Americans have a reputation for never staying long in one place. So long as they stay anywhere, if only for three days, we shall get three marks in a sloping line of the right slope: even two consecutive days will give us the right slope; single days, of course, tell us nothing. The important point is that wherever and whenever we get a few days together, the proper slope shows itself, proving that the marks must have to do with the Moon in some way, though we may not be able to explain why they are so broken up unless we happen to know that they were made by an American.
Now I think we can follow Mr. Maunder's diagram of magnetic storms. Each horizontal line refers, not to the twenty-four hours in which the Earth turns on its axis, but to the twenty-seven days in which the Sun turns on its axis, as we saw at the beginning of the lecture. The marks show when a magnetic storm occurred on the Earth, and you will see at once that they are apt to occur in groups of three or four, one under the other. Whenever they are exactly one under the other (like the marks for lunch time in the other case), it means that when the Sun has rotated exactly once the storm is repeated, just as the striking of a clock is repeated when the minute hand has gone round exactly once. Mr. Maunder claims that the Sun stretches out a long finger like the hand of a clock, rotating as the Sun rotates; and that this finger strikes the Earth and causes a storm; goes round completely once, strikes the Earth again and causes another storm, and so on, until the finger changes its place like the restless American tourist. Some astron-
