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as astronomers have been led to believe of late years. Certainly there is small risk from any effects of collision with comets, since we have learned that the mass even of the largest comet is inconsiderable. But the intermixture of cometic matter with the atmosphere of our earth might not always be a perfectly innocuous process. We do not as yet know what are the gases which are the chief constituents of the vaporous parts of comets, nor the proportion in which solid and liquid matter may be present in the heads and nuclear parts of comets. But there are few gases which, added in very large quantity to our air, would do no harm; and any considerable addition of solid matter, first vaporised and afterwards settling as a fine dust through the air, might severely injure at least those of delicate constitution.

There are some eventualities, indeed, which are very startling to contemplate, yet not altogether inconceivable. Suppose, for instance, that a comet composed in the main of hydrogen should mix with our air, until the oxygen of the air and the hydrogen of the comet were in the proportion in which these gases are present (chemically combined) in water. Then unless every fire and light in the whole earth were extinguished there would be a tremendous explosion, followed instantly by a deluge of water, and leaving the burnt and drenched earth no other atmosphere than the nitrogen now present in the air, together with relatively small quantities of deleterious

vapours.

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We need not greatly trouble ourselves, however, with the fear of such a calamity. If hydrogen comets were common, the earth would probably have encountered one long ago, in which case her condition would certainly not be such as it is at present. It is, however, not at all improbable that the earth is not merely exposed to mischief, but that her inhabitants have actually suffered, more or less seriously, at intervals, from the meteoric. and cometic matter which falls upon her as she journeys onward through showers.1

1 The medal commemorating the great plague of London, which was associated, it will be remembered, with the appearance of a comet, pictures the comet as scattering pestilence from its tail. According to this curious medal, pestilence was scattered in the form of small pothooks. Defoe's remarks about this comet, and the one which appeared a year after, are curious: A blazing star or comet appeared,' he says, 'for several months before the plague, as there did the year after, a little before the fire; the old women and the phlegmatic hypochondriacal part of the other sex, whom I could almost call old women too, remarked, especially afterwards, though not till both those judgments were over, that these two comets passed directly over the city, and that so very near the houses (!!) that it was plain they imported something peculiar to the city alone; and the comet before the pestilence was of a faint dull languid colour, and its motion very heavy and slow; but that the comet before the fire was bright and sparkling, or as others said, flaming, and its motion swift and furious: and that accordingly one foretold a heavy judgment, slow but severe, torrible and frightful as was the plague; but the other foretold a stroke sudden, swift, and fiery, as was the conflagration; nay, so particular some people were, that as they looked upon that comet preceding the fire, they fancied they not only saw it pass swiftly and fiercely, and could perceive the motion with their eye, but even they heard it, that it made a rushing mighty noise, fierce and terrible, though at a distance and but just perceivable. I saw both these stars, and I must confess, had had so much of the common notion of such things in my head, that I was apt to look upon them as the forerunners and warnings of God's judgments, and especially when the plague aal followed the first, I

In my two next chapters I shall consider how the earth and other planets may be regarded as growing under the continual downfall of meteoric matter; and how, although this growth must be regarded as exceedingly slow in these our times, yet in past ages the members of the sclar system, and the sun himself, must have grown appreciably in volume and mass under the continual influx of meteoric matter.

yet saw another of the like kind, I could not but say, God had not yet sufficiently scourged the city.'

HOW THE PLANETS GREW.

I.

IN considering the wonderful processes which are taking place within the limits of the solar system, and still more in endeavouring to trace back the course of events during former ages, we find ourselves surrounded by a hundred sources of perplexity. Nor is it to be expected, perhaps, that we should be able, either now or at any future time, to form clear ideas on a subject so involved in mystery. The powers of man-I am speaking of the race, not of individual men-are wonderful. The Almighty has enabled them to deal successfully with many problems which, even now that they have been solved, seem as though they had been placed beyond all reasonable hopes of mastery. But it has not been given to man to solve all the mysteries that surround him, and it may well be questioned whether it will ever be in his power to solve that great mystery, the origin of the wonderful scheme of worlds of which our earth is a member.

Yet there are steps which man can fairly hope to make on the path leading towards the great secret. There are

processes still taking place which he can gauge and measure, thence inferring the probable nature of the corresponding processes in long past ages of the world's history. There are signs which are full of meaning, traces which can be followed for a great way--we do not yet know how far.

It is certainly a legitimate exercise of the powers given to man to follow out those paths, whether well marked or as yet little trodden, which seem likely to lead to new knowledge. We need not be troubled by doubts as to the way in which such paths may lead us, so that they really lead to the recognition of facts. We may learn many things inconsistent, perchance, with our present ideas as to the way in which it has pleased the Almighty to provide for His worlds. We may have to abandon some conceptions which had appeared very accordant with the might and wisdom of the Creator. But we may be sure of this, that whatever new ideas we may legitimately be led to, will prove not less worthy of Him. Increase of knowledge of His universe-whether of its various parts or of the various periods of its history-will enhance our conceptions of His power and wisdom, though still leaving those conceptions infinitely poor and feeble compared with the reality.

I make these preliminary remarks, because, strangely enough, many persons of religious mind seem afraid to enter upon the course here indicated, and follow with unwilling footsteps those who try to advance some short

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