It is considered that the sun is the princ.i.p.al cause of these great currents. By elevating the surface atmosphere of the equator, a lateral current is induced from the north and south; but on account of the enlarging circles of lat.i.tude, their direction tends more from the north-east and south-east. These currents are usually called the trades.
Without disputing the correctness of this, it may be doubted whether the whole effect is due to the sun. As this principle affects the ocean likewise, it is necessary to look into it; and in order to simplify the question, we will first suppose our globe covered entirely by the ocean, without any protuberant land.
Let us a.s.sign a uniform depth of ten miles to this ocean. In the Fig.
following, the two circles will represent the surface and bottom of the ocean respectively. The axis of rotation is thus represented by the line PP". Let us consider two particles of water at m and n, as feeling the influence of this rotation; they will, of course, be both urged towards the equator by the axifugal force. Now, every particle in the ocean being also urged by the same force, it might be supposed that after a protuberant ma.s.s of water had acc.u.mulated at the equator EE", the whole ocean would be in equilibrium. This would not follow. The particle at m is urged by a greater force than n; consequently the particle at n is overborne by the pressure at m. Considering both in the same direction, yet the particle at n must give way, and move in the opposite direction.
Just as the heaviest scale of the balance bears up the lightest, although both gravitate towards the same point. This is so self-evident that it would seem unnecessary to dwell upon it, had not the scientific world decided that the rotation of the earth can cause no currents either in the atmosphere or in the ocean.
[Ill.u.s.tration: Fig. 22]
The axifugal forces of the two particles m and n are directly as the lines Mm and Nn, and if the gravitating forces were also as the radii Tm and Tn, no motion would be produced. Admitting even the Newtonian law to be rigidly exact, the earth cannot be considered a h.o.m.ogeneous globe, but, on the contrary, the density of the central parts must be nearly thirty times greater than the density of the surface of the ocean. The ratio of the gravitating forces of these two particles is, therefore, less than the ratio of their respective radii, and the axifugal tendency of the particle at n is more than proportionally restrained by the central gravitation; and hence m will move towards the equator, and n towards the poles, as represented in the Fig.
It is on account of the overwhelming momentum of the surface waters of the South Pacific over the North, that the Pacific, at Panama, stands six or seven feet higher than the Atlantic. We shall again allude to this interesting fact.
According to newspaper reports of a lecture, delivered in New York, by Lieut. Maury, U. S. N., this gentleman endeavors to explain the currents of the ocean, by referring them to evaporation in the tropics. The vapor leaves the salt of the water behind, and thus, by continual acc.u.mulation, the specific gravity of the tropical waters is greater than that of the superficial waters nearer the poles; the lighter water, therefore, pa.s.ses towards the equator, and the heavier water below, towards the poles. If this be a correct statement of that gentleman"s theory, fidelity to our standards compels us to question the soundness of the conclusion. The mere fact of the surface water of the ocean being lighter than that of the bottom, cannot on any known principles of science cause any movement of the surface waters towards the equator. When such an acute and practical physicist is driven, by the palpability of the fact that the polar waters are continually tending towards the equator, to seek the cause in the tropical evaporation, it shows that the dogma, which teaches that rotation can produce no motion, is unsound.
Sir John Herschel, in speaking of the solar spots, says: "We may also observe that the tranquillity of the sun"s polar, as compared with his equatorial regions (if his spots be really atmospheric), cannot be accounted for by its rotation on its axis only, but must arise from some cause external to the sun, as we see the belts of Jupiter and Saturn and our trade winds arise from a cause external to these planets combining itself with their rotations, which _alone_ (and he lays an emphasis on the word) can produce no motions when once the form of equilibrium is attained."
