I It may be doubted whether many of the patrons of Mudie's are acquainted with the works of a philosopher, who, in his day, enjoyed no little fame—I mean, Robert Boyle (1627-1691),—and yet there are passages in them well worth attentive perusal, from the lucidity of their style and the soundness of their reflections. He has, for instance, some observations in his "Considerations on the Usefulness of Experimental Philosophy," which are germane to the general purport and tone of our little book. He remarks, that the contemplation of the vastness, beauty, and regular motions of the heavenly bodies, the excellent structure of animals and plants, besides a multitude of other phenomena of nature, and the subserviency of most of these to man, ought, certainly, to induce him, as a rational creature, to conclude that this vast, beautiful, orderly, and, in a word, many ways admirable, system of things, that we call the world, was framed by an Author supremely powerful, wise, and good.

The works of God, he adds, are so worthy of their Author, that, besides the impresses of His wisdom and goodness that are left, as it were, upon their surfaces, there are a great many more curious and excellent tokens and effects of Divine artifice in the hidden and innermost recesses of them; and these are not to be discovered by the perfunctory looks of oscitant and unskilful beholders; but require, as well as deserve, the most attentive and prying inspection of inquisitive and well-instructed considerers. It is not by a slight survey, but by a diligent and skilful scrutiny of the works of God, that a man must be, by a rational and affective conviction, engaged to acknowledge, with the prophet, that the Author of nature is "wonderful in counsel, and excellent in working."

That He is wonderful in counsel and excellent in working must be the conclusion of every devout student of the celestial phenomena; and to those we shall, therefore, devote the first portion of our Spring meditations.

What reception would formerly have been given to any poet who had dared to exclaim—

Would he not have been met with the reproach which Horace, in his Ars Poetica, so epigrammatically formulates?—

As for men of science, they would not have condescended to honour even with a smile such strange and fantastic words.

Let us suppose, now, that our poetical astronomer, thus contemned, had addressed his scientific censors in some such language as the following:—

Do not think, illustrious sirs, that it is by a purely poetical licence I call the firmament a mirror in which the earth may be seen reflected. Only, to prevent all equivoque, we must understand one another. The mirror to which I am alluding does not reflect light, but movement. It is in a particular movement of the stars that the true figure of our planet is reflected, is revealed to us. But before the human mind can appreciate this movement,—especially before it can discover the cause,—we must be prepared to devote ourselves to centuries of assiduous effort. In this long interval, philosophers of every class will allow unrestricted scope to their imagination.

Shall we, then, recall some of these opinions,—some of these truly poetical licences?

Homer and Hesiod represented the earth as a disc, or as a flat rondel, surrounded on all sides by a winding river which they called the Ocean, and which, in the extreme East, communicated with the Phasis, in Colchis. Above this terrestrial disc the outspread sky was arched like a vast dome; a dome supported by two massive pillars, resting on the shoulders of the god Atlas.

Surely the ancient poets must have evolved the earth-disc from their own prolific imagination. Can they never have seen a far-off vessel, showing, as it approached them, at first the tops of its masts, then its swelling sails, and finally its hull? They might have made so simple an observation in any seaport; if they did, why did it not suggest to them the idea that the earth, instead of being level, must be round? Because it is easier to let the imagination speak than the reason.

The fiction of the earth-disc remained long unshaken, with the exception of a few modifications. Thales figured to himself the earth as floating on a humid element. And, six centuries later, we find Seneca still adopting the opinion of the Greek philosopher. "This humid element (humor)," he says, "which sustains the disc of the earth like a ship, may be, perhaps, the ocean, or a liquid of simpler nature than water."[38]

But how, then, was the rising or setting of the stars explained? The ancients supposed that they were extinguished at sunset, and rekindled at sunrise. Thus, an unfounded hypothesis has for its consequence a still more baseless hypothesis; and in this manner we glide down the slope of fiction to fall eventually into an abyss of contradictions. Such is the true punishment of error.

Let us continue.—According to the Chaldeans, who were thought to be profoundly versed in astronomy, the earth was hollow, and shaped like an egg-shell. And,—adds Diodorus, from whom we have this detail,—they adduce numerous and plausible proofs of this assertion.

Yet was this idea in direct opposition to the evidence of our senses when we travel over a wide plain, or navigate the great deep; at least, unless we admit that the earth has the form of a reversed egg-shell, with its convex face uppermost, and its concave one beneath. Heraclitus of Ephesus introduced the Chaldean doctrine into Greece.

