Instead, therefore, of starting with the assumption of a "continuous mass of fluid in equilibrium under hydrostatic pressure," Professor See starts with the assumption of a "cloud of cosmic dust. . . a portion of the universal chaos." He assumes streams of this nebulous matter to be drifting hither and thither through space.

"It is well known that the heavens are full of streams; for there are streams of stars drifting in various directions, and of nebulæ also, but not yet fully investigated. Now suppose two nebulous streams collide while going in opposite directions, or that one overtakes the other owing to differences in their velocities, when moving in the same general direction. As the two masses approach, their figures will be distorted and curved, so that the streams will tend to become entangled. A whirling movement will follow, each stream wrapping and coiling up on the other. Both streams will

remain more or less continuous, and the brightest point will be the place of collision, where the two streams meet. From this point the two streams will wind off and finally have their tails projected in opposite directions, showing how they approached before they coiled up about each other. This explanation is simple and direct and follows from well-known mechanical laws. The winding about the origin usually would not be perfectly symmetrical; but in general, such spirals would be fundamentally double, made up of interwound streams issuing in opposite directions from the nucleus, which is the principal point of contact. If, instead of two streams meeting under differences of proper motion, we imagine a single stream of nebulosity which is not perfectly straight left to its own gravitation-this is about equivalent to separate streams already in collision-it is clear that in time it will coil up on itself, and gradually develop into a spiral nebula. Thus it makes little difference whether there be one or more streams, united or distinct, the final result is the same, the development of a whirlpool nebula.”

Professor See contends that in these vast masses of nebulous matter, minute centers of condensation will sooner or later begin to form through "the precipitation of ions. under the influence of X-rays"; and that these minute centers will gradually increase in size by accretion-attracting to them

selves particles of nebulosity in their neighborhood and engulfing other particles with which they come into contact as they acquire proper motions of their own under the influence of gravity. He pictures a whirlpool nebula, even at a comparatively early stage of its evolution, as containing an infinite number of these incipient globes, each of which is moving under its own developing momentum and at the same time following the movement of the coiling stream to which it belongs. And when (as in the comparatively simple case of the Solar System) the coiling nebular streams have finally evolved a single central nucleus that is in time to become the sun, the outer portions of the nebula will swarm with globules, globes, moons, and planetoids of varying size, moving in varying directions of their own, yet also in the general direction of the original whirlpool; in other words, revolving about the central controlling mass in orbits of every conceivable eccentricity and inclination.

But they will all be moving against the slight friction of the resisting medium furnished by the nebulous matter still pervading the entire system.

This is, in reality, the vital point of the author's argument. For if a smaller body, like a planet, approaches a larger body, like the sun, so that it is caught and held within that larger body's sphere of influence, its orbit round the larger body (which is automatically determined by its speed, mass, etc., etc.) will never be changed unless affected by some outside interference. And, as it has always been assumed that the heavenly bodies moved in a practical vacuum, no such external influence sufficient to account for the gradual reduction of parabolic and elliptical orbits to circular or near-circular ones has ever been found. Note that it was in reality the unquestioning belief that the planets move and always must have moved in a practical vacuum that drove Laplace to seek for a theory of planetary evolution that explained the circularity of their orbits as an original feature of their genesis. But it was not a doubt as to the effect of such a medium, if one could be postulated, that embarrassed investigators.

"The effect of a resisting medium in reducing the major axis and eccentricity of the orbit of the resisted body is fully recognized and has been known for more than a hundred years."

". . . . As the resisting medium is a vera causa, on the secular effects of which all mathematicians are agreed, we may hold that it has as surely rounded up these orbits as if we had witnessed the transformation within the short period of human history covered by exact observations."

"It will doubtless appear very remarkable to future generations," remarks Professor See, "that we should not earlier have perceived that the roundness of the orbits. of the planets and satellites was to be explained by the secular action of a resisting medium and by no other cause whatever. The force and accord of traditional opinion, however, were so great that for a long time no one recognized that a false premise had entirely vitiated Laplace's original reasoning. Babinet's criterion" (a mathematical demonstration of the fact that the solar nebula never has revolved with sufficient rapidity to detach the supposed planetary rings) "lay buried in the Contes-Rendus, and remained unknown to Kelvin, Newcomb, Darwin, Tisserand, Poincarré, and other modern investigators."

