Part 9 (1/2)

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. s.h.i.+p 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.