Anaximander represents the earth as a cylinder, whose upper face alone is inhabited. This cylinder, adds the philosopher, is a third of its diameter in height, and floats freely in the midst of the celestial vault, because there is no reason why it should move more to one side than the other. Leucippus, Democritus, Heraclitus, and Anaxagoras,—names of high repute in the history of philosophy,—adopted Anaximander's system, though it was neither more nor less than a wild phantasy.

Anaximenes and Zenophanes, without pronouncing dogmatically on the form of the earth, represented it as resting,—the one upon compressed air, the other upon roots which were prolonged ad infinitum. But upon what was the compressed air supported? And of what nature were these mysterious roots?

Plato, with a nearer approximation to the probable, gave to the earth the form of a cube. The cube, bounded by six square equal surfaces, appeared to him the most perfect geometrical solid, and consequently the most suitable for the earth, supposed to be the centre of the universe.

Eudoxes, who, in his long travels in Greece and Egypt, must have seen new constellations rising in the south, while others disappeared in the north, never ventured to adduce from his astronomical observations the sphericity of the earth.

Aristotle, bolder than Eudoxes, was led to the conception of this sphericity by simple consideration of mechanics. The earth, he said, must be a sphere, because each particle of matter is carried, by gravity, towards the centre; and as this fact is general, the superficial particles must be at an equal distance from the centre. This theoretical view was adopted by Archimedes, who applied it to the waters covering the terrestrial surface. Aristotle went further; he saw the rotundity of the earth in the shadow thrown by the latter on the bright face of the moon during its eclipses.

It is a noteworthy fact that the arguments of Aristotle, founded on a method to which all the progress of science is due, remained unaccepted for two thousand years. And why?

We shall attempt to explain.

Among those subjects whose comprehension seems to have been specially difficult to the mind of man, we must include the fact that the earth floats without any solid support in the infinity of space, and carries its denizens on its surface, both above and below.

Our creeds, which have ever pretended to explain everything in the physical as well as in the moral order, have here endeavoured to come to the assistance of the weakness of the human mind. And as each creed asserts itself to be the best, to the exclusion of every other, men began to imagine for the earth a navel, and placed it where it was supposed pleasing to the divinity.

The Greek priests dismissed a couple of eagles, one towards the west, the other towards the east; the place where these favourite birds of Jove, of the "Father of gods and men," encountered each other, was to be considered the "navel" of the earth. It chanced to be Delphos, whose oracle was the most esteemed in the ancient world; the sacerdotal caste accumulated there immense wealth. The Greek priests prudently refrained from dealing with the difficult problem of the earth's solid support.

The Hebrew pontiffs, however, were not so reserved. After having made Jerusalem the "navel of the world," they allowed for the earth itself seven solid columns as a foundation. The question of the Antipodes, in which the greatest intellects of antiquity believed,—Pythagoras, Plato, and Aristotle,—was thus pontifically judged and condemned. And Christians who preferred to follow the Judaical observance of external ceremonies to a true comprehension of the spirit of the Gospels, exaggerated the application of this sentence.

The dogmatic condemnation of the existence of the Antipodes long arrested man in his search for that fourth quarter of the world whose inhabitants should have their feet directed towards ours. It was one of the principal obstacles which Columbus was called upon to surmount in the realisation of his sublime idea. When cited before the Council of Salamanca,—composed of prelates and men of science,—he had to meet the revived objection of Lactantius, a Christian apologist of the third century:[39]—"Can there be anything more absurd than a belief in the existence of Antipodes, of inhabitants with their feet opposite to our feet, of people who walk with their feet in the air, and their heads on the ground?—that there is a part of the world where everything is inverted, where trees throw out their branches from top to bottom, while it rains, and hails, and snows, from bottom to top?"

Columbus admirably demonstrated, from an artificial globe, that flies walked as easily on the lower as on the upper surface, and hence pointed out that men, compared with the size of the earth, are much smaller still than flies. But his judges persisted in their conviction, and did not fail to cast in his face the jesting words of Plutarch: "Philosophers, rather than renounce a favourite hypothesis, would make human beings crawl on the lower face of the earth like worms or lizards." But it was principally the authority of St Augustine which they invoked to condemn a belief in the Antipodes. St Augustine had declared such a belief incompatible with the dogmas of the faith; for to admit the existence of inhabitable lands, in the opposite hemisphere, would be to admit the existence of peoples not descended from Adam, since it would have been impossible for him to traverse the ocean lying between Asia and the Antipodes!