So much for the vital point in the author's argument. We now come to the most striking feature of the resultant theory, namely, that instead of the planets being the children of the Sun, and the satellites the children of the planets, they are all brothers, and children of the same nebula. They are one and all, from the Sun itself to the smallest meteor that is "captured" by the Earth, survivors of the original nebular concentrations and 'differ not so much, if at all, in age as in the history of their immemorial careers. As the author puts it in one place:

"We have seen how a nebula grows by the gathering together of cosmical dust. Much of this is very fine, but some of it collects into larger nuclei, and thus arise bodies such as the satellites or planets. Now, in an immense nebula the condensation goes on at an infinitely great number of centers, and the solid globes thus arising are analogous to our Moon. When the larger of these globes have gathered together a vast number of moons and united them into one mass, the result is a planet. But in the early history of a nebula, multitudes of these moons and planets go into the central body to form the governing

sun.

"On the one hand, this builds up the Sun's mass, while on the other, it clears up the inner parts of the nebula. The result is that only planets with orbits of large original perihelion distance survive, and even they have their mean distances and eccentricities greatly reduced. Accordingly, we perceive that the surviving planets were formed at a great distance from the Sun, or in the outer parts of the solar nebula."

The cosmic panorama of a systematized order evolving out of a nebular chaos that this volume presents to our minds is so huge and so difficult to reduce to a summarized statement without destroying its impressive implications of unthinkable time, unimaginable space, and indescribable confusion, that we will perhaps do well to take a few extracts from its author's chapters dealing with the presumable history of the Moon as a guide to our imaginations.

"The true mode of origin of the so-called craters on the Moon has long been a matter of discussion. . . . Nevertheless, it has been generally held that these craters are of volcanic origin. . . . Humboldt remarks with surprise that the central mountains have been found by Schmidt always to lie below the walls by which they are surrounded. According to this astronomer's investigations, it appears that no single central-mountain attains the height of the wall of its crater, but that in all cases it probably lies . . considerably below that surface of the Moon from which the crater is supposed to have been erupted. . erupted. . . . But while the volcanic theory of the Lunar craters is still very generally taught in the universities and other schools of Europe and America, a few sagacious thinkers have always hesitated to accept this orthodox interpretation of the most. remarkable phenomena presented by the Moon's surface. In his Popular Astronomy, edition of 1878, Newcomb hints at a new theory of the Lunar craters, as follows: 'The Mountains consist, for the most part, of round saucer-shaped elevations, the interiors being flat, with small conical mounds rising here and there. Sometimes there is a single mound in the center. It is very curious that the figures of these inequalities in the Lunar surface can be closely imitated by throwing pebbles upon the surface of some smooth, plastic mass, as mud or

mortar." . . . . We now come to a statement of the general theory of the so-called Lunar craters, namely, that they are simple satellitic indentations in a surface of loose and largely uncemented cosmical dust and fragmentary rock, and not volcanic at all. It It is to be remembered that the Moon's gravity is only about one-sixth that of the Earth, and therefore any layer of dust or rock on the Moon would be less settled than on the Earth in this proportion; so that the material would be only very slightly compressed by gravity. Moreover, as there has never been any water or other fluid on the Moon, except perhaps melted lava in places, there has been no chemical agency which could cement the dust or fragmentary rock into a coherent mass, and it has therefore retained the form of a loose mass of stony character, somewhat analogous to the ashes blown out of a terrestrial volcano. Falling satellites, whether large or small, could therefore easily indent the surface by compression of the uncemented material under the force of impact.

"Accordingly, if a satellite fifty miles in diameter collided with the Moon, it would sink down into the soft and uncompacted surface, and at the same time be flattened and spread out at the base. This flattening and basal spreading of the satellite would make a broad saucer-shaped crater, steepest on the inside and surrounded by more or less débris driven out and scattered by the force of the impact, exactly as shown by the Lunar craters. In flattening and spreading at the base, the satellite would force the walls of the crater outward, and itself be reduced to fragments, resting on a fluid base, with the highest peak in the center. This is exactly the aspect presented by the larger craters. . . . The considerable number of craters that are simple depressions without sensible walls are to be explained by the comparative looseness of the material of the Moon's surface layers -which allows the mass to yield downward without throwing up much of a wall about the depression produced. . . . The clefts. are paths cut by glancing satellites, which thus leave a straight or curved line, according to the nature of the surface and the resistence and rebound. . . . The covering up of ancient cities on the Earth is due to deposits of waste, rubbish, and dust traceable to meteorological causes connected with the atmosphere, such as sand borne