With respect to the origin of the solar spots, we have no disposition to question the conclusion; but, as regards the _principle_ laid down, that rotation can produce no motions when once the form of equilibrium is attained, we must unequivocally dispute it. If our atmosphere were of uniform density, the rotation of the earth would cause no current such as we have described; with our atmosphere as it is, the result will be different. The momenta of two portions of matter are the products of their inertiae by their motions, and, in the present case, we must take the inertiae of equal s.p.a.ces. A cubic inch of air at the surface, and at three miles above the surface, is as 2 to 1; but their centrifugal velocity varies only as the radii of the respective spheres, or as 1320 to 1321. In the polar regions, therefore, the momentum of the surface air preponderates, and, in this case, the _surface_ current is towards the equator, and the upper current towards the poles. When, however, the centrifugal velocity is considerably increased in a lower lat.i.tude, and the curvature of the surface becomes more and more inclined to the direction of that resolved part of the centrifugal force, which is always _from_ the axis, the surface layers will evince a tendency to leave the surface, and an intermingling will then take place in the s.p.a.ce between lat.i.tude 70 and 50, or in lat.i.tude 60. As this layer is continually urged on in the same direction by the surface layer of lat.i.tudes above 60, the upper layer now becomes a current setting _towards_ the equator, and, consequently, the back current occupies the surface. Now, considering that the rarefying action of the sun is elevating the air under the equator, there must necessarily be an upper current from the equator to the poles; so that if we conceive the two currents to meet about lat.i.tude 30, there will be a second intermingling, and the current from the poles will again occupy the surface. Thus, we regard a part of the effect of the trades to the rotation of the earth, which is the chief impelling power at the poles, as the sun is at the equator; and the lat.i.tudes 60 and 30 will be marked by some especial phenomena of temperature, and other meteorological features which do actually obtain. These would be much more marked if the irregular configuration of land and sea, the existence of mountain chains, and the different heating power of different lat.i.tudes, owing to the unequal distribution of the land, did not interfere; and the currents of the air (disregarding the deflection east and west) might then be represented by a treble link or loop, whose nodes would vary but little from lat.i.tudes 30 and 60. As it is, it has, no doubt, its influence, although unimportant, when compared with the disturbing action of the ethereal vortices.
There is another phenomenon due to the action of the radial stream, which has given much trouble to the physicist, and which has yet never been explained. This is the horary oscillations of the atmospheric pressure which, in some countries are so regular that the time of day may be ascertained by the height of the barometer. According to Humboldt, the regularity of the ebb and flow in the torrid regions of America, is undisturbed by storms or earthquake. It is supposed that the maxima occur at 9 A.M. and 10 P.M., and the minima at 4 A.M. and 4 P.M. From the morning minimum to the morning maximum is, therefore, five hours; from the evening minimum to the evening maximum is 6 hours; from the evening maximum to the morning minimum is 5 hours, and from the morning maximum to the evening minimum is 7 hours. Again, these oscillations are greatest at the equator, and diminish with the increase of lat.i.tude.
[Ill.u.s.tration: Fig. 23]
If we suppose the earth"s axis perpendicular to the plane of the vortex, and P the pole in the above figure, and SP the line joining the centre of the earth and sun, M and m will represent the points in the earth"s equator where it is midday and midnight respectively. The solar stream penetrates the terral vortex; and strikes the earth"s atmosphere along the lines parallel to SP. The direct effect would be to pile up the atmosphere at N and n; and therefore, were the earth at rest, the maximum would be at 6 A.M. and 6 P.M., and the minimum at midday and midnight; but the earth rotating from N towards M, carries along the acc.u.mulated atmosphere, being more sluggish in its motions than the producing cause, which cause is still exercised to force it back to N.