Some authorities denied the Antipodes on the ground taken by certain classic writers, that the regions of the opposite hemisphere were uninhabitable under the tropics, on account of the extreme heat, and near the Poles, on account of the extreme cold. Others cited Epicurus, affirming that the earth was inhabitable and roofed with a celestial vault only in our western hemisphere, the other half being an inaccessible chaos. Others pretended that no traveller could reach the east by way of the west, because the earth, being pear-shaped, he would encounter on his road an insurmountable tuberosity,—undoubtedly the tail or stalk of the pear! Finally, there were some who dwelt upon the magnitude of the earth, whose circuit would occupy a voyage of upwards of three years.

It was to this objection, as the most serious, that Columbus principally addressed his reply. And he replied by discovering the New World. But had not this daring genius been supported in his projects by the Spanish sovereigns, Ferdinand and Isabella, he would have been handed over to the Inquisition, and condemned as a heretic. It was then so dangerous to believe in the Antipodes, that a Bishop of Salzburg was deposed from his episcopal throne, and deprived of his ecclesiastical dignity, by the Pope Zacharias, for having countenanced the heresy.

We now know why, for a whole series of centuries, men would not follow in the footsteps of Aristotle, who was the first to establish theoretically the sphericity of the earth.

The discovery of the New World, and the voyages of circumnavigation which rapidly succeeded one another, demonstrated, not only that inhabitants there are whose feet are opposite to ours, but that the earth does not rest upon any species of support; that it floats, like a star, freely in space.

The ice was broken. The question of the earth's figure was revived, and, this time, discussed in a new light.

Is the earth perfectly round?

Copernicus never doubted it; he who was the first, after Pythagoras, to represent our planet as revolving round the sun. The geometrical sphericity of the earth wonderfully harmonised with the perfect circles in which he supposed the planets to move. Kepler, who had first laid a sacrilegious hand on the holy figure of the circle, and on the circular orbits of the stars, never ventured, however, to dispute the perfect rotundity of the earth; it appeared to him a matter beyond all controversy. Galileo was the first to hazard a doubt. But this doubt became a certainty only through the labours of Huygens.

Galileo, who died in the very year that Newton was born (1642), had discovered, as we know, that all bodies, in falling, obey an uniformly accelerative force, called gravitation, and that the space traversed increases as the square of the time occupied in their descent. Huygens perceived that gravity varies according to the parallels of latitude, and it was not long before he demonstrated, by the number of oscillations which a pendulum of a certain length performs in a certain time, that it diminishes in a regular ratio as we approach the Equator, where it reaches its minimum, and that it augments, on the contrary, in due proportion as we approach the Poles, where it must attain its maximum. Strong in this knowledge, and knowing, moreover, that the material molecules, uniformly distributed in the volume of a sphere, act upon a point of its surface as if they were all reunited in the centre of that sphere, Huygens deduced from it the inequality of the equatorial and the polar radius; he attempted even to determine how much the former exceeded the latter. We know, now-a-days, that this difference is 139,670 feet (41,848,380—41,708,710 feet).

Newton admits, with Huygens, that the earth bulges out at the Equator and is flattened at the Poles; that, in a word, it is a spheroid of revolution. He went much farther: he made the precession of the Equinoxes depend upon this flattening; but he did not furnish its mathematical demonstration. What has been the result? A free skirmishing ground for all opinions.

While Newton maintained that the form of the earth was that of a spheroid flattened at the poles, as a necessary sequence of the great natural law which bears his name, Jacques Cassini declared himself in favour of an elongated spheroid. The difference between these two illustrious teachers originated a controversy which lasted for upwards of fifty years. The Academy of Sciences of Paris pronounced, not unnaturally, in favour of the opinion of their colleague, though it was far from having the authority of Dominique Cassini, father of Jacques, and, still less, that of the illustrious President of the Royal Society of London. But patriotic ardour supplemented the weakness of their arguments. The flattened spheroid and Newton's law were rejected by France, because they were an English invention. Undoubtedly, no one openly acknowledged so paltry a reason, but it was certainly true as a sentiment. As everybody knows, it was Voltaire who first removed the prohibition, and popularised the Newtonian philosophy in France.

How did our astronomers finally succeed in demonstrating mathematically the veritable form of our planet?