by the wind from the desert, etc. On the Moon, however, there is no atmosphere sufficiently dense to carry dust, and it must therefore be scattered by impacts and by direct descent from celestial space. The fact that the older craters are visibly covered up is a tangible proof of the important part played by cosmical dust in the course of ages. . . . It might be asked why, if the Lunar craters are due to the impact of satellites, the Earth does not exhibit batterings and indentations of the same kind. We answer that the geological changes on the Earth, due to the actions of the oceans and the atmosphere, have entirely obliterated the original indentations of our globe; and since the continents were formed, the Earth has suffered no important collisions with satellites, because the region in which it moves was long ago cleared of small bodies. This again indicates that the conspicuous indentations still visible on our Moon probably were made prior to its capture by the Earth; which affords us an impressive illustration of the enormous age of the Solar System. . . . As the Moon could not at present retain air or water, it is obvious that it has never had these elements. . . . And as our Moon is a captured planet on which surface changes are nearly insensible, owing to the absence of water and atmosphere, the study of its well-preserved surface phenomena is of the highest importance for throwing light upon the terrific process of planetary development by the capture of satellites."

According to the Capture Theory, then, the Solar System as we see and know it to-day is the central portion (Professor See estimates that the orbit of Neptune represents perhaps one-thirty-third of the diameter of the parent nebula) of the original whirlpool nebula, swept, garnished, and reduced to order. Each of the planets and satellites has done its part of the sweeping and has had its orbit more or less completely reduced to circularity in the process. And the result has left the system so nearly free from nebulous matter and small particles that their action as a resisting medium has escaped observational recognition. But the asteroids survive to testify to the millions of small planets that formerly whirled about the central nebular mass and later filled the whole of the present planetary system; and one has but to watch the sky for

meteors on any summer night to see the capture of still smaller surviving bodies being consummated and the final sweeping in process being carried on. And when we look at the pictures of the battered surface of the Moon in Professor See's volume, it almost seems as if we could hear an echo of those titanic impacts, see the turmoil of the resulting upheavals, and follow the long and perilous journey of our sidereal companion from the outer regions of its origin, through planetoid-filled space and past threatening major planets, to its final surrender to the attraction of the Earth.

Of course, this is but the flimsiest and most inadequate outline of Professor See's theory as developed in this stupendous volume of his; and it entirely ignores scores of vital and frequently fascinating problems that he has tackled and disposed of by the way. Let us take a look at one of thesethe problem of the retrograde satellites of Jupiter, the discovery of which threw so much doubt upon the validity of the Laplacian hypothesis. The author points out that in a system made up of two equal stars, or of two unequal bodies like the Sun and Jupiter, the space about them is divided into three spheres of gravitational influence, -one near each body and completely dominated by it, and one about both of them together and controlled by them conjointly. This is graphically illustrated by the diagram on page 1668. If, now, a small body from without enters the joint sphere of influence at a speed that does not enable it to escape, it will begin to revolve about both, in an orbit represented by the outermost line of the diagram. Later on, under the influence of the resisting medium pervading the system, it will drop down to the second line of indicated orbit. Later still, it will drop down to the third line, then to the fourth, and so on. finally, it will reach a point where, instead of traveling round, both it will begin to cut a figure eight, circling first one and then the other. And although, when it first entered the system, it was traveling in the same direction around the Sun as all the other bodies of the nebular family—that is to say, in the direction assumed by the original whirl of the nebular vortex-it only requires a moment's study of the diagram to see that when it ultimately abandons its figure eight orbit and drops down into the closed sphere of influence of either the Sun or of Jupiter,

And

it is entirely a matter of chance as to whether it assumes a direct or a retrograde motion about its chosen center.

"But why," we are inclined to ask at this point, "if this is the case, is it that there are not as many anti-clock-wise revolutions in the solar system as clock-wise ones?"

The answer is absurdly simple. The retrograde revolution of a small planet around the sun, or of a small satellite around a planet is always the result of a special mix-up between the joint influences of the Sun and the planet as described above. All bodies captured directly by the Sun and all bodies captured directly by the planét are revolving in the normal direction of the original spiral nebula. The occasional retrograde body therefore finds itself moving against the stream and is almost certainly destroyed in collision. Only here and there, in the sheltered closed space about a big planet does a retrograde satellite survive.

