Chapter III

GRAVITATION.

The Forms of Matter—Shape of the Earth—Probability of the Mass forming this Planet having existed in a Nebulous State—Zodiacal Lights—Comets—Volatilization of Solid Matter by Artificial means—The principle of Gravitation—Its Influence through Space and within the smallest Limits—Gravitating powers of the Planets—Density of the Earth—Certainty of Newton’s Law of the Inverse Square—Discovery of Neptune—State of a Body relieved from Gravitation—Experiment explaining Saturn’s Ring, &c.—General inference.

Let us suppose the earth—consisting of three conditions of matter; the solid, the fluid, and the aëriform—to be set free from that power by which it is retained in its present form of a spheroid flattened at the poles, but still subject to the influences of its diurnal and annual rotations. Agreeably to the law which regulates the conditions of all bodies moving at high velocities, the consequence of such a state of things would be, that our planet would instantly spread itself over an enormous area. The waters and even the solid masses of this globe would, in all probability, present themselves amidst the other phenomena of space in a highly attenuated state, revolving in an orbit around the sun, as a band of nebulous matter, which might sometimes be rendered sensible to sight by still reflecting solar light, or by condensation in the form of flights of shooting stars.[13]

This may be illustrated by experiment. If upon a rapidly revolving disc we place a ball of dust, it will be almost immediately spread out, and its particles will arrange themselves in a series of regular curves, varying with the velocity of the motion. In addition to the disintegration which would arise from the tendency of the atoms to fly from the centre, the motion, in space, of the planetary mass would naturally occasion a trailing out, and the only degree of uniformity which this orb could, under these imaginary conditions, possibly present, would be derived from the combined effects of motions in different directions.

Amid the remoter stars, some remarkable cloud-like appearances are discovered. These nebulæ, presenting to the eye of the observer only a gleaming light, as from some phosphorescent vapour, were long regarded as indications of such a condition as that which we have just been considering. Astronomers saw, in those mysterious nebulæ, a confirmation of their views, which regarded all the orbs of the firmament as having once been thin sheets of vapour, which had gradually, from irregular bodies traversing space, been slowly condensed about a centre, and brought within the limits of aggregating agencies, until, after the lapse of ages, they become sphered stars, moving in harmony amid the bright host of heaven.[14] Geologists seized on those views with eagerness, as confirming theoretical conclusions deduced from an examination of the structure of the earth itself, and explained by them the gradual accretion of atoms into crystalline rocks from a cooling mass.

The researches of modern astronomers, aided by the magnificent instruments of Lord Rosse,[15] have, however, shown that many of the most remarkable nebulæ are only clusters of stars; so remote from us, that the light from them appears blended into one diffused sheet or luminous film. There are, however, the Magellanic clouds, and other singular patches of light, exhibiting changes which can only be explained on the theory of their slow condensation. There is no evidence to disprove the position that world-formation may still be going on; that a slow and gradual aggregation of particles, under the influence of laws with which we are acquainted, may be constantly in progress, to end, eventually, in the formation of a sphere.

May we not regard the zodiacal light as the remains of a solar luminiferous atmosphere, which once embraced the entire system of which it is the centre?[16] Will not the strange changes which have been seen to take place in cometary bodies, even whilst they were passing near the earth,—as the division of Biela’s comet and the ultimate formation of a second nucleus from the detached portion,—strongly tend to support the probability of the idea that attenuated matter has, in the progress of time, been condensed into solid masses, and that nebulous clouds must still exist in every state of tenuity in the regions of infinite space,[17] which, in the mysterious processes of world-formation, will, eventually, become stars, and reflect across the blue immensity of heaven, in brightness, that light which is the necessary agent of organisation and all manifestations of beauty?