To obtain a clear and accurate conception, we are obliged to transport ourselves back two thousand years. Let us recall, in the first place, that, owing to the diurnal movement, all the stars progress from east to west; that they rise and set, to recommence the same rotation. This is a general and conspicuous fact, which everybody can confirm for himself. But now for another, whose observation requires a little more time and patience. During the diurnal movement, which carries on all the stars and the sun himself, the latter progresses independently, in the inverse direction of the celestial vault, as a fly might do upon a revolving globe. But this second fact is complicated with a third: While advancing on his own account, from west to east, the circle which the sun traverses is not parallel to the Equator; the radiant luminary transports himself alternatively into the northern and southern hemispheres, accomplishing this rotation in 365 days and a fraction of a day, in an oblique plane, which cuts that of the Equator under an angle of about 23½°.

Let us here take advantage of a parenthesis to explain a few astronomical technicalities, necessary for the due comprehension of our subject.

It is in the plane, or oblique circle,—? ?????? ?????;, as PtolemÆus called it,—that eclipses occur, owing to the relative positions of the sun, earth, and moon; and it is for this reason modern astronomers have denominated it the Ecliptic. The Ecliptic is the Equator of the oblique sphere (sfa??a ???e??????), properly so called, as the Equator is that of the sphere of the world, or the right sphere (sfa??a ????). The circles parallel to the Ecliptic, which continue to diminish in diameter up to the poles of the oblique sphere, bear the name of parallels of latitude; and we give that of meridians of longitude or oblique ascensions (??af??a? ???a?) to the great circles which cut the first rectangularly as they all pass through the axis and the poles of the Ecliptic. The same division by circles cutting each other rectangularly has been made on the right sphere, or sphere of the world. Only, there the latitudes are named declinations, and the longitudes right ascensions. The general diurnal movement is a movement in right ascension; it is measured upon the Equator. The individual annual movement of the sun is a movement in longitude; it is measured upon the Ecliptic.

The zone, or belt, which the sun seems to trace in its annual march, from the limit of its southern excursion (the winter solstice) to the limit of its boreal excursion (the summer solstice), and in returning from that limit to the other, after having twice passed through the equinoctial line (or Equator),—this zone is marked on the firmament by a belt of constellations known as the Zodiac.

These constellations are named, according to the figurative grouping of the stars (on which we have commented in Book I.),—the Ram, the Bull, the Twins, the Crab, the Lion, the Virgin, the Balance, the Scorpion, the Archer, the Cow, the Water-bearer, the Fishes. There are twelve, three for each season. The constellations represented by these figures, so singularly chosen, spread over the whole celestial vault,—that is, over an extent of 360°.

To resume.

The heavens, like earth, have their annals: everything changes there as in the human world. In the age of Hipparchus,—or some two thousand years ago,—the sun entered, at the spring equinox, into the zodiacal sign of Aries; in the summer solstice, it entered into that of Cancer; at the autumnal equinox, into Libra; and at the winter solstice, into Capricorn. These signs then corresponded exactly to the constellations which they represent.

Now, whatever Aristotle and his disciples may say, the firmament is not incorruptible (?f?a?t??) and immovable; even the fixed stars, as we call them, change their place in time. We have seen that the whole celestial vault or "right sphere" runs, from east to west, around the poles of the world; we have seen also that the sun moves, on his own account, from west to east, around the poles of the oblique sphere, or the Ecliptic. Well, this does not suffice; there is a third movement to be observed,—that of the right sphere itself round the poles of the Ecliptic; and this, not like that of the sun, from west to east, but inversely, from east to west. Only, this movement of the starry sphere in longitude, or parallel to the plane of the Ecliptic, is extremely slow, compared with the movement of the same sphere in right ascension, or parallel to the Equator of the world. While the former traverses in twenty-four hours the 360° of the circle, the latter occupies (in round numbers) 25,000 years.

Who was the first discoverer of the slow movement of the heavens? Hipparchus. This great astronomer, on comparing his own observations with the more ancient ones of Aristillus and Timocharis, succeeded in ascertaining that the constellation which, 150 years before him, corresponded to the spring equinox, did not, in his time, any longer coincide exactly with the same equinoctial point, but had outstripped or preceded it about 2°. This is what we mean by the precession of the equinoxes.

Hipparchus was at first of opinion that this movement affected only the constellations of the Zodiac; but he soon became assured of its universality. He perceived that if it does not alter the parallels of latitude; because it has occurred parallel with the Ecliptic, it makes the position of the equinox retrograde from east to west, and the sun pass slowly through the same constellations in the reverse of the order in which he annually traverses them.

