I received a very kind letter from Mr.Christie, in which he said that it would be very difficult to make the results obtained from drawings, however accurate, at all comparable with those derived from photographs; especially as regards the accurate size of the spots as compared with the diameter of the sun.

And no doubt he is right.
"What, do I suppose, is the cause of these spots in the sun?
Well, that is a very difficult question to answer.

Changes are constantly going on at the sun's surface, or, I may rather say, in the sun's interior, and making themselves apparent at the surface.

Sometimes they go on with enormous activity; at other times they are more quiet.

They recur alternately in periods of seven or eight weeks, while these again are also subject to a period of about eleven years--that is, the short recurring outbursts go on for some years, when they attain a maximum, from which they go on decreasing.