Of the inner planets, Mercury and the moon seem destitute of any considerable atmospheres, and are therefore lacking in the supply of water necessary for life, and the same is almost certainly true of all the asteroids. There remain Venus, Mars, and the satellites of the outer planets, which latter, however, we must drop from consideration as being too little known. On Venus there is an atmosphere probably containing vapor of water, and it is well within the range of possibility that liquid water should exist upon the surface of this planet and that its temperature should fall within the prescribed limits. It would, however, be straining our actual knowledge to affirm that such is the case, or to insist that if such were the case, life would necessarily exist upon the planet. On Mars we encounter the fundamental difficulty of temperature already noted in § 152. If in some unknown way the temperature is maintained sufficiently high for the polar caps to be real snow, thawing and forming again with the progress of the seasons, the necessary conditions of life would seem to be fulfilled here and life if once introduced upon the planet might abide and flourish. But of positive proof that such is the case we have none. On the whole, our survey lends little encouragement to the belief in planetary life, for aside from the earth, of all the hundreds of bodies in the solar system, not one is found in which the necessary conditions of life are certainly fulfilled, and only two exist in which there is a reasonable probability that these conditions may be satisfied. CHAPTER XII COMETS AND METEORS 158. VISITORS IN THE SOLAR SYSTEM.--All of the objects--sun, moon, planets, stars--which we have thus far had to consider, are permanent citizens of the sky, and we have no reason to suppose that their present appearance differs appreciably from what it was 1,000 years or 10,000 years ago. But there is another class of objects--comets, meteors--which appear unexpectedly, are visible for a time, and then vanish and are seen no more. On account of this temporary character the astronomers of ancient and mediæval times for the most part refused to regard them as celestial bodies but classed them along with clouds, fogs, Jack-o'-lanterns, and fireflies, as exhalations from the swamps or the volcano; admitting them to be indeed important as harbingers of evil to mankind, but having no especial significance for the astronomer. The comet of 1618 A. D. inspired the lines-- "Eight things there be a Comet brings, When it on high doth horrid range: Wind, Famine, Plague, and Death to Kings, War, Earthquakes, Floods, and Direful Change," which, according to White (History of the Doctrine of Comets), were to be taught in all seriousness to peasants and school children.