We know this movement now to nearly the fraction of a second. By an inappreciable daily quantity, it rises, at the end of the year, to 50".3,—in a century, to about 1½°,—in twenty centuries, to 30°, or the twelfth part of the Zodiac. It is for this reason the Ram, which, in the days of Hipparchus, was occupied by the sun in spring, has no longer any value as a commemorative sign; it gives place now-a-days to the constellation of the Fishes, and corresponds to the constellation of Taurus, or the Bull; the constellation of Taurus to Gemini, or the Twins; the constellation of the Twins to Cancer, and so on. But little more than a month, then (a month of 2000 years!), of the great year (a year of 25,000 years!) has elapsed since the epoch of Hipparchus. It is to astronomy especially that, with a slight variation, we may apply the aphorism of Hippocrates—"Brevis vita, ars longa" (Life is short, and art long).

The precession of the equinoxes explains why the pole of our starry vault does not occupy invariably the same point of the firmament, and why the constellations which we now see shining during the nights of a given season change their places as time glides by.

But what is the cause of this movement?

Before this question, as before a sovereign tribunal, appear the two opposite doctrines which have been enunciated on the value of the earth and the sun in the world's system. According to the doctrine at once the oldest and most intolerant, the earth occupies immovably the centre of the world; the sun and the planets are only its satellites; they, like the moon, revolve around the earth; finally, all the starry sphere, the whole celestial vault, rotates upon its own axis in four-and-twenty hours. We have been speaking as if this were really and truly the condition of things. If we admit this doctrine, which bears the name of the Ptolemean system,—though, in truth, it is probably as old as humanity itself,—how shall we explain the precession of the equinoxes? We cannot do otherwise than suppose, that while the celestial sphere executes its diurnal movement round the poles of the world, it executes another and much slower movement round the poles of the Ecliptic.

But this assuredly is a most singular supposition. What! the same starry sphere revolves at one and the same time parallel to the plane of the Equator, and parallel to another plane (the Ecliptic) inclined upon the first? After having imagined eight spheres of crystal to explain the movements of the moon, the sun, the planets (Mercury, Venus, Mars, Jupiter, Saturn), and the stars, do we require a ninth? Where will you stop, if you begin to discover additional movements? You are condemned to wander from hypothesis to hypothesis, until you fall into an abyss of contradictions!

Such is the language employed by the tribunal of posterity, in addressing itself to the error which would substitute appearance for reality.

According to the other theory, it is the sun which occupies the centre of the system, and it is the earth which, accompanied by the remainder of the planets, revolves around it. This theory is likewise of considerable antiquity, though generally known as the Copernican system. But four-and-twenty centuries prior to the epoch of Nicholas Copernicus, it was taught by the "Samian sage," Pythagoras, and his disciples. The system then in acceptance, however, imposed upon them the necessity of silence. PtolemÆus was acquainted with it, but endeavoured to turn it into ridicule. "There are people," he says, "who pretend that heaven is immovable, and that it is earth which revolves on its own axis; evidently these individuals are unaware how supremely absurd is their opinion (p?? ?e????tat??)." And it was in the name of logic and mathematics that PtolemÆus thus treated the Pythagoreans!

In the system of Copernicus,—the diurnal movement of the right sphere,—it is the earth's rotation upon its own axis which, being prolonged into the heavens, marks there, by its extremities, what are called the Poles of the world, just as the Equator of the world is simply the prolongation of the terrestrial Equator. As for the Equator of the oblique sphere (the Ecliptic), in which the sun apparently moves, it is, in reality, the identical plane in which the earth moves during its annual revolution round the sun. Now, in this movement of translation, the axis of the earth does not remain constantly parallel to itself; it deviates,—very slightly, it is true,—and so as to be scarcely perceptible to several generations of men. It is then quite natural that our successors should see, for a long time to come, the northern pole of the starry sphere near the extremity of the tail of Ursa Minor. But, two thousand years hence, this slow deviation will have become very perceptible; astronomers will then see the pole of the world in another constellation, and, as this displacement is continuous, the prolonged axis of the earth will have traced on the firmament, in 25,000 to 26,000 years, a circle parallel to the plane of the Ecliptic, and having for its centre the pole of that plane. This circle is the base of a cone whose summit rests upon the earth. (Fig. 27, a.)

Fig. 27.