not being often employed in
ordinary life, and giving us no very precise idea of the magnitudes
which they imply, I recall here a remark that will convey a better
understanding of the immensity of the solar volume. If we imagine the
centre of the sun to coincide with that of the earth, its surface
would not only reach the region in which the moon revolves, but would
extend nearly as far again beyond.' By the transit of Venus in 1769,
it was demonstrated that the sun is 95,000,000 miles from the earth;
and yet, distant as it is, its physical constitution has been
determined; and the history of the successive steps by which this
proof has been arrived at, forms one of the most interesting chapters
in the progress of science.

It was in 1611 that Fabricius, a Dutch astronomer, first observed
spots on the eastern edge of the sun, which passed slowly across the
disk to the western edge, and disappeared after a certain number of
days. This phenomenon having been often noted subsequently, the
conclusion drawn therefrom is, that the sun is a spherical body,
having a movement of rotation about its centre, of which the duration
is equal to twenty-five days and a half. These dark spots, irregular
and variable, but well defined on their edge, are sometimes of
considerable dimensions. Some have been seen whose size was five times
that of the earth. They are generally surrounded by an aureola known
as the _penumbra_, and sensibly less luminous than the other portions
of the orb. From this penumbra, first observed by Galileo, many
apparently singular deductions have been made: namely, 'The sun is a
dark body, surrounded at a certain distance by an atmosphere which may
be compared to that of the earth, when the latter is charged with a
continuous stratum of opaque and reflecting clouds. To this first
atmosphere succeeds a second, luminous in itself, called the
_photosphere_. This photosphere, more or less remote from the inner
cloudy atmosphere, would determine by its outline the visible limits
of the orb. According to this hypothesis, there would he spots on the
sun every time that there occurred in the two concentric atmospheres
such corresponding clear spaces as would allow of our seeing the dark
central body uncovered.'

This hypothesis is considered by the most competent judges to render a
very satisfactory account of the facts. But it has not been
universally adopted. Some writers of authority have lately represented
the spots