erb; above the Rocky Mountains
the sky was of such purity as to permit the detection of Jupiter's
satellites with the naked eye on several successive nights. The
opportunity for advancing knowledge was made the most of. Nearly a
hundred astronomers, including several Englishmen, occupied twelve
separate posts, and prepared for an attack in force.

The question had often suggested itself, and was a natural one to ask,
whether the corona sympathises with the general condition of the sun?
whether, either in shape or brilliancy, it varies with the progress of
the sun-spot period? A more propitious moment for getting this question
answered could hardly have been chosen than that at which the eclipse
occurred. Solar disturbance was just then at its lowest ebb. The
development of spots for the month of July, 1878, was represented on
Wolf's system of "relative numbers" by the fraction 0.1, as against
135.4 for December, 1870, an epoch of maximum activity. The
"chromosphere"[534] was, for the most part, shallow and quiescent; its
depth, above the spot zones, had sunk from about 6,000 to 2,000
miles;[535] prominences were few and faint. Obviously, if a type of
corona corresponding to a minimum of sun-spots existed, it should be
seen then or never. It _was_ seen; but while, in some respects, it
agreed with anticipation, in others it completely set it at naught.

The corona of 1878, as compared with those of 1869, 1870, and 1871, was
generally admitted to be shrunken in its main outlines and much reduced
in brilliancy. Lockyer pronounced it ten times fainter than in 1871;
Harkness estimated its light at less than one-seventh that derived from
the mist-blotted aureola of 1870.[536] In shape, too, it was markedly
different. When sun-spots are numerous, the corona appears to be most
fully developed above the spot-zones, thus offering to our eyes a rudely
quadrilateral contour. The four great luminous sheaves forming the
corners of the square are made up of rays curving together from each
side into "synclinal" or ogival groups, each of which may be compared to
the petal of a flower. To Janssen, in 1871, the eclipsing moon seemed
like the dark heart of a gigantic dahlia, painted in light on the sky;
and the similitude to the ornament on a compass-card, used by Airy in
1851, well conveys the decorative effect of the beamy, radiated kind of
aureola, never, it would appear, absent when solar activity is at a
tolerably high pitch. In his s