A History of Science - v.3 (Williams)

constantly directed towards this body. But this constancy of areas necessitates an increase of velocity proportional to the distance. It is thus seen

that the same cause would diminish the velocity of the molecules which form the inner part of the ring.

"If all the molecules of the ring of vapor continued to condense without disuniting, they would at length form a ring either solid or fluid. But this formation would necessitate such a regularity in every part of the ring, and in its cooling, that this phenomenon is

extremely rare; and the solar system affords us, indeed, but one example--namely, in the ring of Saturn.

In nearly every case the ring of vapor was broken into several masses, each moving at similar velocities, and continuing to rotate at the same distance around the sun. These masses would take a spheroid form with a rotatory movement in the direction of the revolution, because their inner molecules had less velocity than the outer. Thus were formed so many planets in a condition of vapor. But if one of them were powerful

enough to reunite successively by its attraction all the others around its centre of gravity, the ring of vapor would be thus transformed into a single spheroidical mass of vapor revolving around the sun with a rotation in the direction of its revolution. The latter case

has been that which is the most common, but nevertheless

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the solar system affords us an instance of the

first case in the four small planets which move between Jupiter and Mars; at least, if we do not suppose,

as does M. Olbers, that they originally formed

a single planet which a mighty explosion broke up

into several portions each moving at different velocities.

"According to our hypothesis, the comets are strangers to our planetary system. In considering them,

as we have done, as minute nebulosities, wandering from solar system to solar system, and formed by the condensation of the nebulous matter everywhere

existent in profusion in the universe, we see that when they come into that part of the heavens where the sun

is all-powerful, he forces them to describe orbits either elliptical or hyperbolic, their paths being equally possible in all directions, and at all inclinations of the

ecliptic, conformably to what has been observed. Thus the condensation of nebulous matter, by which we

have at first explained the motions of the rotation and revolution of the planets and their satellites in the same direction, and in nearly approximate planes, explains

also why the movements of the comets escape this general law."[3]

The nebular hypothesis thus given detailed completion by Laplace is a worthy complement of the grand cosmologic scheme of Herschel. Whether true or false,

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the two conceptions stand as the final contributions of the eighteenth century to the history of man's ceaseless efforts to solve the mysteries of cosmic origin and cosmic structure. The world listened eagerly and without prejudice to the new doctrines; and that attitude tells

of a marvellous intellectual growth of our race. Mark the transition. In the year 1600, Bruno was burned

at the stake for teaching that our earth is not the centre of the universe. In 1700, Newton was pronounced

"impious and heretical" by a large school of philosophers for declaring that the force which holds the planets

in their orbits is universal gravitation. In 1800, Laplace and Herschel are honored for teaching that gravitation built up the system which it still controls; that our universe is but a minor nebula, our sun but

a minor star, our earth a mere atom of matter, our race only one of myriad races peopling an infinity of worlds. Doctrines which but the span of two human lives before would have brought their enunciators

to the stake were now pronounced not impious, but sublime.

ASTEROIDS AND SATELLITES

The first day of the nineteenth century was fittingly signalized by the discovery of a new world. On the evening of January 1, 1801, an Italian astronomer,

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Piazzi, observed an apparent star of about the eighth magnitude (hence, of course, quite invisible to the unaided eye), which later on was seen to have moved,

and was thus shown to be vastly nearer the earth than any true star. He at first supposed, as Herschel had done when he first saw Uranus, that the unfamiliar body was a comet; but later observation proved it a tiny planet, occupying a position in space between Mars and Jupiter. It was christened Ceres, after the tutelary goddess of Sicily.

Though unpremeditated, this discovery was not unexpected, for astronomers had long surmised the existence

of a planet in the wide gap between Mars and Jupiter. Indeed, they were even preparing to make concerted search for it, despite the protests of philosophers, who argued that the planets could not possibly exceed the magic number seven, when Piazzi forestalled their efforts. But a surprise came with the sequel; for the very next year Dr. Olbers, the wonderful physicianastronomer of Bremen, while following up the course

of Ceres, happened on another tiny moving star, similarly located, which soon revealed itself as planetary.

Thus two planets were found where only one was expected.

The existence of the supernumerary was a puzzle, but Olbers solved it for the moment by suggesting that Ceres and Pallas, as he called his captive, might be fragments of a quondam planet, shattered by internal

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explosion or by the impact of a comet. Other similar fragments, he ventured to predict, would be

found when searched for. William Herschel sanctioned this theory, and suggested the name asteroids

for the tiny planets. The explosion theory was supported by the discovery of another asteroid, by Harding,

of Lilienthal, in 1804, and it seemed clinched when Olbers himself found a fourth in 1807. The new-comers were named Juno and Vesta respectively.

There the case rested till 1845, when a Prussian amateur astronomer named Hencke found another asteroid, after long searching, and opened a new epoch of discovery. From then on the finding of asteroids became a commonplace. Latterly, with the aid of photography, the list has been extended to above four

hundred, and as yet there seems no dearth in the supply, though doubtless all the larger members have been revealed. Even these are but a few hundreds of miles

in diameter, while the smaller ones are too tiny for measurement. The combined bulk of these minor

planets is believed to be but a fraction of that of the earth.

Olbers's explosion theory, long accepted by astronomers, has been proven open to fatal objections. The

minor planets are now believed to represent a ring of cosmical matter, cast off from the solar nebula like the

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rings that went to form the major planets, but prevented from becoming aggregated into a single body by the perturbing mass of Jupiter.

