and so does the skin of our Earth as it shrinks. The mountains and valleys are the crinkles: the shrinkage is always trying to crumple them closer together with a stronger and stronger grip: they may hold out for a time, but at last they give way suddenly and then we have an earthquake. If we knew how much our Earth shrinks each year, we might be able to look forward and calculate how much smaller it will be in a million years from now; but we do not really know enough as yet to make the calculation. Or we could look backward and calculate how much bigger it was a million years ago, or two million or fifty million: and similarly with the Sun. The case of the Sun is easier because we have something to go upon; we can measure the amount of heat he is now giving out, and that tells us how fast he must be shrinking, whereas we have no such information about the Earth. In that way Lord Kelvin calculated backwards and thought he could find how many million years ago the Sun could have started being a Sun at all. You see as you go backwards you must suppose him bigger and bigger until he is spread out so much that he would no longer be like a Sun at all, and Lord Kelvin thought this was about 100,000,000 years ago. But he was assuming all the time that the heat we receive all comes from the shrinkage: that wonderful substance radium had not then been discovered, and there seemed to be no escape from Lord Kelvin's conclusion. Since the discovery of radium, however, the matter has assumed an entirely different aspect; we have learnt of the existence of a new kind of power-house
