RAM SHOWING THE MAIN LAYERS OF THE SUN

Compare with frontispiece.]

[Illustration: _Photo: Royal Observatory, Greenwich._

FIG. 6.--SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, May 29, 1919.
TAKEN AT SOBRAL, BRAZIL.

The small Corona is also visible.]

[Illustration: FIG. 7.--THE VISIBLE SURFACE OF THE SUN

A photograph taken at the Mount Wilson Observatory of the Carnegie
Institution at Washington.]

[Illustration: FIG. 8.--THE SUN

Photographed in the light of glowing hydrogen, at the Mount Wilson
Observatory of the Carnegie Institution of Washington: vortex phenomena
near the spots are especially prominent.]

The fourth and uppermost layer or region is that of the corona, of
immense extent and fading away into the surrounding sky--this we have
already referred to. The diagram (Fig. 5) shows the dispositions of
these various layers of the sun. It is through these several transparent
layers that we see the white light body of the sun.

Sec. 2

The Surface of the Sun

Here let us return to and see what more we know about the
photosphere--the sun's surface. It is from the photosphere that we have
gained most of our knowledge of the composition of the sun, which is
believed not to be a solid body. Examination of the photosphere shows
that the outer surface is never at rest. Small bright cloudlets come and
go in rapid succession, giving the surface, through contrasts in
luminosity, a granular appearance. Of course, to be visible at all at
92,830,000 miles the cloudlets cannot be small. They imply enormous
activity in the photosphere. If we might speak picturesquely the sun's
surface resembles a boiling ocean of white-hot metal vapours. We have
to-day a wonderful instrument, which will be described later, which
dilutes, as it were, the general glare of the sun, and enables us to
observe these fiery eruptions at any hour. The "oceans" of red-hot gas
and white-hot metal vapour at the sun's surface are constantly driven by
great storms. Some unimaginable energy streams out from the body or
muscles of the sun and blows its outer layers into gigantic shreds, as
it were.

The actual temperature at the sun's surface, or what appears to us to be
the surface--the photosphere--is, of course, unknown, but careful
calculation suggests that it is from 5,000 deg. C. to 7,000 deg. C. The interior
is vastly hotter. We can form no conception of such temperatures as must
exist there. Not even the most obdurate solid cou