eruptions upon his surface, like that seen by Professor Hall in
1876. On December 7th of that year a bright spot was seen upon
Saturn's equator. It elongated itself from day to day, and
remained visible for several weeks. Such a thing had never before
been known upon this planet, and had it not been that Professor
Hall was engaged in observations upon the satellites, it would not
have been seen then. A similar spot on the planet was recorded in
1902, and much more extensively noticed. On this occasion the spot
appeared in a higher latitude from the planet's equator than did
Professor Hall's. At this appearance the time of the planet's
revolution on its axis was found to be somewhat greater than in
1876, in accordance with the general law exhibited in the
rotations of the sun and of Jupiter. Notwithstanding their
transient character, these two spots have afforded the only
determination of the time of revolution of Saturn which has been
made since Herschel the elder.

[Illustration with caption: THE GREAT REFRACTOR OF THE NATIONAL
OBSERVATORY AT WASHINGTON]

Of the satellites of Saturn the brightest is Titan, which can be
seen with the smallest telescope, and revolves around the planet
in fifteen days. Iapetus, the outer satellite, is remarkable for
varying greatly in brilliancy during its revolution around the
planet. Any one having the means and ability to make accurate
photometrical estimates of the light of this satellite in all
points of its orbit, can thereby render a valuable service to
astronomy.

The observations of Venus, by which the astronomers of the last