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which it appears to be to the naked eye, or when viewed in the telescope, but that it is a double sun, the two bodies revolving continuously about their mutual center of mass. These hundreds of binary systems are so far away that even under the highest telescopic magnification they blend into a common and essentially mathematical point. It is the expectation that the future, possibly the present century, will establish that one star in three, on the average, is a double solar system. It may even prove to be the truth that our solar system, consisting of one great central sun and many attendant planets, is not the average and prevailing system, but is the exception and not the rule. However, we have no good reason to doubt that tens of thousands, more probably tens of millions, of distant suns are the centers of planetary systems, and that countless planets are the abode of life. As our sun is but one of hundreds of millions of suns, it is absurd and essentially inconceivable that our planet, or two or three of our planets, should be the only bodies throughout the universe supporting life. It is vastly more probable that if our vision could penetrate to other stellar systems, lying in all directions from us, we should there find life in abundance, with degrees of intelligence and civilization from which we could learn much, and with which we could sympathize. The spectroscope proves absolutely that dozens of chemical elements in the earth's surface strata exist in our sun : that iron, the silicon of our rocks, hydrogen, helium, magnesium and so forth exist in the distant reaches of our stellar system. If there is a unity of materials, unity of laws governing those materials throughout the universe, why may we not speculate somewhat confidently upon life universal?

In the days of my youth, here in northern Ohio, the opinion prevailed throughout the community, and widely over the earth, that comets were the forerunners of wars, plagues or other forms of dire distress. Did not the great comet of 1811 herald the war of 1812, and that of 1843 the Mexican War and Donati's comet of 1858 our Civil War.? Even in the twentieth century the fear that a comet may collide with the earth and destroy its inhabitants comes to the surface, here and there, every time a comet is visible to the naked eye. The findings of astronomers concerning these visitors to our region of space have taught that we have nothing to fear from them, and that their close approaches may be welcomed, for they are interesting members of our sun's family. They revolve around our sun as the planets do, and render unto it homage and obedience. It is undoubtedly true that the earth has plunged through the tails of comets many a time and without appreciable effects upon our health and happiness. In fact, the inhabitants have at the time been blissfully unaware of the passage. It is true that a collision of the con

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densed head of a comet with the earth is not impossible; it may some time occur; but comprehensive studies of this question, based upon observational data concerning many of these bodies, lead indubitably to the conclusion that we must not expect these collisions to occur, on the average, more than once in 15 or 20 million years. The so-called shooting stars, which we have all observed in the night sky, are in many cases, perhaps in all, though we do not know, the burning of minute pieces of comets which have disintegrated and disappeared as comets forever from our sight. Colliding with the earth, rushing through the upper strata of our atmosphere with speeds up to 40 or more miles per second, the frictional resistance of the air heats them to the burning point, and they are turned into ashes and the vapors of combustion. A very few get through to the earth's surface and are found and placed in our museums. It is not certain that any of those in the museums are parts of disintegrated comets, but some of them probably are. The number of small foreign bodies which collide with our planet every day is very great; a conservative estimate is 20,000,000. Except for our beneficent atmosphere man would suffer many tragedies from the bombardment. There is reason to believe that the earth is growing larger very slowly, from these accretions, and this may have been the process by which the earth grew from a small nuclear beginning up to its present size.

Astronomers have determined that our solar system is very completely isolated in space. We are widely separated from our neighbors. I shall not try your patience by quoting the tremendous distances in miles, for they are incomprehensible to all of us. Rays of light sent out by the sun require a little more than eight minutes to reach the earth. The outermost known planet in our system, Neptune, would be reached in four hours and a half. Rays of light leaving the sun at the same time and traveling at the same rate, 186,000 miles per second, must travel continuously during four years and a half to reach our nearest known neighbor in space, the bright double star, Alpha Centauri. If the distance from the sun to the earth is 1, the distance to our outer planet is 30, and the distance to Alpha Centauri is 275,000. There appears to be an abundance of room in the great stellar system to meet the requirements of all. The spectrograph attached to the Lick telescope has determined that our sun and its family of planets is traveling through the great stellar system with a speed of 12 miles per second, equivalent to 400,000,000 miles per year. The earth is certainly hundreds of millions of years in age, the sun is no doubt at least as old, and the early youth of the earth was lived, not where we now are, but far

elsewhere in the stellar system; and its future journeyings will lead to quite other points of observation.

