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198 old drawings show. Later Herschel thought that they were holes in the bright surface of the Sun through which his dark interior could be seen; but the idea of the sun being a dark body surrounded by a bright envelope has long been given up. We scarcely know as yet what the spots are, or how they are caused; but there is no doubt at all that they are regions of fierce disturbance where terrible tornadoes are raging.

On one famous occasion (represented in Fig. 58) two independent observers, Carrington and Hodgson, saw an especially noteworthy disturbance. Two intensely bright spots appeared at the positions marked A and B and then travelled in about five minutes to the new positions C and D. The distances do not look very large on the picture, perhaps, but on the Sun himself they represent nearly 100,000 miles, so that the rate of travel was about 300 miles a second! That will give you some idea of the fury of the storms that must be raging in the Sun.

A curious thing about these sunspots is that they wax and wane about every eleven years. At the present time (i. e. Christmas 1913–14, when these lectures were delivered) the Sun has been nearly free from spots for at least two years. It is what is called a time of "minimum." But we are expecting spots to begin again soon, and they will increase in number till they reach a "maximum," and then they will die away again to a minimum about eleven years from now: say about 1924, since we are already probably past the minimum. We have records which show us that this waxing and waning