And here, perhaps, we might be content to leave the matter; calling attention to the photographs of spiral nebulæ reproduced from the fine plates in Professor See's volume, and to the startling evidence they present to us of the actual existence and that in every stage of development of the stupendous processes here outlined. But to do so would be to glimpse but one side of the author's farreaching speculative grasp. For he has not only reversed and reorganized the Laplacean doctrine of the Solar System's mode of development from a nebula, but has promulgated a logically constructed and ably defended theory that seems to bring into one gigantic and unbroken sequence the most outwardly dissimilar systems of the stellar universe and even to suggest the possible order of that universe itself.

He not only traces the double star systems, disclosed by our telescopes and spectroscopes, to an analyzed nebular origin; but applies the same theory to the infinitely complex star clusters that are scattered along the Milky Way, and argues that these huge and powerful centers of gravitational attraction must inevitably be applying the capture process to independent stars that approach them in the heaven. He points to the visible breaking up of the Milky Way itself as the result of the capture process, carried to its highest power, and shows that, if the stellar universe itself

is a whirlpool of a higher order, and if our sun with its satellites lies at the point indicated by the bright dot, the exact configuration of the heavens as we see it would be explained.

And finally, arguing from the demonstrated repulsive power of light and electric energy upon matter, he concludes that just as the star systems are developed from nebulæ, so the nebulæ themselves are gradual accumulations of the cosmic matter expelled by these repulsive forces from the stellar systems.

"The past ten years have brought to light the most conclusive evidence of the prominent part played by the repulsive forces in nature. The delicate laboratory experiments independently made by Lebedow, Nichols, and Hull, have established the radiation pressure of light on an observational basis, and this result has been confirmed by the further theoretical researches of Arrhenius, Schwartzchild, and others. . . . Moreover, the observational researches now being made by various astronomers of the visible repulsion from the Sun of the matter in the tails of comets, are continually adding specific examples of cosmical repulsion based on the exact measurement and photography of celestial objects.

If, then, very finely divided matter is continually being expelled from all well-developed stars, . . . it manifestly suffices to imagine these fine particles drifting hither and thither throughout the universe, and here and there forming immense clouds of cosmical dust. Thus have the nebulæ arisen."

In the illustration on page 1667, we see a map of one hemisphere of the heavens. The small crosses indicate the position of catalogued star clusters. The small dots indicate the position of catalogued nebulæ. The Quaker Meeting arrangement of this celestial congregation is patent to the most casual eye. But it requires a See to make the following deduction from it:

"That this extraordinary distribution of the nebulæ, as far as possible from the stars of the Milky Way, has some deep physical significance can scarcely be doubted. What then is the meaning of the observed distribution? To answer this question in the simplest and most unbiased manner, we may ask ourselves how Democritus, Anaxagoras, Aristotle, or any of the Greek nat

ural philosophers would have answered such. a question, had they known that cosmical dust is constantly expelled from the stars. by electric forces and by the radiationpressure of their light and driven away from the Milky Way, which they also knew to be composed of small stars too dense to be seen individually? Can any one doubt that the Athenian sages would have said that the nebulæ are formed of cosmical dust expelled from the stars, and are therefore located as far away from the Milky Way as possible, being collected principally at its poles?

"It is clear that the Physical Universe is governed by two antagonistic principles, both constantly at work: one being the secular condensation of matter under gravity to form stars and systems, to be followed eventually by another of the opposite character; namely, an expulsion of cosmical dust, under electric forces and the radiationpressure of the light of the stars, to form nebulæ. . . .

"It follows that there is a cyclic process by which stars and systems arise from nebulæ, while nebulæ in turn are formed from the stars."

It is evident that the Capture Theory comes very near to fulfilling the three conditions which we have seen that a new scientific hypothesis dealing with questions of general concern must meet. It brings into apparent harmony all the outwardly discordant discoveries that have been made since the old theory was put forward. It pushes speculative constructiveness far enough ahead to insure itself a lease of life while its myriad implications are being tested. And it gives to the whole mass of these observations and speculations a dramatic unity that at once. fires the imagination and satisfies the reason. And meanwhile, that must indeed be an earth-bound and unresponsive mind that "takes no interest in astronomy" when astronomy presents to it so vast, so ordered, and so magnificently inspiring a panorama as the one Professor See here opens out for our contemplation; "enabling an ephemeral mortal, dwelling on a tiny planet attached to a sun of inferior magnitude, to survey the uncounted millions of similar suns scattered throughout the immensity of space, and trace the progress of an orderly development extending over immeasurable eons of time."