a strange excrescence, suggesting the
budding-out of a fresh comet in that incongruous situation.[1354] Some
of these changes, Professor Barnard thought, might possibly be explained
by a rotation of the tail on an axis passing through the nucleus, and
Pickering, who formed a similar opinion on independent grounds, assigned
about 94 hours as the period of the gyrating movement.[1355] He,
moreover, determined accelerative velocities outward from the sun of
definite condensations in the tail, indicating for its materials, on
Bredikhine's theory, a density less than one half that of
hydrogen.[1356] This conclusion applied also to Rordame's comet, which
exhibited a year later phenomena analogous to those remarked in Swift's.
Their photographic study led Professor Hussey[1357] to significant
inferences as to the structure and rapid changes of cometary appendages.

PLATE IV.

[Illustration:
Photographs of Swift's Comet.
By Professor E. E. Barnard.
No. 1. Taken April 4, 1892; exposure 1 h.
No. 2. Taken April 6, 1892; exposure 1 h. 5 m ]

Seven comets were detected in 1892, and all, strange to say, were
visible together towards the close of the year.[1358] Among them was a
faint object, which unexpectedly left a trail on a plate exposed by
Professor Barnard to the stars in Aquila[1359] on October 12. This was
the first comet actually discovered by photography, the Sohag comet
having been simultaneously seen and pictured. It has a period of about
six years. Holmes's comet is likewise periodical, in rather less than
seven years. Its path, which is wholly comprised between the orbits of
Mars and Jupiter, is less eccentric than that of any other known comet.
Subsequently to its discovery, on November 6, it underwent some curious
vicissitudes. At first bright and condensed, it expanded rapidly with
increasing distance from the sun (to which it had made its nearest
approach on June 13), until, by the middle of December, it was barely
discernible with powerful telescopes as "a feebly luminous mist on the
face of the sky."[1360] But on January 16, 1893, observers in Europe and
America were bewildered to find, as if substituted for it, a yellow star
of the seventh magnitude, enveloped in a thin nebulous husk, which
enclosed a faint miniature tail.[1361] This condensation and recovery of
light lasted in its full intensity only a couple of days. The almost
evanescent faintness of Holmes's comet at its next return accounted for
its