From this cause the maximum is now found at K. For a like reason the minimum at M would be found at L, but on account of the motion of the earth being now in the same direction as the solar stream, the minimum is found still more in advance at k; so that, according to the theory, the interval between the morning maximum and the evening maximum, should be greater than the interval between the evening maximum and the morning maximum; and so it is, the first being 13 hours and the last 10 hours. The morning minimum should also be less marked than the evening minimum, and this also is a fact. The effect also should be greater in the tropics than in high lat.i.tudes, which again also obtains; being 1.32 French lines at the equator, and only 0.18 at lat.i.tude 70. Had the earth no obliquity, the effect would be as the squares of the cosines of the lat.i.tude; but the ratio is diminished by the inclination of the axis. But there are other variations of the barometer of longer period, apparently depending on the phases of the moon, but which cannot be reconciled to the attracting power of the moon as an atmospheric tide; and Arago concluded that they were due to some _special cause_, of which the nature and mode of action are unknown. Perhaps this theory will obviate the difficulty, as although the central vortex comes to the meridian at the same time as the moon, its effect will be different on the inferior meridian to what it is on the superior one; whereas the moon"s attraction should be the same on both. That the pa.s.sage of a vortex over or near a particular place should affect the barometer, is too obvious to need explanation, and therefore we may say that the theory will explain all those varieties both small and great, which have caused so much speculation for the last fifty years.
TERRESTRIAL MAGNETISM.
In applying the theory to the magnetism of the earth, we must bear in mind that the earth is probably magnetic by induction, and not in virtue of its own specific action. The rotation of the surrounding ether, and the consequent production of a radial stream, calls the ether into motion within the earth"s interior, as well as on the surface; but it does not follow that the ether shall also enter the earth at its poles and escape at its equator, for the obliquity of the vortex would interfere with this result. It is sufficient that this does occur in the terral vortex immediately surrounding the earth. From late experiments it is pretty well established that the axial direction of the needle, (and of other bodies also,) is due to peculiar internal arrangement in laminae or layers, the existence of which is favorable to the pa.s.sage of the magnetic current.
According to the experiments[27] of Dr. Tyndal, it is found that the magnetism of a body is strongest along the line of greatest density. As, therefore, the laminae of bodies may be considered planes of pressure, when these planes are suspended horizontally, the directive force is greatest, and the longest diameter of the body sets axial. On the other hand, when the body was suspended so that the laminae were vertical, the longest diameter set equatorial. Now, we know that the crust of the earth is composed of laminae, just as the piece of shale in Doctor Tyndal"s experiments, and that these layers are disposed horizontally.
And whatever force originally arranged the land and water on our globe, it is evident that the continents are longest from north to south, and therefore correspond to the natural direction of the magnetic force.
In consequence of the intrinsic difficulties of this question, and the mystery yet attaching to it, we may be permitted to enter a little more minutely into it, and jointly consider other questions of interest, that will enable us to refer the princ.i.p.al phenomena of terrestrial magnetism to our theory.
We have before adverted to the discrepancies in the earth"s compression, as determined by the pendulum, and also to the uncertainty of the moon"s ma.s.s, as deduced from the nutation of the earth"s axis. It is also suspected that the southern hemisphere is more compressed than the northern; and other phenomena also point out the inadequacy of the law of gravitation, to account for the figure of the earth.
From the invariability of the axis of rotation, we must conclude that whatever form is the true form, it is one of equilibrium. In casting our eyes over the map of the world, we perceive that the surface is very unequally divided into land and sea; and that the land is very unequally arranged, both north and south, and east and west. If we compare the northern and southern hemisphere, we find the land to the water about 3 to 1. If we take the Pacific portion, and consider the north end of New Zealand as a centre, we can describe a great circle taking in one half the globe, which shall not include one-tenth of the whole land. Yet the average height of the remaining nine-tenths, above the level of the sea, is nearly 1,000 feet. Call this nine-tenths nearly equal to one-fourth of the whole surface, and the protuberant land in the hemisphere, opposite the South Pacific, amounts to 1/30,000 part of the whole ma.s.s of the earth, or about 1/700 of the ma.s.s of the moon. Again, the mean density of the earth is about 5--water being unity,--and the mean density of the surface land is only about half this: but three-fourths of the whole surface is water. Hence, we see that the materials of the interior of the earth must be either metallic or very compressible. To a.s.sign a metallic nucleus to the earth, is repugnant to a.n.a.logy; and it is not rendered even probable by facts, as we find volcanic emissions to contain no heavier elements than the sedimentary layers. Besides, there are indications of a gradual increase of density downwards, such as would arise from the compressibility of the layers. Seeing, therefore, the equilibrium of the whole ma.s.s, and the consequent hydrostatic balance of the land in the sea,--seeing also the small compressibility of the solid portions, and the great compressibility of the fluid, the inference is legitimate that the whole is hydrostatically balanced, and that our globe is a globe of water, with an intermediate sh.e.l.l of land, specifically lighter than the fluid in which it is suspended. Where this sh.e.l.l is of great thickness, it penetrates to greater depths, and attains to greater elevations above the surface of the aqueous globe; where it is less thick, it is found below the surface, and forms the bottom of the upper ocean. Recent soundings give much greater depths to some parts of the ocean, than the most elevated land upon the globe.