The question of greatest interest to present-day astronomers is that of stellar systems other than our own. The chances seem strong that the hundreds of thousands of spiral nebulæ known to exist, in very distant space are other and independent systems of stars. many of them perhaps containing as many stars as our stellar system. In other words, our stellar system may be but one of hundreds of thousands of isolated stellar systems distributed through endless space. This is not an established fact, but the evidence seems to run in its favor.

I have referred to some of the problems and results of astronomical science. The list of interesting items is a long one, but available time has its limits. In brief, it is the astronomer's duty to discover the truth about his surroundings in space, and make it a part of the knowledge of his day and generation. The ultimate and real value of his work lies in its influence upon the lives of the people of the world, in the change for the better which it induces in their modes of thought, and in the impulse which it gives to an advancing civilization.

Would that the attractions of the sky to the average man were more potent. It is a curious comment upon the attributes of city life that hundreds of thousands of people, especially children, in London and Paris, in the darkness which gave them semiconcealment from the enemy's destructive airships, should have obtained their first real vision of the starry heavens. What must have been their sensations? On the other hand, those who can view its beauties and wonders are prone to neglect it; to look down instead of up. Emerson has said somewhere in his immortal essays that if our sky should be clear of clouds but one night in a century, the people of this globe would look forward to the rare event, and not only prepare to behold its beauties themselves, but make sure that their friends far and wide were likewise minded. How the beauties of the night sky would surpass the expectations of the most lively imagination! The wondrous vision would be the prevailing subject of conversation for years and years, and the repetition of the vision, 100 years later, would need no advertising.

Our knowledge of the heavens is in its infancy. We have but made a start upon the discovery of the truth about the stars, and the results of astronomical research are not so widely known amongst the people as they should be. This splendid institution, The Warner & Swasey Observatory, presented by men who were masters in telescope and observatory design and construction, by men who have thought much of relative

values in life, this institution has a field of great usefulness lying before it. In their administration of the generous gift, the trustees, the president and the faculty of the Case School of Applied Science, whether for research, for school instruction or for community education, will have the sympathetic interest of astronomers, of all lovers of the truth. This observatory may assist in the solution of important problems concerning the universe of which we form a part. The universities, the colleges, and the technical schools of our country, and of other countries, are graduating every year many hundreds of young men, ready to start upon the more serious phases of their lives, who can tell us all about the lights in our houses, but not one word about the lights in our sky. This institution will do its quota in approximating to a liberal education. The casual visitor who enters its portals in search of knowledge, yea, the passerby in the street who merely sees a dignified and purposeful observatory set upon a hill, will have his thoughts directed to higher levels.

COSMOGONY AND STELLAR EVOLUTION.1

By J. H. JEANS, F. R. S.

EVOLUTIO

I. THE EVOLUTION OF GASEOUS MASSES.

The progress of observational astronomy has made it abundantly clear that astronomical formations fall into well-defined classes: they are almost "manufactured articles" in the sense in which Clerk Maxwell applied the phrase to atoms. Just as atoms of hydrogen or calcium are believed to be of similar structure no matter where they are found, so star-clusters, spiral nebulæ, binary stars are seen to be similar, although in a less degree, no matter in what part of the sky they appear. The problem of cosmogony is to investigate the origins of these comparatively uniform formations and the process of transition from one class to another.

In attacking this problem the cosmogonist of to-day stands upon the shoulders not only of previous cosmogonists, but also, what is of even greater importance, upon the shoulders of the brilliant and industrious astronomical observers of the past century. We shall find it convenient to take as our starting point the most famous theory of cosmogony ever propounded-the nebular hypothesis of Laplaceand we shall examine to what extent it remains tenable in the light of modern observational and theoretical research.

Laplace's hypothesis referred primarily to the genesis of the solar system, which he believed to have originated out of a hot nebulous mass that shrank as it cooled. The nebula was supposed to be in rotation, so that the principle of conservation of angular momentum required that as the mass cooled its speed of rotation should increase. It is well known that a mass either of gas or of liquid in rotation can not rest in equilibrium in the spherical shape which would be assumed in the absence of rotation. If the rotation is very slow the equilibrium shape will be an oblate spheriod of small eccentricity. As the rotation increases, the ellipticity will increase, but it is found that the spheroidal shape is soon departed from. Laplace believed, as a matter of conjecture rather than of reasoned proof, that with con

1 Lectures delivered at King's College, London, on May 3 and 10, 1921. Reprinted by permission from Nature, June 30 and July 7, 1921.

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