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attracted to the Sun and falling into him with tremendous speed, would develop enough heat to keep him going. You see, the Sun is giving out enormous quantities of heat; we get a good deal on our Earth, and some goes to Mercury and Venus and the other planets. But vastly more goes straight out into space because there is no planet in the direct line to catch it. From the amount our Earth receives we can calculate the total output, and it is truly terrific, enough to melt in a single second a solid column of ice two miles thick stretching from the Earth all the ninety-three million miles to the Sun. How is this outpouring kept up? One idea used to be that meteors fall in; but it was calculated that so many would be required that the Sun would grow visibly larger, which is not the case. Hence, though meteors may account for a part of the heat, there must be some other supply too. I must mention here that the Sun is not burning like a fire or a gas jet: it is glowing like an electric glowlamp. You know the difference between the two cases: a gas jet or candle gets quickly used up; but the filament of an electric light bulb, thin though it is, lasts for hundreds of hours without being consumed. The light comes, not from the consumption of the filament, but from the energy supplied to it from the electric power-house. The question we are considering is—what is the Sun's power-house, what is the source of the energy supplied which makes it glow?

The explanation considered sufficient until recently is almost the opposite of that we have just