ae."[133]

The view, confidently upheld by Lalande,[134] that spots were rocky
elevations uncovered by the casual ebbing of a luminous ocean, the
surrounding penumbrae representing shoals or sandbanks, had even less to
recommend it than Derham's volcanic theory. Both were, however,
significant of a growing tendency to bring solar phenomena within the
compass of terrestrial analogies.

For 164 years, then, after Galileo first levelled his telescope at the
setting sun, next to nothing was learned as to its nature; and the facts
immediately ascertained, of its rotation on an axis nearly erect to the
plane of the ecliptic, in a period of between twenty-five and twenty-six
days, and of the virtual limitation of the spots to a so-called "royal"
zone extending some thirty degrees north and south of the solar equator,
gained little either in precision or development from five generations
of astronomers.

But in November, 1769, a spot of extraordinary size engaged the
attention of Alexander Wilson, professor of astronomy in the University
of Glasgow. He watched it day by day, and to good purpose. As the great
globe slowly revolved, carrying the spot towards its western edge, he
was struck with the gradual contraction and final disappearance of the
penumbra _on the side next the centre of the disc_; and when on the 6th
of December the same spot re-emerged on the eastern limb, he perceived,
as he had anticipated, that the shady zone was now deficient _on the
opposite side_, and resumed its original completeness as it returned to
a central position. In other spots subsequently examined by him, similar
perspective effects were visible, and he proved in 1774,[135] by strict
geometrical reasoning, that they could only arise in vast photospheric
excavations. It was not, indeed, the first time that such a view had
been suggested. Father Scheiner's later observations plainly
foreshadowed it;[136] a conjecture to the same effect was emitted by
Leonard Rost of Nuremburg early in the eighteenth century;[137] both by
Lahire in 1703 and by J. Cassini in 1719 spots had been seen as notches
on the solar limb; while in 1770 Pastor Schuelen of Essingen, from the
careful study of phenomena similar to those noted by Wilson, concluded
their depressed nature.[138] Modern observations, nevertheless, prove
those phenomena to be by no means universally present.

Wilson's general theory of the sun was avowedly tentative. It took the
modest form of an