Captain Denham, of H. B. M. ship Herald, lately sounded in 37 south and 37 west, and found bottom at 7,706 fathoms, or about nine English miles.
As the interior portions of our globe are totally unknown, and the compressibility of water is well established, it is just as sane to consider water the most abundant element of nature, as solid land. The great question to ask is, whether there may not be other phenomena incompatible with this supposition? It is plain that the permanency of terrestrial lat.i.tudes and longitudes would be unaffected by the conditions we have supposed. Would the precession of the equinoxes be also unaffected? Mr. Hopkins has entered into such an investigation, and concludes: "Upon the whole, then, we may venture to a.s.sert that the minimum thickness of the crust of the globe, which can be deemed consistent with the observed amount of precession, cannot be less than one-fourth or one-fifth of the radius of the earth." These investigations were made on the hypothesis of the interior fluidity being caused by the fusion of the central portions of a solid globe; but it is evident that the a.n.a.lytical result would be the same if these central parts were water, inclosed by an irregularly-spherical sh.e.l.l of land. Nor would the result be affected, if we considered certain portions of the interior of this solid sh.e.l.l to be in a state of fusion, as no doubt is the case.
May not the uncertainty of the ma.s.s of the moon, be owing to the fact that this sh.e.l.l is not so rigidly compacted but that it may yield a little to external force, and thus also account for the tides in the Pacific groups, rather obeying the centrifugal force due to the orbit velocity of the earth, than the attraction of the moon?
Since the days of Hipparchus the sidereal day has not diminished by the hundredth part of a second; and, consequently, seeing that the contraction of the ma.s.s must be limited by the time of rotation, it is inferred that the earth has not lost 1/508th of one degree of heat since that time. This conclusion, sound as it is, is scarcely credible, when we reflect on the constant radiation into a s.p.a.ce 60 below zero. Admit that the globe is a globe of water, whose average temperature is the temperature it receives from the sun, and the difficulty vanishes at once. Its diameter will be invariable, and the only effect of the cooling of the solid parts will be to immerse them deeper in the water, to change the _relative_ level of the sea without changing its volume.
This is no puerile argument when rightly considered; but there is another phenomenon which, if fairly weighed, will also conduct us to the same views.
It is now a fact uncontroverted, that the sea does actually change its level, or rather, that the elevation of continents is not only apparent but real. The whole coast of Sweden and Finland is rising at the present day at the rate of four feet in a century, while on the south a contrary effect is produced. Various hypotheses have been formed concerning this interesting fact. Yet from the indications of geology, it must have been an universal phenomenon in the early ages of the world, in order to account for the emersion of sedimentary deposits from the fluid which deposited them. May not internal fires be yet spreading, and the continents expanding instead of contracting? And may there not be an inequality in this process, so as necessarily to immerse in one direction nearly as much as to elevate in another? One fact is certain, the elements are scattering the materials of the land along its Oceanic coasts, which of itself must produce a very minute effect in disturbing the hydrostatic balance; but a more efficient agent is the earthquake and volcano.