The Discovery of Neptune

As we have seen, the discovery of the first asteroid confirmed a conjecture; the other important planetary discovery of the nineteenth century fulfilled a prediction. Neptune was found through scientific prophecy.

No one suspected the existence of a trans-Uranian planet till Uranus itself, by hair-breadth departures from its predicted orbit, gave out the secret. No one

saw the disturbing planet till the pencil of the mathematician, with almost occult divination, had pointed

out its place in the heavens. The general predication of a trans-Uranian planet was made by Bessel, the great

Konigsberg astronomer, in 1840; the analysis that revealed its exact location was undertaken, half a decade

later, by two independent workers--John Couch Adams, just graduated senior wrangler at Cambridge, England, and U. J. J. Leverrier, the leading French mathematician of his generation.

Adams's calculation was first begun and first completed.

But it had one radical defect--it was the work

of a young and untried man. So it found lodgment in a pigeon-hole of the desk of England's Astronomer Royal,

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and an opportunity was lost which English astronomers have never ceased to mourn. Had the search

been made, an actual planet would have been seen

shining there, close to the spot where the pencil of the mathematician had placed its hypothetical counterpart. But the search was not made, and while the

prophecy of Adams gathered dust in that regrettable pigeon-hole, Leverrier's calculation was coming on, his tentative results meeting full encouragement from

Arago and other French savants. At last the laborious calculations proved satisfactory, and, confident of the result, Leverrier sent to the Berlin observatory, requesting that search be made for the disturber of Uranus in a particular spot of the heavens. Dr. Galle received the request September 23, 1846. That very night he turned his telescope to the indicated region,

and there, within a single degree of the suggested spot, he saw a seeming star, invisible to the unaided eye, which proved to be the long-sought planet, henceforth

to be known as Neptune. To the average mind, which finds something altogether mystifying about abstract

mathematics, this was a feat savoring of the miraculous.

Stimulated by this success, Leverrier calculated an

orbit for an interior planet from perturbations of Mercury, but though prematurely christened Vulcan, this

hypothetical nursling of the sun still haunts the realm

of the undiscovered, along with certain equally hypothetical

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trans-Neptunian planets whose existence has

been suggested by "residual perturbations" of Uranus, and by the movements of comets. No other veritable additions of the sun's planetary family have been made in our century, beyond the finding of seven small moons, which chiefly attest the advance in telescopic powers. Of these, the tiny attendants of our Martian neighbor, discovered by Professor Hall with the great Washington refractor, are of greatest interest, because of their small size and extremely rapid flight. One of them is poised only six thousand miles from Mars, and whirls about him almost four times as fast as he revolves, seeming thus, as viewed by the Martian, to rise in the west and set in the east, and making the month only one-fourth as long as the day.

The Rings of Saturn

The discovery of the inner or crape ring of Saturn, made simultaneously in 1850 by William C. Bond, at the Harvard observatory, in America, and the Rev.

W. R. Dawes in England, was another interesting optical achievement; but our most important advances

in knowledge of Saturn's unique system are due to the mathematician. Laplace, like his predecessors, supposed these rings to be solid, and explained their stability as due to certain irregularities of contour which Herschel bad pointed out. But about 1851 Professor

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thought them they must fall of their own weight. Then Professor J. Clerk-Maxwell, of Cambridge, took

the matter in hand, and his analysis reduced the puzzling rings to a cloud of meteoric particles--a "shower

of brickbats"--each fragment of which circulates exactly as if it were an independent planet, though of

course perturbed and jostled more or less by its fellows. Mutual perturbations, and the disturbing pulls

of Saturn's orthodox satellites, as investigated by Maxwell, explain nearly all the phenomena of the rings in

a manner highly satisfactory.

After elaborate mathematical calculations covering many pages of his paper entitled "On the Stability of Saturn's Rings," he summarizes his deductions as follows:

"Let us now gather together the conclusions we

have been able to draw from the mathematical theory of various kinds of conceivable rings.

"We found that the stability of the motion of a

solid ring depended on so delicate an adjustment, and at the same time so unsymmetrical a distribution of

mass, that even if the exact conditions were fulfilled, it could scarcely last long, and, if it did, the immense

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preponderance of one side of the ring would be easily observed, contrary to experience. These considerations, with others derived from the mechanical structure of

so vast a body, compel us to abandon any theory of solid rings.

"We next examined the motion of a ring of equal satellites, and found that if the mass of the planet is sufficient, any disturbances produced in the arrangement of the ring will be propagated around it in the

form of waves, and will not introduce dangerous confusion. If the satellites are unequal, the propagations

of the waves will no longer be regular, but disturbances of the ring will in this, as in the former case,

produce only waves, and not growing confusion. Supposing the ring to consist, not of a single row of large satellites, but a cloud of evenly distributed unconnected particles, we found that such a cloud must

have a very small density in order to be permanent, and that this is inconsistent with its outer and inner parts moving with the same angular velocity. Supposing the ring to be fluid and continuous, we found that

it will be necessarily broken up into small portions.

"We conclude, therefore, that the rings must consist of disconnected particles; these must be either solid or liquid, but they must be independent. The

entire system of rings must, therefore, consist either of a series of many concentric rings each moving with

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