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The upper cut gives a view of the nebule and star clusters in the northern hemisphere, the nebulæ being represented by dots, the clusters by crosses. An enlarged view of a star cluster is shown in the smaller cut. The location of the groups of nebula relative to the group of star clusters is one of the interesting points dealt with in the new theory

and that the pub

lic has meanwhile

come to regard as al

most a familiar fact. Professor See's Capture Theory is at once a profound modification of this theory and a startlingly illuminating extension of it. And whatever may be the ultimate attitude of the scientific world toward its claims, it opens up to our minds such vistas of cosmic perspective that a glance at its scope and general nature can hardly fail to interest every mind that ever questions the unknown or feels itself responsive to the marvels of the visible universe's mechanical perfection.

Laplace seized upon one of the latest discoveries of telescopic astronomy at his time-namely, the nebula-to explain one of the oldest and most recalcitrant puzzles of the skies-namely, the circularity of planetary orbits.

"Over two thousand years ago," says Professor See in his introduction, "this

beautiful property" (the circularity of plan-
etary orbits) "still so characteristic of our
system as fully explored by the most pow-
erful telescopes of the present time, was
remarked with admiration
by Aristarchus,
Apollonius, Arch-
imedes, and

other an

cient geometers. It was likewise a subject of

con

stant

discus

sion

among the natural philosophers of the classic period. But, although they were eloquent in their descriptions

This diagram

illustrates the action

of a small astronomical body
which enters the joint sphere
of influence of two larger
bodies at a speed that does
not enable it to escape. It
will first revolve about both
in an orbit represented by
the outermost line of the

of the beauty and order of the cosmos, and made this feature of the heavenly motions one of the leading doctrines of the schools of Athens and Alexandria, yet in general the Greek sages were content to follow Plato in ascribing this property to a wise

place undertook to solve with the new nebular key that he found ready to his hand. "The word nebula," Professor See tells us, "is the Latin equivalent of nephele, which is used by Greek

provision of the deity, and formulator of

it not unnaturally escaped their attention that there might be a profound physical reason for this almost

writers, from Aristotle to Ptolemy,

diagram.
Later, it will drop

to the second line, then
the third, then the
fourth, and so on until it
finally drops into the
closed sphere of influ-
ence of either one or the
other of the larger bodies
and revolves about that

the new nebular hypothesis

stars.

to denote a cloud or

cloud-like object. Thus in the catalogue of fixed

stars given in the "Almagest," Ptolemy describes five objects as cloudy stars . . . because

they presented a somewhat blurred

appearance, due

to the proximity of other stars which made a small cluster and therefore could not be distinctly separated by the naked eye. . . . After the invention of the telescope by Galileo in 1610, they were all readily resolved into separate and distinct Two years later, in 1612, Simon Marius discovered the Great Nebula of Andromeda and found it to be an object of totally different character from those previously known. . . . On examining it with the telescope, Marius was surprised to find that the same nebulous aspect was preserved, and that, unlike the objects previously classed as nebulous, owing to the compression of individual stars, it did not seem to be of sidereal construction. justly compared the light of this great nebula to that of a candle shining at night through a transparent horn, which is a very This was the world-old puzzle that La- appropriate description of the general char

perfect circularity of the planetary paths. This circularity was also the principal circumstance leading to the final adoption of the ancient system of eccentrics and epicycles. . . . It has exercised an equal fascination over the minds of the greatest modern geometers, but has always been erroneously explained on Laplace's original hypothesis of a gradually accelerated rotation which detached these bodies quite gently from the central masses which now govern their motions.”

He

acter of this remarkable object. The earliest true nebula were all discovered by accident, and for a long time the list of them was very small. . . . In 1714, Halley presented to the Royal Society an account of the six nebulæ then known to astronomers ... up to the time of Sir William Herschel the total number of nebulæ known did not exceed 150. This unrivalled man was the first to attempt a systematic exploration of the heavens with powerful instruments. . . . In 1786, he presented to the Royal Society a catalogue of 1000 new nebulæ and clusters of stars; and three years later, in 1789, a second catalogue of 1000 additional nebula."

And Laplace applied this new key to the old puzzle in the following fashion: He assumed a nebula to be an enormous mass of gaseous matter (he figured its density at something like two hundred and sixty million times less than that of atmospheric air at sea level) revolving about its own axis and forming what the scientists call "a figure of equilibrium"-which is to say that it was held together as a unified whole by a balance between the centripetal forces of its gravity and the centrifugal forces developed by its rotation. Starting from this assumption, Laplace argued that as the nebula condensed into a smaller and less tenuous mass, it became more and more heated and revolved faster and faster about its axis. Finally, its outer and more fluid layer, unable any longer to cling to the whirling central body, was gently detached from it, but continued to revolve at the same rate of speed that it possessed at the moment of separation; while the central mass, continuing to shrink in size, to increase its temperature, and to augment its speed of revolution, repeated the process until it had shed as many rings as there are planets. It was then supposed that the matter composing these rings somehow (the possibility of such a thing was never very clearly demonstrated) gathered itself together into a single globe which, on its own hook, proceeded to repeat the proc

ess-revolving more rapidly on its own axis as it condensed and throwing off rings which, in their turn, condensed into still smaller bodies. Thus, after the elapsing of enormous ages, a central portion of the original nebula was supposed to have produced the sun, the rings that the nebulæ had cast off and left behind it in the process of shrinking were supposed to have produced the planets, and the rings that these planets had cast off in the course of their condensation were supposed to have produced the satellites of those planets. And as all these masses were supposed to have been revolving upon their axes at the time that the rings were detached from them, these rings and the bodies that evolved from them continued to revolve in orbits nearly circular.