The inferences drawn from a careful study of the condition of our own globe are in favour of the assumption of the existence of nebulous matter. By the processes of art and manufacture, by the operation of those powers on which organisation and life depend, solid matter is constantly poured off in such a state that it cannot be detected, as matter, by any of the human senses. Yet a thousand results, daily and hourly accumulating as truths around us, prove that the solid metals, the gross earths, and the constituents of animal and vegetable life, all pass away invisible to us, and become “thin air.” We know that, floating around us, these volatilized bodies exist in some material form, and numerous experiments in chemistry are calculated to convince us, that the most attenuated air is capable, with a slight change of circumstances, of being converted into the condition of solid masses. Hydrogen gas, the lightest, the most ethereal of the chemical elements, dissolves iron and zinc, arsenic, sulphur, and carbon; and from the transparent combinations thus formed, we can with facility separate those ponderous bodies. Such substances must exist in our own atmosphere; why not in the regions of space? Whether this planet ever floated a mass of nebulous matter, only known by its dim and filmy light, or comet-like rushed through space with widely eccentric orbit, are questions which can only receive the reply of speculative minds. Whether the earth and the other members of the Solar System were ever parts of a Central Sun,[18] and thrown from it by some mighty convulsion, though now revolving with all the other masses around that orb, chained in their circuits by some infinite power, is beyond the utmost refinements of science to discover. This hypothesis is, however, in its sublime conception, worthy of the master-mind that gave it birth.

All we know is, that our earth is an oblate sphere, which, by the effects of its rotation around an axis, is somewhat enlarged at the equator and flattened at the poles;—that it maintains its regular course around the sun, in virtue of the operation of two forces, one of which, acting constantly, would eventually draw it into the body of the sun itself; but that it is opposed by the other, centrifugal force, and the varying momentum of the revolving mass;—that the same force acting from the centre of the earth itself, and from the centre of every particle of its substance, resolves the whole into a globular form.

The principle of Gravitation[19] is that force which resides in every form of matter, by which particle is attracted by particle, and mass by mass, the less towards the greater. What this may be, we scarcely dare to speculate. In the vast area of its action, which opens before the eye of the mind, we see a power spanning all space, and linking together every one of those myriads of worlds which spangle the robe of the Infinite, and we are compelled to pause. Is this principle of gravitation a property of matter, or is it a power higher than the more tangible forces, is the question which presses on the mind. If we regard it as a subtile principle pervading all space, we compel ourselves to look beyond it for another power yet more refined; and we cannot halt until, ascending from the limitable to the illimitable, we resolve gravitation and its governing influences to the centre of all power—the will of the eternal Creator.

Science has developed the grand truth, that it is by the exercise of this all-pervading influence that the earth is retained in its orbit—that the pellucid globe of dew which glistens on the leaf is bound together—that the débris which float upon the lake accumulate into one mass—that the sea exhibits the phenomena of the tides—and the aërial ocean its barometric changes. In all things this force is active, and throughout nature it is ever present. Our knowledge of the laws which it obeys, enables us to conclude that the sun and distant planets are consolidated masses like this earth. We find that they have gravitating power, and by comparing this influence with that exerted by the earth, we are enabled to weigh the mass of one planet against another. In the balance of the astronomer, it is as easy to poise the remote star, as it is for the engineer to calculate the weight of the iron tunnel of the Menai Straits, or any other mechanical structure. Thus throughout the universe the balance of gravitating force is unerringly sustained. If one of the most remote of those gems of light, which flicker at midnight in the dark distance of the starry vault, was, by any power, removed from its place, the disturbance of these delicately balanced mysteries would be felt through all the created systems of worlds.

From the peculiarity of the laws which this power called gravity obeys, it has been inferred that it acts from centres of force; it is proved that its power diminishes in the inverse ratio of the square of the distance, and that the gravitating power of every material body is in the direct proportion of its mass. In astronomical calculations we have first to learn the mass of our earth. Experiment informs us that the density of our hardest rock is not above 2·8; but from the enormous pressure to which matter must be subjected, at great depths from the surface, the weight of the superincumbent mass constantly increasing, it is quite certain that the earth’s density must be far more than this. Maskelyne determined the attraction of large masses by a plummet and line on the mountain Schehallion.[20] Cavendish, with exceedingly delicate apparatus, observed the attraction of masses of known weight and size upon each other. Applying the powers of arithmetical calculation, and the data obtained from the small experiments to the larger phenomena, Maskelyne determined the earth’s mean density to be 4·71, whilst Cavendish made it 5·48, but the more recent refined investigations of Baily have determined it to be 5·67.[21]

