scovered by Galileo in 1610), showing that the sun rotates on an axis
once in about every twenty-five days. There are many interesting facts
connected with this rotation; especially that the sun does not rotate as a
solid body, the parts near the (Sun's) Equator flowing quicker than those
nearer the Poles; but for the present we cannot stop to dwell upon them.
What interests us particularly is the history, not from day to day, but
from year to year, as Schwabe has already given it for a series of years.

[Illustration:

FEB. 20, 1894. FEB. 21, 1894.

XI.--PHOTOGRAPHS OF THE SUN TAKEN AT THE ROYAL OBSERVATORY, GREENWICH,
SHEWING SUNSPOTS]

[Sidenote: Wolf's numbers.]

[Sidenote: Greenwich areas.]

[Sidenote: Magnetic fluctuations.]

When it became generally established that this periodicity existed, Rudolf
Wolf of Zurich collected the facts about sun-spots from the earliest
possible date, and represented this history by a series of numbers which
are still called Wolf's Sun-Spot Numbers. You will see from the diagram
the obvious rise and fall for eleven years,--not ten years, as Schwabe
thought, but just a little over eleven years. The facts are, however,
given more completely by the work done at the Royal Observatory at
Greenwich. It is part of the regular daily work of that Observatory to
photograph the sun at least twice. Many days are of course cloudy or wet,
so that photographs cannot be obtained; but there are available
photographs similarly taken in India or in Mauritius, where the weather is
more favourable, and from these the gaps are so well filled up that very
few days, if any, during the whole year are left without some photograph
of the sun's surface. On these photographs the positions and the areas of
the spots are carefully measured under a microscope, and the results when
submitted to certain necessary calculations are published year by year. It
is clearly a more accurate estimate of the spottedness of the sun to take
the total _area_ of all the spots rather than their mere _number_, for in
the latter case a large spot and a small one count equally. Hence the
Greenwich records will perhaps give us an even better idea of the
periodicity than Wolf's numbers. Now, at the same observatory magnetic
observations are also made continuously. If a magnet be suspended freely
we are accustomed to say that it will point to the North Pole; but this is
only very roughly true. In the first place, it is pro