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a strictly regular supply; but Mr. C. G. Abbot has now found out for us that this is not so: the light and heat do vary, and it is most important for us to watch the variations, considering that the future history of the world may depend on them.

But let us return to the spots and let us look a little more closely at the diagram (Fig. 59) showing their fluctuations. When the curve rises to a peak or maximum there were numerous spots: when it falls to a valley or minimum there were very few. The dates are shown along the bottom line every ten years. You will see that the interval between two minima is not always exactly eleven years: thus there is only about nine years between the minima of 1775–6 and 1784–5; but as much as thirteen years between the minima of 1811 and 1824. The variation is not regular, and there must be some reason for the want of regularity. I have been studying this matter specially for the last year and have found what I think is the key to the puzzle: I think there is a swarm of meteors revolving round the Sun, not in a nearly circular track like our Earth, but in an elongated track like that of a comet. I hope you remember the way in which a comet moves—loitering along slowly when it is far from the Sun, quickening up as it comes nearer, and whizzing round the sharp turn when it is closest to the Sun—what is called perihelion. Now I think this meteor swarm whizzes round so close to the Sun's surface that some of the meteors actually graze the surface and make the sunspots. The swarm is collected mostly at one part of the track, like a lot of people running