The upheaving of tracts of land by earthquakes, as on the coast of Chili would thus be satisfactorily explained, by attributing a certain resistance due to cohesion or friction preventing a _gradual_ change of level, but producing it suddenly by the jar of the earthquakes. May we not inquire also, whether the facility with which the earth seems moved by this destructive agent, does not point to the same solution as the irregularity of the figure of the earth?
This is a subject on which it is allowable to speculate, especially if any light can be thereby thrown on the still more mysterious source of terrestrial magnetism. It is for such a purpose that we have permitted ourselves to digress from that subject. In this connection we also may acknowledge our indebtedness to the sacred volume for the first germ of this theory of the weather.
Believing in the authenticity of the Mosaic history of the deluge, the author found it difficult to refer that event to other than natural causes, called into action by the operation of other causes, and all simultaneous with the going forth of the fiat of Omnipotence. Thus reasoning, he was led to regard the deluge as a physical phenomenon inviting solution, and as a promising exponent to the climatology of the early world. He looked upon the bow of promise, as the autograph of the Creator, the signature to a solemn bond, upon which the eye of man had never before rested. But if there was no rainbow before the deluge, there was no rain; and following up this clue, he was not only enabled to solve the problem, but also led to the true cause, which produces the princ.i.p.al commotions in our atmosphere.
Science boasts of being the handmaid of religion; yet there are names of note in her ranks who have labored rather to invest this phenomenon with the mantle of fable, and to force it into collision with the records graven on the rocky pages of geognosy. But the world is ever p.r.o.ne to be captivated by the brilliancy of misapplied talents, instead of weighing merit by its zeal in reconciling the teachings of those things which are seen, with those which are revealed.
If our globe be const.i.tuted as we suppose, the land might experience repeated submersions, without involving the necessity of any great departure from established laws. And we might refer to the historical record of one of these, with all the minute particulars as positive data, imposing on us the necessity of admitting that the solid parts of the globe are hydrostatically balanced in the sea. But, modern science is not always correctly defined when called the pursuit of truth, nor human learning the means of discovering it.
If we could divest ourselves of this prejudice, we should have a ready solution of the difficulty presented by the earth having two north magnetic poles, and probably two also in the south. For, by regarding the old and new continents as two distinct ma.s.ses of land whose bases are separated by 6,000 miles of water, we recognize two great magnets, dependent, however, for their magnetism, on the rotation of the terral vortex.
This is no place to enter into a lengthy discussion of such a difficult subject as magnetism, but we may be allowed to enter a protest against the current theory of electro-magnetism, viz.: that a force is generated by a galvanic current at right angles to the producing cause, which is contrary to the fundamental principles of mechanics. We may conceive that a current is induced from or to the surrounding s.p.a.ce by the rarefaction or condensation attending the transmission of such a current along a wire, and that rotation should follow, just as a bent pipe full of small holes at the lower end, and immersed in water as a syphon, will generate a vorticose motion in the water; but mere juxtaposition, without partic.i.p.ation and communication with the general current, is irrational, and, therefore, not true.
We have always regarded a magnetic needle as a part of the great natural magnet, the earth; that its north pole actually points to the north, and its south pole to the south; and, being free to move, it is affected by the circular motion of the surrounding ether, and by every motion by which the ether is directed. If there was any attraction between the earth and the needle, opposite poles would be presented, but it is not so--the force is merely directive.
MAGNETIC VARIATIONS.
Let us now see whether we cannot a.s.sign an adequate cause for the secular and periodic variations in the inclination and declination of the needle. These have been generally referred to changes of temperature, as in fact, also, the magnetism of the earth is sometimes ascribed to galvanic or electric currents, called forth by a daily change of temperature. Our theory gives a totally different explanation of these variations.