From data thus obtained by severe inductive experiments and mathematical analyses, the astronomer, by observing the deviations of a distant star, is enabled to determine the influence of those stellar bodies near which it passes, and, hence, to calculate the relative magnitudes of each. The accuracy of the law is in this way put to the severest test, and the precision of astronomical prediction is the strongest proof of its universality and truth.

Rolling onward its lonely way, in the far immensity of our system, the planet Uranus was discovered by the elder Herschel,—so great its distance that its diminished light could scarcely be detected by the most powerful telescopes; but since its discovery its path has been carefully watched, and some irregularities noticed. Most of these disturbances were referable to known causes; but a little alteration in its rate of motion observed when the planet was in one portion of its vast orbit was unexplained. Convinced of the certainty of Newton’s law, and having determined that the attraction of known masses was insufficient to produce the disturbance observed, these deviations were referred to the gravitating influence of a mass beyond the known limits of our Solar System. By the investigations of Adams in England,[22] and Le Verrier in France,[23] the place of the hypothetical mass was determined, and its size computed. As a grand confirmation of the great law, and to the glory of those two far-searching minds, who do honour to their respective countries and their age, the hypothesis became a fact, in the discovery of the planet Neptune in the place determined by rigorous calculation. Astronomy affords other examples of the sublime truth of the law of gravitation, than which science can afford no more elevated poetry.

So completely is all nature locked in the bonds of this infinite power, that it is no poetic exaggeration to declare, that the blow which rends any earthly mass is conveyed by successive impulses to every one of the myriads of orbs, which are even too remote for the reach of telescopic vision.

An illustrative experiment must close our consideration of relative operations of rotation and gravitation. We well know that a body in a fluid state would, if suspended above the earth, it being at the same time free to take any form, naturally assume that of a flattened spheroid, from the action of the mass of the earth upon it: whereas the force of cohesive attraction acting equally from all sides of a centre, would, if uninfluenced, necessarily produce a perfect sphere. The best method of showing that this would be the case, is as follows:—

Alcohol and water are to be mixed together until the fluid is of the same specific gravity as olive oil. If, when this is effected, we drop globules of the oil into the mixed fluid, it will be seen that they take an orbicular form;—and, of course, in this experiment the power of the earth’s gravitating influence is neutralized. The same drops of oil under any other conditions would be flattened. Simple as this illustration is, it tells much of the wondrous secret of those beautifully balanced forces of cohesion and of gravitation; and from the prosaic fact we rise to a great philosophical truth. Our experiment may lead us yet farther in exemplification of known phenomena. If we pass a steel wire through one of those floating spheres of oil, and make it revolve rapidly and steadily, thus imitating the motion of a planet on its axis, the oil spreads out, and we have the spheroidal form of our earth. Increase the rapidity of this rotation, and when a certain rate is obtained the oil widens into a disc, a ring separates itself from a central globe, and at a distance from it still revolves around it.[24] Here we have a miniature representation of the ring of Saturn. This is a suggestive experiment, the repetition of which, by reflective minds, cannot fail to lead to important deductions. The phenomena of cohesion, of motion, and gravitation, are all involved; and we produce results resembling, in a striking manner, the conditions which prevail in the planetary spaces, under the influence of the same powers. If we take a glass globe, and having filled it with a fluid of the proper density, drop into it large and small globules of oil, we may produce an instructive representation of the stellar vault, with its beautiful spheres of light revolving in their respective orbits; and though crossing each other’s paths, still moving in obedience to attracting and repelling forces—onward in perfect harmony.

From the centre of our earth to the utmost extremity of the universe—from the infinitely small to the immensely vast—gravitation exerts its force. It is met on all sides by physical powers acting in antagonism to it, but, like a ruling spirit, it restrains them in their wildest moods.

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