There are, however, grave difficulties in identifying the two kinds of phenomena. One (already mentioned) is the total absence of the regular swirling motion--in a direction contrary to that of the hands of a watch north of the solar equator, in the opposite sense south of it--which should impress itself upon every lineament of a sun-spot if the cause assigned were a primary producing, and not merely (as it possibly may be) a secondary determining one. The other, pointed out by Young,[467] is that the cause is inadequate to the effect. The difference of movement, or _relative drift_, supposed to occasion such prodigious disturbances, amounts, at the utmost, for two portions of the photosphere 123 miles apart, to about five yards a minute. Thus the friction of contiguous sections must be quite insignificant.
A view better justified by observation was urged by Secchi in and after the year 1872, and was presented in an improved form by Professor Young in his excellent little book on _The Sun_, published in 1882.[468] Spots are manifestly associated with violent eruptive action, giving rise to the faculæ and prominences which usually garnish their borders. It is accordingly contended that upon the withdrawal of matter from below by the flinging up of a prominence must ensue a sinking-in of the surface, into which the partially cooled erupted vapours rush and settle, producing just the kind of darkening by increased absorption told of by the spectroscope. Round the edges of the cavity the rupture of the photospheric shell will form lines of weakness provocative of further eruptions, which will, in their turn, deepen and enlarge the cavity. The phenomenon thus tends to perpetuate itself, until equilibrium is at last restored by internal processes. A sun-spot might then be described as an inverted terrestrial volcano, in which the outbursts of heated matter take place on the borders instead of at the centre of the crater, while the cooled products gather in the centre instead of at the borders.
But on the earth, the solid crust forcibly represses the steam gathering beneath until it has accumulated strength for an explosion, while there is no such restraining power that we know of in the sun. Zöllner, indeed, adapted his theory of the solar constitution to the special purpose of procuring it; yet with very partial success, since almost every new fact has proved adverse to his assumptions. Volcanic action is essentially spasmodic. It implies habitual constraint varied by temporary outbreaks, inconceivable in a gaseous globe, such as we believe the sun to be.