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202 probability that it would return by the figures for the years 1531, 1607 and 1682 being so much alike: he inferred that it must be the same comet running regularly round a track. Then he noticed that the returns to the Sun were not quite regular: there was an interval of more than seventy-six years between the first pair and of less than seventy-five years between the second pair. This was against him unless he could give a reason for the difference. With great acuteness he assigned the reason in general terms: he said that when the comet was far away from the Sun, loitering slowly along, any of the planets which happened to come by might attract it out of its course a little, upsetting the regularity of the return. It will do us no harm to look at his actual words—

Now, the meteor swarm which I consider responsible for sunspots can be pulled out of its course in the same kind of way as Halley's comet: and if we look for occasions when it was so attracted we