In the northern hemisphere, the north point of the needle moves from east to west in the morning from about 8 A.M. to 1 P.M., and returns to its mean position about 10 P.M. It then pa.s.ses over to the east, and again returns to its mean position about 8 or 9 A.M. The a.n.a.logy of this motion, with the horary changes in the barometer, indicate a common origin. Humboldt, in the instructions he drew up for the Antarctic Expedition under Sir James Ross, says: "The phenomena of periodical variations depend manifestly on the action of _solar heat_, operating probably through the medium of thermo electric currents induced on the earth"s surface. Beyond this rude guess, however, _nothing is yet known of their physical cause_. It is even still a matter of speculation whether the solar influence be a princ.i.p.al or only a subordinate cause." That the sun may exert a modifying influence on the phenomenon is not unlikely, but that he cannot be the princ.i.p.al cause, is evident from the following considerations. These horary variations of the magnetic needle are as great at the bottom of deep mines far removed from solar influence, as on the surface. They are as great, _ceteris paribus_ on a small island in the midst of the ocean, as in the interior of continents, where the heating power of the surface is vastly greater. They are extremely regular, so that between the tropics, according to the sagacious Humboldt, "the time of the day may be known by the direction of the needle, as well as by the height of the barometer."
But what is the cause of these variations? This question is the most difficult of all physical problems, and we shall only aim at indicating the causes which are yet perhaps too intricately involved to afford a positive numerical determination. Admitting the existence of two princ.i.p.al solid ma.s.ses whose general direction is from south to north, and that these ma.s.ses are more susceptible of permeation by the ethereal fluid than the waters in which they are suspended, we have a general solution of the position of the magnetic poles, and of the isogonic, isoclinic, and isodynamic lines. Considering, too, that the southern poles of these ma.s.ses are the points of ingress, and the northern poles the points of egress, it is easily understood that the ethereal medium having the temperature of s.p.a.ce, will cause the southern hemisphere to be colder than the northern, and also that the magnetic poles will be the poles of maximum cold, and the centres respected by the isothermal and isogeothermal lines.
The general direction of the magnetism of the earth may be considered as the controlling influence, therefore, in determining the position of the magnetic needle; but there are other causes which, to some extent, will modify the result. That half of the globe turned away from the sun will partake of the density of the ether at that distance, which is greater than on the side next the sun; the magnetic intensity ought, therefore, to be greater in the night than in the day. The poles of the great terrestrial magnets, or even the position of a magnetic needle on the surface, are continually placed by the earth"s rotation in a different relation to the axes of the terral vortex, and the tangential current, which is continually circulating around the globe, has its inclination to a given meridian in a perpetual state of change. If we conceive that there is a tendency to force the needle at right angles to this current, we shall have an influence which varies during the day, during the year, and during the time occupied by a complete revolution of the node. The princ.i.p.al effect, however, of the horary variation of the needle is due to the radial stream of the sun, which not only penetrates the atmosphere, but also the solid crust of the earth. Its princ.i.p.al influence is, however, an indirect influence, as we shall endeavor to explain.
No fact in the science of electro-magnetism is, perhaps, better established than the disposition of an ethereal current to place itself at right angles to the magnetic meridian, and conversely, when the current is not free to move, to place the needle at right angles to the current. Now, the terrestrial magnet or magnets, may be considered to be surrounded by a body of ether in rotation, which, in the earth, on its surface, and for some distance from the surface, is made to conform to the general rule, that is, to circulate at right angles to the magnetic meridian. Outside this again, the ether more and more conforms to the position of the axis of the vortex, and this position varying, it must exert _some_ influence on the surface currents, and, therefore, change in some degree the position of the magnetic meridian. The radial stream comes from the sun in parallel lines, and strikes the globe and its superficial ethereal envelope just as we have shown its action on the atmosphere; but in this last case the magnetic equator is not a great circle, neither can we suppose its effects to be an acc.u.mulation of a fluid which is imponderable at points 90 from the plane pa.s.sing through the centre of the earth and sun, and coincident with the plane of the central meridian, and a depressing effect on that meridian. Its precise influence must be, from the nature of the cause, to deflect the circular current