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this idea: I think they will come round in time, though at present they have not done so. On the other hand, I may be mistaken, and some other origin of sunspots may be found. But in any case I hope you may have been interested to follow the course of the story, seeing how one idea leads to another. Unless we can get some chain of reasoning of this kind, which can be checked at various points, we cannot advance our knowledge: when, on the other hand, we can see our way to some check and find that it works, it gives us confidence that we are on the right road. It was a great pleasure to me when, having seen that collisions between Saturn and the Leonids ought to recur in about 265½ years, I looked at the Chinese observations for the check, and found it very completely shown.

Although the particular idea I have just sketched is new, it is by no means a new idea that meteors should fall into the Sun. At one time it was thought that his heat and light were kept up in this way. If a shot is fired into a target, both shot and target are warmed. The heat comes from the stoppage of the motion of the shot. Do you remember our experiment to illustrate why meteors shine when they strike our air? We whirled an electric junction through the air and it became warm, because the air was resisting the motion and stopping it partly. If a bullet went right through the target and continued its course, nevertheless it would be partly stopped—it would scarcely continue its course so quickly as before: and we should get some heat: when the motion is wholly stopped we get more. Thus it was thought that meteors