Thus, in attempting to devise an explanation of the circularity of planetary orbits, Laplace evolved a speculative theory of the origin of the Solar System itself. And for something like a century, this hypothesis, so striking in the grandeur of its conception compared with what went before it, not only satisfied the imaginative curiosity of the world in general, but fully occupied the attention of the scientific world as well. was, in a manner of speaking, shot full of holes by destructive criticism and by such discoveries as those of the retrograde satellites of Jupiter and Saturn. But as a structure it stood unshaken. And although, as time has gone on, it has become more and more evident, even to interested laymen, that the Laplacean hypothesis was destined

According to Professor See's theory, a spiral nebula is the result of the coming together of two streams of cosmical dust which coil up on each other with a whirling movement

It

to be superseded, no constructive theory of sufficiently inclusive scope has been brought forward actually to supersede it.

It is the most striking feature of the present volume that it not only offers a theory that seems to harmonize all the discrepancies developed in a century's examination of the old doctrine, but that it broadens the scope of evolutionary speculation so as to include in one theory the origin of all known orders of sidereal systems and even to glimpse a possible self-perpetuating cycle in celestial mechanics.

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A diagram that illus

trates Professor See's

spiral theory of the sidereal

universe. Our sun is supposed to

be a member of a cluster that is repre

sented by the bright point below the center of the spiral

Just as Laplace used the then recent revelations of the telescope in regard to the - existence of nebula as the speculative basis of his hypothesis, so Professor See uses the revelations made during the past twenty years by astronomical photography in regard to the diffusion of nebulosity throughout space and in regard to the structure of the Spiral, or Whirlpool Nebulæ, as the speculative basis of his modifications and extensions of that theory.

. . . For a long time after the epoch of Laplace and Herschel the impression prevailed that the nebulæ were continuous masses of fluid in equilibrium under hydrostatic pressure. This assumption underlies

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The Great Nebula of Andromeda. Simon Marius, who discovered this nebula in 1612, compared its light to the light of a candle shining at night through a transparent horn. This was one of the first true nebulæ to be discovered, although previous to the invention of the telescope, star clusters had been taken for nebulæ

Laplace's formulation of the nebular hypothesis, and has held its ground almost up to the present time. . . . It is true that a note of dissent was occasionally heard, as when sagacious thinkers began to meditate over the vast extent of the nebulæ, their great transparency, and their enormous distance from the earth. This seemed to make it difficult to believe that the nebulæ are continuous masses of fluid in equilibrium under conditions of hydrostatic pressure. For, with such extreme rarity, hydrostatic pressure could not be exerted unless the mass was at enormous temperature, and a very high temperature could not be maintained in a mass of small density and great transparency, since the heat would be almost instantly

of such regions of nebulosity. At the very lowest estimate of distance they must cover billions of times the space occupied by the solar system, though always so exceedingly

Professor See concludes that just as the star systems are developed from nebulæ, so the nebulæ themselves are gradual accumulations of the cosmic matter that is expelled from the stellar systems by the repulsive forces of light and electric energy

radiated away into space. The dissenters, however, were embarrassed by the fact that Laplace's theory of detachment alone enabled them to account for the roundness of the orbits of the planets and satellites, and their mutual relationships, all of which were supposed to follow from the condensation of rings of vapor."

"The amazing extent of the larger nebulæ has long been known, but has become more obvious with the progress of astronomic photography during the past twenty years. Barnard's photographs of the Milky Way, taken at the Lick Observatory from 1888 to 1894, mark a new epoch in bringing out the cloud forms shown to exist among the stars; but along with this interesting revelation came another, showing that the background of the sky is almost everywhere covered with faint nebulosity. In fact, it has proved extremely difficult to find any part of the sky which was perfectly black. The nebulosity is everywhere spread among the stars and in the nebulæ proper becomes so conspicuous as to attract instant attention.... Some of the regions of nebulosity are of about the same extent as the cloud forms of the Milky Way, thus covering whole constellations. It is scarcely necessary to dwell on the absolute extent

tenuous that the nebulosity often shows merely as the faintest haze on the background of the sky.

"If instead of diffuse nebulosity we consider actual nebulæ, such as the great nebula of Orion, or Andromeda, we shall perceive that the space occupied by these masses cannot well be less than a billion times that occupied by the solar system.

.. Obviously, such nebulæ are much too tenuous and widely extended to be regarded as gaseous masses in equilibrium under the pressure and attraction of their parts. . . . the forces of attraction exerted by one part of such a nebula upon another must be excessively feeble. Different parts of such a mass have different proper motions, and there may thus arise a streaming movement, as where dark lanes are shown in our photographs.

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"We must regard them as clouds of cosmical dust produced by the expulsion of particles of matter from the stars by the radiation pressure of their light and by electric forces, thus constituting portions of the universal chaos in which order has not yet been introduced.

"The first spiral nebula noticed in the telescope was disclosed in April, 1845, by the great reflector of Lord Rosse. . . . after Lord Rosse's unexpected discovery in 1845 and his publication of a list of fourteen of these objects five years later, no further advance of importance was made for nearly forty years.

"New interest, however, was awakened. by Dr. Isaac Roberts' unexpected discovery, in 1887, that the great nebula in Andromeda is really annular or spiral in character, with dark lanes between the whirls. . . . But it was chiefly Keeler's work at the Lick Observatory, with the Crossley Reflector, which emphasized the prevalence of the spiral form as most typical of the nebula."

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