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O single man, if he spent his life in travel, could hope to gain more than an imperfect know

ledge of a small portion of the earth's surface. No man, whatever his gifts and industry, could hope to solve a millionth of the problems still unanswered concerning the earth and its life as we know it to-day. No one can say how old it is, yet we know that it is of an age so great that we cannot grasp it. We cannot conceive the slow processes that it passed through before it became a fit abode for life at all, nor the countless succeeding ages during which life was assuming its myriad forms. It took these vast stretches of time and these innumerable stages of development to make. the earth and the life on it.

The earth itself is the merest speck in the universe. It belongs to a system of which the star we call the sun is the centre; it depends upon the sun for its light and heat and life. The earth and other planets revolve round the sun, and the whole system, the solar system, as we call it, moves through space.

The sun is one star. There are countless others. Some of these are centres of light and heat like the sun; others are travelling about in space, cold and dark. In relation to the visible universe the solar system is

small; yet the earth is some 98,000,000 miles distant from the sun, and takes a measurement of time which we have called a year to travel round it, while Neptune, an outer member of the system, takes not one of our


THE GREAT NEBULA IN ORION From In Starry Realms,by Sir Robert Ball By permission of Messrs Sir Isaac Pitman and Sons, Ltd.

years, but one hundred and sixty-four, to complete its orbit—that is, the path of its revolution.

Distances in the solar system, then, are so great that the mind cannot grasp them. But they are small compared with the distances that usually separate star from star. For the sun to reach the star that is its nearest neighbour it would take, moving at its estimated speed of about ten miles a second, as far as we can reckon, about 70,000 years, and there is no particular reason for believing that, except in star clusters, the stars are usually closer together than this.

Besides single stars and star clusters, the heavens are sprinkled with luminous smoke-like patches, called nebulæ i.e. fogs or mists. Only a few of these can be seen by the naked eye—the clearest is the faint patch of light round the middle star in the line of three known as the Sword of Orion—but with powerful telescopes about 500,000 are visible. Some of these under the telescope prove to be star clusters—that is, clouds of stars in which the stars appear so close that their light is blent into a common radiance. Others are of very different composition. Some of these are believed to consist of gas so intensely hot that it gives out light, others of luminous solid material surrounded by a darker, cooler, gaseous zone or atmosphere, the latter nebulæ perhaps representing a stage into which the former have passed by cooling. Another theory is that a nebula does not consist of luminous gas, but of countless solid meteorites—those bodies which, entering our atmosphere and heated by the friction of their passage to incandescence, we call shooting stars--which, continually colliding, give off a constant supply of luminous vapour due to the heat generated by collision.

One of the nebulæ visible to the naked eye is the Great Nebula in the constellation of Andromeda. A photograph of it taken by the telescope shows that it consists of a glowing central mass surrounded by a darker outer zone, and that this outer portion is breaking up into rings. There are knots or patches in these rings which may be looked upon as possible future planets, destined to revolve round the glowing centre, which would be their sun. It is probable that the solar system, one member of which is our earth, was once such a nebula, though the exact nature of it is uncertain.


The spinning mass of the sun with its group of spinning planets revolving in their orbits about it, sweeping through space at about ten miles a second, belongs, as far as we can tell, to a star cluster, or cloud of stars, the members of which are comparatively near to one another. The Milky Way, the luminous track across the sky, consists of hundreds of distinct star clusters. The nebulæ are most numerous where the stars are fewest. The solar system, the stars, and the nebulæ make up the visible universe—all, that is, that the telescope brings to our knowledge. Beyond its range there may be a thousand such universes. We have no proof of life on any other member of the solar system. Yet life may be there, resembling the life of the earth, or quite different. There may be life in some form in countless solar systems of countless universes.

The solar system—this infinitely small portion of the visible universe-consists of the sun, eight great planets with their moons, a great number of much smaller planets, certain comets, and innumerable meteorites.

The names of the planets in the order of their distance from the sun are: Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune. They travel round the sun, and the sun keeps them in their course by the power of gravitation-the force of attraction which all bodies exert on one another in proportion to their mass and the distance between them—the force which we know from experience has to be overcome if we are to leave the surface of the earth, and which produces what we know familiarly as weight.

It has been said that we have no proof of any form of life on any planet of the solar system except our

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