Another theory has been that it would perish by a comet. That it will not be by the shock we have already seen from the light weight of the comet and from experience; but it has been suggested that the gas may combine with the air, and an explosion take place that would destroy us all; but is not that also contradicted by experience?
Another idea is that we shall finally fall into the sun by the resistance of the ether to our motion. Encke's comet loses in thirty-three years a thousandth part of its velocity. It appears then that we should have to wait millions of centuries before we came too near the sun.
In reality, however, we are simply dependent on our sun, and our destiny depends upon that.
In the first place, in its voyage through space it might encounter or come within the range of some dark body we at present know nothing of, and the attraction might put out of harmony all our solar system with calamitous results. Or since we are aware that the sun is a radiating body giving out its heat on all sides, and therefore growing colder, it may one day happen that it will be too cold to sustain life on the earth. It is, we know, a variable star, and stars have been seen to disappear, or even to have a catastrophe happen to them, as the kindling of enormous quantities of gas. A catastrophe in the sun will be our own end.
Fontenelle has amusingly described in verse the result of the sun growing cold, which may be thus Englished:—
"Of this, though, I haven't a doubt,
One day when there isn't much light,
The poor little sun will go out
And bid us politely—good-night.
Look out from the stars up on high,
Some other to help you to see;
I can't shine any longer, not I,
Since shining don't benefit me.
"Then down on our poor habitation
What numberless evils will fall,
When the heavens demand liquidation,
Why all will go smash, and then all
Society come to an end.
Soon out of the sleepy affair
His way will each traveller wend,
No testament leaving, nor heir."
The cooling of the sun must, however, take place very gradually, as no cooling has been perceived during the existence of man; and the growth of plants in the earliest geological ages, and the life of animals, prove that for so long a time it has been within the limits within which life has been possible—and we may look forward to as long in the future.
It is not of course the time when the sun will become a dark ball, surrounded by illuminated planets, that we must reckon as the end of the earth. Life would have ceased long before that stage—no man will witness the death of the sun.
The diminution of the sun's heat would have for its natural effect the enlargement of the glacial zones! the sea and the land in those parts of the earth would cease to support life, which would gradually be drawn closer to the equatorial belt. Man, who by his nature and his intelligence is best fitted to withstand cold climates, would remain among the last of the inhabitants, reduced to the most miserable nourishment. Drawn together round the equator, the last of the sons of earth would wage a last combat with death, and exactly as the shades approached, would the human genius, fortified by all the acquirements of ages past—give out its brightest light, and attempt in vain to throw off the fatal cover that was destined to engulf him. At last, the earth, fading, dry, and sterile, would become an immense cemetery. And it would be the same with the other planets. The sun, already become red, would at last become black, and the planetary system would be an assemblage of black balls revolving round a larger black ball.
