of ten cells, effulgent strata were drawn out from the positive pole, and passed along the under or upper surface of the receiver according to the direction of the current. On making the circuit of the magnet and breaking it immediately, the luminous strata rushed from the positive, and then retreated, cloud following cloud with a deliberate motion, and appearing as if swal lowed up by the positive electrode&quot; This, as Mr Marsh remarks, bears a very considerable resemblance to the conduct of the auroral arches, which almost invariably drift slowly southward; and we cannot do better than sum up his theory in his own words:&mdash;&quot;The foregoing considerations seem to render it probable that the aurora is essentially an electric discharge between the magnetic poles of the earth leaving the immediate vicinity of the north magnetic pole in the form of clouds of electrified matter, which float southward through the atmosphere at a height of 40 miles or more from the earth, sometimes to a distance of more than 30 from the pole; that whilst they are thus moving forward, with a comparatively slow and steady motion, or sometimes even remaining almost stationary for a long time, bright streams of electricity are from time to time suddenly shot out from them in a nearly vertical direction, that is to say, in the magnetic curves corresponding to the points from which they originate; that these curves, ascending to a great height beyond the atmosphere, then bending more and more southward and downward until they finally reach corresponding points in the southern magnetic hemisphere, are the pathways by which the electric currents pass to their destination; and that for several hundred miles from the earth these curves are thus traced through space and illuminated with bright electric light; and further, that the magnetism of the earth also causes these luminous currents and the electrified matter 