towards the poles, in places less than 90 from the meridian, and a contrary effect must be produced in places greater than 90 from the meridian. Let us a.s.sume, for argument"s sake, that the magnetic poles of the earth correspond to the poles of rotation, the parallels of lat.i.tude will, therefore, represent the ethereal currents circulating around the globe. Now, at sunrise, the radial stream of the solar vortex is tangential to the surface, and, therefore, can produce no change in these currents. As the sun ascends say about 8 or 9 A.M., the radial stream striking only the surface of the earth perpendicularly in that place where the sun is vertical (which we will suppose at the equator), streams off on every side, as the meridians do from the pole, and the circles of lat.i.tude (that is the ethereal currents) being parallel to the equator, they are met by the radial stream obliquely, and deflected towards either pole. By this deflection they are no longer at right angles to the meridians. But, from the principle of reaction above noticed, the magnetic meridians will place themselves at right angles to the current, or, in other words, the magnetic pole will change its position on the surface of the earth with respect to that particular place. But, in other parts of the world, the meridians are in opposite phases at the same instant of absolute time; therefore, the magnetic poles are not points, but wide areas enclosing the magnetic poles of all the countries under the sun. As this conforms to observation, it is worthy our especial attention, and may be understood by the subjoined figure, in which the oblique curves represent the course of the tangential current in the different positions of the sun, the parallel lines representing the solar radial stream.
[Ill.u.s.tration: Fig. 24]
As the sun gains alt.i.tude the action of the radial stream is at a greater and greater angle to the circular currents, and attains its maximum at noon, still acting, however, after noon; but seeing that the circular current possesses a force of re-action, that is, that the magnetism of the earth is ever striving to bring these currents to their natural direction, an hour or two after noon, the currents tend again to the equator, and the maximum deflection is pa.s.sed, and finally ceases a few hours after sunset. Now let us attend to what is going on on the opposite side of the world. The radial stream pa.s.sing over the polar regions, now produces a contrary effect; the ethereal atmosphere of the great magnet is acc.u.mulated on the farthest side from the sun, by the action of the radial stream pa.s.sing over the polar region, the parallel currents are now bent towards the equator, being at a maximum in places where it is an hour or two past midnight. Before they were concave to the equator, and now they are convex; the magnetic meridian is therefore deflected the contrary way to what it was in the day time, by the same principle of reaction. After the maximum, say at 4 A.M., the deflection gradually ceases, and the magnetic meridian returns to its mean position at 8 or 9 A.M. These times, however, of maximum and minimum, must vary with the time of the year, or with the declination of the sun, with the position of the moon in her orbit, with the perigee of the orbit, and with the place of the ascending node; there are also minor influences which have an effect, which present instrumental means cannot render appreciable.
What says observation? The needle declines from its mean position in the whole northern hemisphere to the westward, from about 8.30 A.M., until 1.30 P.M.; it then gradually returns to its mean position by 10 A.M.
After 10 P.M., it pa.s.ses over to the eastward, and attains its maximum deflection about three or four hours after midnight, and is found again at its mean position about 9 A.M. Now, this is precisely the direction of the deviation of the magnetic meridian, the needle therefore only follows the meridian, or still continues to point to the temporary magnetic pole. And although we have a.s.sumed, for the sake of simplicity, that the mean magnetic pole corresponds to the pole of rotation; in truth there are two magnetic poles, neither of which correspond; yet still the general effect will be the same, although the numerical verification will be rendered more difficult.
In the southern hemisphere the effect is the reverse, (this southern hemisphere, however, must be considered separated from the northern by the magnetic equator, and not by the geographical one,) the needle declines to the eastward in the morning, and goes through the same changes, subst.i.tuting east for west, and west for east. Does observation decide this to be to be a fact also? Most decidedly it does; and this alone may be considered a positive demonstration, that the theory which explains it is true. The contrary deflection of the needle in the northern and southern hemisphere may be formally proclaimed as utterly beyond the reach of the common theory of magnetism to explain. This difficulty arises from considering the needle as the disturbed body instead of the earth; and also from the fact that the effect of solar heat must be common to needles in both hemispheres, and act upon similar poles, and consequently the deflection must be in the same direction.
