we may be respecting catastrophes threatened as nearer at hand-on account of their improbability.1

Admitting the possibility that, at the remote epoch when the change has been effected, there may be reasoning beings upon this earth, we may accept the fanciful ideas suggested by Dr. Ball. 'Our remote posterity,' he says, 'will have a night 700 hours long, and when the sun rises in the morning, 700 hours more will elapse before he can set. This,' he adds (though we should suppose he can hardly be very confident on this point), 'they will find a most suitable and agreeable arrangement. They will look back on our short periods of rest, and short periods of work, with mingled curiosity and pity. Perhaps they will even have exhibitions of eccentric individuals able to sleep for eight hours, work for eight hours, and play for eight hours. They will look on such curiosities in the same way as we look on the man who undertakes to walk a thousand miles in a thousand hours.' ('All which propositions,' as Carlyle words it, ‘I, for the present, content myself with modestly but peremptorily and irrevocably denying.')

But although, immediately after telling us these things, the Astronomer Royal for Ireland adds, 'I am beyond all things anxious to give you the impression that I am not indulging in any mere romance,' we may indeed place a great deal more reliance on what he says later respecting the evidence given by the moon's present rate of rotation. It is utterly incredible that the moon, when first formed, no

1 It has been stated in the Spectator that I believe in the probability that all life will be destroyed from off the face of the earth a few years hence. This was at least news to myself. I have discussed the probability that a certain comet will be absorbed by the sun, mentioning some one else's suggestion that such destruction might be effected a few years from the present time; but I have also been careful to explain that what has already happened in the case of this very comet, shows how very small is the chance that the final absorption of the comet will in any way affect the earth's inhabitants. I have scarcely ever mentioned such fears except to ridicule them.

matter what theory of her formation we accept, rotated anything like so slowly as she does at present. It is to all intents and purposes certain that then-whenever 'then' was-she rotated in much less than 24 hours. Now she requires 27 days for each rotation. There is here evidence of an enormous amount of work done by the earth in raising and maintaining lunar tides, for by such work alone could the moon's rotation rate have been changed to what it now is.

Whether the moon formerly had oceans, as most astronomers believe, or not, matters little. We see from her present aspect that she was once intensely hot, insomuch that the greater part of her substance, if not fluid, must have been viscous and plastic. In that plastic mass the earth raised tidal vibrations, swaying the moon's rotation rate into accordance with her period of revolution round the earth. In the constancy with which the unjustly called 'inconstant moon' turns ever the same face towards the earth, we recognise the long-continued action of these tidal vibrations. As Dr. Ball well says-Those tides have ceased for ages; their work is done; but they have raised a monument in the moon to testify to the tidal sufferings which the moon has undergone.'

What the earth has done, effectively though slowly, to the moon, the moon will do as effectively, though even more slowly, to the earth. It is this cause of change, of the efficiency of which the moon's calm face is ever speaking to us, that will produce the lengthening of the day, and of the lunar month, which we have already considered.

I do not altogether agree with Dr. Ball as to the future of the earth and moon lying beyond the sufficiently distant future to which we have already carried our thoughts. He points out that besides the lunar there is a solar tide, and that after the former has done its work in bringing the earth's rotation period to coincidence with the lunar month, the latter still checking the earth's rotation, will cause the terrestrial day to exceed in length the lunar month. He

considers that in the case of Mars's internal satellite such a change has already been brought about, the satellite revolving around Mars in a period shorter than that of the planet's rotation. It appears to me that the two cases are not analogous. The mystery of the inner satellite, Dr. Ball tells us, 'has never been explained: it is due to the action of the solar tides on Mars; nay, more, we can actually foresee that at some incredibly remote future time our earth and moon are destined to present the same movements which have seemed so anomalous in Mars.' He appears to overlook the effects which the outer satellite would tend to produce, and also what we notice in the case of our own moon. We can readily understand how, with an outer moon travelling in longer period, the Martian day would have increased in length beyond the time of the inner moon's rotation: whereas we see in our own moon clear evidence that the solar tide has not the power which Dr. Ball here assigns to it—or rather, that whatever effects it may exert in that way, are overborne by greater forces working in an opposite direction. Ever since the moon's rotation-period was brought into agreement (by her earthraised tides) with her period of revolution, she has been subject to the sun's influence in still further lengthening her period of rotation-this influence being somewhat stronger on her than on Mars, despite her smaller globe. Yet during the millions of years that this force has been at work, it has not in the slightest degree availed to lengthen the rotation period beyond the period of revolution. These periods were, and remain, absolutely coincident. The reason is

obvious: the earth has exerted a greater force to prevent such an increase of the moon's period of rotation than the sun has exerted to produce it. In like manner, we may safely conclude that, whenever the moon has wrought the terrestrial day into coincidence with the lunar month, she will continue thenceforth to maintain that coincidenceoverruling all the efforts which the sun will make to still further lengthen the terrestrial day.

For my own part, however, I believe that long before that time arrives, every particle of water will have disappeared from the earth's surface-the seas and oceans being withdrawn into the earth's interior as her mass parts with its heat. That any living creatures will exist on the earth at the remote time to which our thoughts have been carried, seems to me altogether improbable.

55

BIRTH AND DEATH OF WORLDS.

To the child in its nurse's arms the room in which his life is passed seems the whole world. But as the child grows older he finds the room to be but part of a house, the house part of a street, the street part of a city, and so forth. Gradually he gets larger and larger ideas of the earth which is his home. What is true of each individual child, is true of the childhood of the human race. In long-past ages men judged the narrow tract, island, or valley where their lives were passed as the whole world. For that region the heavens were made; the sun rejoiced as a giant to run his course, that he might illuminate that abode by day; the moon was made to be its light by night; sun, moon, and stars to be for signs and for seasons, and for days and years. But gradually men widened their conceptions of the world. They found the home of their tribe to be but part of the region which their race inhabited, this to be but part of a larger region. Wider and wider became their knowledge of the earth, until they found that what they had regarded as the whole world was the merest point on the great globe which is the home of the human race.

But this was little. It may be said that this was nothing. Men had found the earth, their home, to be much vaster than they had imagined; but they presently found that in another sense it is exceedingly minute. They had judged it to be the great fixed centre of the universe, even after they had found that it is not a great level tract of land and water, but a globe. They now found that it is not fixed, but