Part 10 (1/2)
From the investigations of Lamont, it would appear, that the period of the variations of magnetic declination is about 10? years, while, more recently, R. Wolfe has suggested the connection between this variation and the solar spots, and a.s.signs a period of 11.11 years, and remarks, that it ”corresponds more exactly with the variations in magnetic declination than the period of 10? years established by Lamont. The magnetic variations accompany the solar spots, not only in their regular changes, but even in their minor irregularities: this latter fact is itself sufficient to prove definitely the important relations between them.”[28]
As the planet Jupiter exerts the greatest influence on the sun, in forcing the centre from the mechanical centre of the system, the longitude of the sun will in a great measure depend on the position of this planet; and, in consequence, the sun will generally revolve around this centre in a period nearly equal to the period of Jupiter. The sidereal period of Jupiter is about twelve years, but the action of the other planets tend to shorten this period (at least, that has been the effect for the last twenty or thirty years), and bring it nearly to the period a.s.signed by M. Wolfe to the variations in the magnetic declinations. As this has its influence on the radial stream, and the radial stream on the declination, we see at once the connection between them. When we come to a consideration of the solar spots, we shall exhibit this influence more fully.
AURORA BOREALIS.
Let us now examine another phenomenon. The Aurora Borealis has been generally considered to be in some way connected with the magnetism of the earth, and with the position of the magnetic pole. It is certain that the appearance of this meteor does affect the needle in a way not to be mistaken, and (although not invariably) the vertex of the luminous arch will usually conform to the magnetic meridian. Yet (and this is worthy of attention), the observations made in the North Polar Expeditions[29] ”appear to prove, that in the immediate vicinity of the magnetic pole the development of light is not in the least degree more intense or frequent than at some distance from it.” In fact, as the American magnetic pole is, as stated, in lat.i.tude 73, the central vortex will seldom reach so high, and, consequently, the aurora ought at such times to be more frequent in a lower lat.i.tude. In a late work by M. de la Rive, this gentleman expresses the opinion, that the cause of the aurora is not due to a radiation of polar magnetism, but to a purely electrical action.[30] His explanation, however, is not so satisfactory as his opinion. Now, we have examined numerous cases of auroral displays, and never yet found one which could not be legitimately referred to the action of ethereal vortices. Generally, the aurora will not be visible, when the upper surface of the atmosphere of that lat.i.tude in which the vortex is known to be (reckoning in the direction of the magnetic meridian) is below the horizon, which shows that the brightest portion is in the atmosphere. In lat.i.tude 41 even, it may show itself when the vortex is three days north, more frequently when one or two days north; but when the vortex pa.s.ses centrally, or south, it rarely is seen, and this is the only difficulty in explaining it by the theory. But, when we reflect that the ether shoots out in straight lines, and at an angle corresponding to the magnetic dip, we are at no loss to perceive the reason of this. If each minute line composing the light were seen endwise, it would be invisible; if there were millions such in the same position, they could add nothing to the general effect; but, when viewed sideways, the case would be different, there would be a continued reduplication of ray upon ray, until in the range of some hundreds of miles an effect might be produced amounting to any degree of intensity on record. Now, this is the case when the aurora is immediately overhead, it will be invisible to those below, but may be seen by persons a hundred miles south; so, also, when it is to the south, it is too oblique to the line of vision to be seen, especially as all the rays to the northward of the observer can contribute nothing to increase the effect. That it is of the nature of rays very much diffused, can hardly be doubted; and, therefore, if only of a few miles in depth, its impressions are too faint to be sensible. By referring to the record of the weather in the second section of this work, an auroral display will be found on July 12th, the central vortex having pa.s.sed a little to the northward the same evening, and the next day pa.s.sing south _descending_. On that occasion the author saw an inclined column, in profile, due east, and between himself and a line of bluffs and timber, about eight miles distant; And, he has not any doubt that the ma.s.s of rays began where he stood. As in a shower, every drop, pa.s.sing through a conical surface, whose axis pa.s.ses through the sun and through the eye, contributes to form the apparently distant rainbow.
The alt.i.tude of this meteor has been much exaggerated, especially of those rings or luminous arches, which are often detached completely from the luminous bank. On the 24th of May, a bright aurora was visible at Ottawa, but the author's attention was engrossed by the most brilliant arch of light he had ever seen. It was all the time south of the zenith, and had no visible connection with the aurora north. At 9 hours, 59 minutes, 30 seconds mean solar time, Arcturus was in the exact centre of the band, at which time it was very bright, and full 7 wide. At the same time, Prof. G.W. Wheeler observed the aurora in Perryville, in the State of Missouri, only 1 of longitude to the westward, but did not see the arch.[31] The difference of lat.i.tude between the two places being 3 30', and the weather, as he states, clear and still, there is only one reason why he did not see the arch: it must have been too _low_, and had become merged in the bank of light. At the time mentioned, the alt.i.tude of Arcturus was 68 30', and, as Prof. Wheeler a.s.signs only 10 as the alt.i.tude of the bank, the maximum elevation of the arch, on the supposition of its composing a part of the bank, was 43 miles. At Perryville, the bank and streamers had disappeared at 10 o'clock. At Ottawa, the arch or bow disappeared at 10 h. 5 m., differing only the fraction of a minute from the time at Perryville; but, the bank was still visible, but low and faint, the greatest alt.i.tude having been over 30. To show the rapid fluctuations in width and position of this bow, we will add a few of the minutes taken at the time with great care, in hopes some other observer had been equally precise. When first seen, there were three luminous patches, or elongated clouds of light; one in Leo, one in Bootes, and another in Ophinchus, all in line. This was about 9h. 15m. The times following are correct to 30 seconds:
9h. 42m. 30s. Bow complete; south edge 2 north of Arcturus.
