Page:A Voyage in Space (1913).djvu/245

Rh lecture, there was a furious magnetic storm on Earth, which was naturally attributed in some way to the solar disturbance. But now there is this difficulty: other disturbances in the Sun have been noticed without any corresponding magnetic storm on Earth; while on the other hand we have had storms which have driven the poor telegraphists nearly frantic while the surface of the Sun has betrayed no emotion whatever, so far as we could see. Hence, the question was in a very unsatisfactory state until Mr. Maunder made his ingenious suggestion. To explain the nature of it, perhaps you will let me first give an illustration of a more familiar kind. You know that when you are at the seaside the best time for bathing alters during the day, according to the tide. The time of high-tide falls later and later by about fifty minutes each day, because the tides are caused by the Moon and they follow it round. When the Moon has made a complete turn round the Earth, that is to say, in a month—the tides have changed by twenty-four hours, which comes to about fifty minutes each day. Let us make a diagram (Fig. 71) in which the twenty-four hours go from left to right, putting the days below one another; and let us mark the high tides, or the best bathing times on the diagram: there will be two of them each twenty-four hours, though one of them may come in the middle of the night when not many people want to bathe; but put them all down. Then the fifty minutes alteration each day will cause the marks to slope to the right. If we were to mark lunch time, which depends on the Sun, the marks would fall exactly below