Another interesting but groundless hypothesis is that of Adhemar on the periodicity of deluges. This theory depends on the fact of the unequal length of the seasons in the two hemispheres. Our autumn and our winter last 179 days. In the southern hemisphere they last 186 days. These seven days, or 168 hours, of difference, increase each year the coldness of the pole. During 10,500 years the ice accumulates at one pole and melts at the other, thereby displacing the earth's centre of gravity. Now a time will arrive when, after the maximum of elevation of temperature on one side, a catastrophe will happen, which will bring back the centre of gravity to the centre of figure, and cause an immense deluge. The deluge of the north pole was 4,200 years ago, therefore the next will be 6,300 hence. It is very obvious to ask on this--_Why_ should there be a _catastrophe_? and why should not the centre of gravity return _gradually_ as it was gradually displaced? 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.