9 45 30 Northern edge diffuse south; edge bright, and well defined; 10 wide in zenith; north edge on Alphacca.
9 47 30 South edge 5 north of Arcturus; north edge close to Cor. Caroli.
9 53 30 Eastern half composed of four detached bands _s.h.i.+ngling_ over each other.
58 30 Arcturus on south; bow narrower.
9 59 30 Arcturus in the middle of the band; very bright and regular in outline, and widest at the zenith.
10 0 30 Arcturus on northern edge; north side better defined than the southern.
10 2 0 Arcturus 1 north; very bright.
10 2 30 Gamma and Delta Leonis, northern edge.
10 3 Regulus on southern age; getting faint.
10 5 Fast fading away.
10 5 30 Scarcely visible; bank in north faint.
This aurora was due to the _inner vortex ascending_, whose period was at this time 28 days.
There are several circ.u.mstances to be observed in this case. The bow brightened and faded simultaneously with the aurora, and respected the vertex of the auroral bank, being apparently concentric with it. The bow, therefore, depends on the same cause, but differs from the aurora in being limited to the _surface_ of the atmosphere in which the vortex has produced a wave to the southward of its central path, as may be understood by inspecting Fig. 2, Sec. 1,--the figure representing the polar current of the central vortex. On the 29th of May, 1840,[32] the author saw a similar phenomenon, at the same time of night, and pa.s.sing over the same stars southward until it reached within 5 of Jupiter and Saturn, to which it was parallel. This atmospheric wave offers a greater resistance to the pa.s.sage of the ether: hence the light. On this account it is, also, that when the pa.s.sage of a vortex is attended by an auroral display there will be no thunder-storm. There may be an increase of wind; but the atmosphere at such times is too dry to make a violent storm, and there is a silent restoration of the equilibrium, by the ether pa.s.sing through the dry atmosphere, without meeting any condensable vapor, and becoming luminous on account of the greater resistance of the air when unmixed with vapor. We thus see also the connection between the aurora and the linear cirri, and we have a triumphant explanation of the fact, that when the observer is north of the northern limit of the vortices, he sees the aurora to the south and not to the north; for, to see it to the northward, he would have to see it in the same lat.i.tude as it appears in the south, and, consequently, have to see across twice the complement of the lat.i.tude. We thus see, also, why the temperature falls after an aurora; for, the pa.s.sage of electricity in any shape, must have this effect on account of the great specific caloric of this fluid. We see, also, why the aurora should be more frequent where the magnetic intensity is greatest and be consequently invisible at the equator, and why the magnetic needle is so sensibly affected at the time of its occurrence. We may, perhaps, here be allowed to allude to another phenomenon connected with terrestrial magnetism and electricity.
EARTHQUAKES.
The awful and destructive concussions which sometimes are produced at great depths beneath the surface of the soil, would seem to indicate that no force but that of electricity is adequate to account for the almost instantaneous desolation of wide tracts of the earth's surface.
But we do not mean to say that the action of the terral vortices, combined with the internal conditions of our planet, is the only cause; although it is far from improbable that the same activity of the ether, which generates through these vortices, the full fury of the hurricane in the tropics, may be simultaneously accompanied by a _subterranean_ storm. And physicists are too rash to reject the evidence on which the connection of the phenomena rests.
In the extract given by Colonel Reid, in his ”Law of Storms,” from Sir George Rodney's official report of the great hurricane of 1780, it is stated, that, ”Nothing but an earthquake could have occasioned the _foundations_ of the strongest buildings to be rent; and I am convinced that the violence of the wind must have prevented the inhabitants from feeling the earthquake which certainly attended the storm.”[33] Again, in the Savannah-la-Mar hurricane, which occurred the same year and month, the Annual Register, published at Jamaica, states, that at the same time, ”a smart shock of an earthquake was felt.” The general serenity of equatorial regions is due to the fact that they are beyond the limit of the vortices, as in Peru, where neither rain nor lightning nor storm is ever seen. Thunder and rain, without storms, however, are common in other tropical countries, also out of the reach of the vortices. But even in those parts, (as the Antilles,) lying in the track of these vortices, the weather is not as _frequently_ disturbed as in higher lat.i.tudes. The storms of the Antilles, when they do occur, however, are fearful beyond any conception, showing the presence of some cause, auxiliary to the ordinary disturbing action of the vortices, which, when simultaneously occurring, adds tremendously to their force.
That earthquakes are preceded _sometimes_ by a peculiar haziness and oppressiveness, similar to that which sometimes precedes a storm, is a current opinion in volcanic countries. And Humboldt, who doubts the connection, has to confess that sudden changes of weather have _succeeded_ violent earthquakes, and that ”during the great earthquake of c.u.mana, he found the inclination of the needle was diminished 48'.”
He also mentions the simultaneous occurrence of shocks, from earthquakes, and a clap of thunder, and the agitation of the electrometer during the earthquake, which lasted from the 2d of April to the 17th of May, 1808; but concluding that ”these indications presented by clouds, by modifications of atmospheric electricity, or by calms, cannot be regarded as _generally_ or _necessarily_ connected with earthquakes, since in Peru, Canada, and Italy, earthquakes are observed, along with the purest and clearest skies, and with the freshest land and sea breezes. But if no meteorological phenomena indicates the coming earthquake, either on the morning of the shock or a few days previously, the influence of certain periods of the year, (the vernal and autumnal equinoxes,) the commencement of the rainy season in the tropics, after long drought, cannot be overlooked, even though the genetic connection of meteorological processes, with those going on in the interior of our globe, is still enveloped in obscurity.”[34]