not then ride at anchor, but floated free. Some held that its
surface was depressed below the average cloud-level, and that the cavity
was filled with vapours. Professor Wilson, on the other hand, observing
with the 16-inch equatorial of the Goodsell Observatory in Minnesota,
received a persistent impression of the object "being at a higher level
than the other markings."[1079] A crucial experiment on this point was
proposed by Mr. Stanley Williams in 1890.[1080] A dark spot moving
faster along the same parallel was timed to overtake the red spot
towards the end of July. A unique opportunity hence appeared to be at
hand of determining the relative vertical depths of the two formations,
one of which must inevitably, it was thought, pass above the other. No
forecast included a third alternative, which was nevertheless adopted by
the dark spot. It evaded the obstacle in its path by skirting round its
southern edge.[1081] Nothing, then, was gained by the conjunction,
beyond an additional proof of the singular repellent influence exerted
by the red spot over the markings in its vicinity. It has, for example,
gradually carved out a deep bay for its accommodation in the gray belt
just north of it. The effect was not at first steadily present. A
premonitory excavation was drawn by Schwabe at Dessau, September 5,
1831, and again by Trouvelot, Barnard, and Elvins in 1879; yet there was
no sign of it in the following year. Its development can be traced in
Dr. Boeddicker's beautiful delineations of Jupiter, made with the
Parsonstown 3-foot reflector, from 1881 to 1886.[1082] They record the
belt as straight in 1881, but as strongly indented from January, 1883;
and the cavity now promises to outlast the spot. So long as it survives,
however, the forces at work in the spot can have lost little of their
activity. For it must be remembered that the belt has a shorter
rotation-period than the red spot, which, accordingly (as Mr. Elvins of
Toronto has pointed out), breasts and diverts, by its interior energy, a
current of flowing matter, ever ready to fill up its natural bed, and
override the barrier of obstruction.

The famous spot was described by Keeler in 1889, as "of a pale pink
colour, slightly lighter in the middle. Its outline was a fairly true
ellipse, framed in by bright white clouds."[1083] The fading
continuously in progress from 1887 was temporarily interrupted in 1891.
The revival, indeed, was brief. Professor Barnard wrote