n considers that
the photospheric cloudlets change their shape and character with the
progress of the sun-spot period;[511] but this is as yet uncertain.

The "grains," or more brilliant parts of the photosphere, are now
generally held to represent the upper termination of ascending and
condensing currents, while the darker interstices (Herschel's "pores")
mark the positions of descending cooler ones. In the penumbrae of spots,
the glowing streams rushing up from the tremendous sub-solar furnace are
bent sideways by the powerful indraught, so as to change their vertical
for a nearly horizontal motion, and are thus taken, as it were, in flank
by the eye, instead of being seen end-on in mamelon-form. This gives a
plausible explanation of the channelled structure of penumbrae which
suggested the comparison to a rude thatch. Accepting this theory as in
the main correct, we perceive that the very same circulatory process
which, in its spasms of activity, gives rise to spots, produces in its
regular course the singular "marbled" appearance, for the recording of
which we are no longer at the mercy of the fugitive or delusive
impressions of the human retina. And precisely this circulatory process
it is which gives to our great luminary its permance as a _sun_, or
warming and illuminating body.

FOOTNOTES:

[Footnote 405: _Mem. R. A. S._, vol. xxi., p. 157.]

[Footnote 406: _Ibid._, p. 160.]

[Footnote 407: _Month. Not._, vol. xxi., p. 144.]

[Footnote 408: _Le Soleil_, t. i., pp. 87-90 (2nd ed., 1871).]

[Footnote 409: See _ante_, p. 58.]

[Footnote 410: _Observations at Redhill (1863)_, Introduction.]

[Footnote 411: _Month. Not._, vol. xxxvi., p. 142.]

[Footnote 412: _Cape Observations_, p. 435, _note_.]

[Footnote 413: _Month. Not._, vol. x., p. 158.]

[Footnote 414: _Rosa Ursina_, lib. iii., p. 348.]

[Footnote 415: _Observations at Redhill_, p. 8.]

[Footnote 416: _Op._, t. iii., p. 402.]

[Footnote 417: _Rosa Ursina_, lib. iv., p. 601. Both Galileo and
Scheiner spoke of the _apparent_ or "synodical" period, which is about
one and a third days longer than the _true_ or "sidereal" one. The
difference is caused by the revolution of the earth in its orbit in the
same direction with the sun's rotation on its axis.]

[Footnote 418: _Rosa Ursina_, lib. iii., p. 260.]

[Footnote 419: Faye, _Comptes Rendus_, t. lx., p. 818.]

[Footnote 420: _Ibid._, t. xii., p. 648.]

[Footnote 421: _Proc. Am. Ass. Adv. o