But a still more capital feature is presented by the discovery of Colonel Sabine, that the deflection is in contrary directions at the Cape of Good Hope, at the epoch of the two equinoxes. This arises from the great angle made by the magnetic meridian at this place, with the terrestrial meridian--the variation being by Barlow"s tables, 30 to the westward. The sun varies in declination 47 throughout the year. At the southern solstice, therefore the radial stream strikes the circular current on the southern side, and deflects it towards the equator, rendering the declination to the westward in the morning; but at the northern solstice the radial stream strikes the current on its northern side, and the deflection is eastward in the morning. And the vicinity of the Cape of Good Hope is, perhaps, the only part of the world where this anomaly will obtain; as it is necessary not only that the declination shall be considerable, but also that the lat.i.tude shall not be very great.
Observation also determines that the amount of the horary variation increases with the lat.i.tude. Near the equator, according to Humboldt, it scarcely amounts to three or four minutes, whilst it is from thirteen to fourteen minutes in the middle of Europe. The theory explains this also; for as the circles recede from the equator, the angles made by their planes with the direction of the radial stream increases, and hence the force of deflection is greater, and the effect is proportioned to the cause. We have also a satisfactory explanation of the fact that there has not yet been discovered a line of _no variation of horary declination_ as we might reasonably antic.i.p.ate from the fact that the declinations are in _contrary directions_ in the northern and southern hemisphere. This is owing to the ever-varying declination of the sun.
There would be such a line, no doubt, if the axis of the earth were perpendicular to the plane of the orbit, and the magnetic pole coincided with the pole of rotation: for then the equator would be such a line.
MAGNETIC STORMS.
But there are also irregular fluctuations in the direction of the magnetic needle. These depend on the moon, and are caused by the pa.s.sage of the vortices over or near to the place of observation. The action of these vortices is proved to be of variable force, whether arising from atmospheric conditions, or due to an increased activity of the ethereal medium throughout the whole system, is at present immaterial. They do vary, and sometimes the pa.s.sage of a vortex will deflect the needle a whole degree. At other times, there are magnetic storms extending over a great part of the earth"s surface; but there is reason to suppose, that the extent of these storms has been over estimated. Thus, on the 25th of September, 1841, a magnetic storm was observed in Toronto, and at the same time there was one felt at the Cape of Good Hope. There is no great mystery in this. If we suppose the axis of the central vortex, for instance, to have pa.s.sed Toronto in lat.i.tude 43 33" north, in ordinary positions of the moon, in her orbit, the southern portion of the axis would be in 33 or 34 south lat.i.tude, and consequently would have pa.s.sed near the Cape of Good Hope on the same night. Now, we certainly could not expect the northern portion of the vortex to be intensely active, without the southern portion being in the same state of activity. That this is the true explanation is proved by magnetic storms in the same hemisphere being comparatively limited in extent; as, according to Gauss and Weber, magnetic storms which were simultaneously felt from Sicily to Upsala, did not extend from Upsala to Alten. Still it would not be wonderful if they were felt over a vast area of thousands of miles as a consequence of _great_ disturbance in the elasticity of the ether in the terral vortex; as the solid earth must be permeable to all its motions, and thus be explicable on the general principles we have advanced.
But besides these variations which we have mentioned, there are changes steadily going on, by which the isodynamic, isogonic and isoclinic lines are permanently displaced on the surface of our planet. These must be attributed to changes of temperature in the interior of the globe, and to the direction in the progress of subterranean fires, which it may also be expected will change the isogeothermal lines. But there are changes, which although of long period, are yet periodic, one of which is obviously due to the revolution of the lunar nodes in eighteen and a half years, and the revolution of the apogee in nine years. The first is continually changing the obliquity of the axis of the vortex, and they both tend to limit the vortices in their extreme lat.i.tudes; but the planet Jupiter has an indirect influence, which is probably equal, if not greater, than the action of the moon, in changing the magnetic declination.