.

HELICON.--The companion ring-plain on the E. It is 13 miles in diameter,
and is very similar, though not quite so deep. There is a crater on the
S.E. wall, and, according to Neison, another on the outer slope of the N.
border. Webb records a central crater. If Helicon is observed when on the
morning terminator, it will be seen to be traversed by a curved ridge
which cuts through the walls, and runs up to a bright crater S.E. of
Leverrier. It appears to be a "fault," whose "downthrow," though slight,
is clearly indicated by an area of lower ground on the E. There is a
great number of small craters in the neighbourhood of this formation.

EULER.--The most easterly of the row of great ring-plains, which,
beginning on the W. with Autolycus, and followed by Archimedes,
Timocharis, and Lambert, extends almost in a great circle from the N.W.
to the S.E. side of the Mare Imbrium. It is about 19 miles in diameter,
and is surrounded by terraced walls, which, though of no great height
above the Mare, rise 6000 feet above the floor. There is a distinct
little gap in the S. wall, easily glimpsed when it is close to the
morning terminator, which probably represents a small crater. Euler has a
bright central mountain, and is a centre of white silvery streaks.

BRAYLEY.--A very conspicuous little ring-plain E.S.E. of Euler, with two
smaller but equally brilliant objects of the same class situated
respectively E. and W. of it.

DIOPHANTUS.--Forms with Delisle, its companion on the N., a noteworthy
object. It is about 13 miles in diameter, with a wall, which has a
distinct break in its continuity on the N., rising about 2500 feet above
the Mare. A rill-valley runs from the E. side of the ring towards the W.
face of a triangular-shaped mountain on the E. of a line joining the
formation with Delisle. North are three bright little craters in a line,
the middle one being much the largest. From the most easterly of these
objects a light streak may be traced under a high sun, extending for many
miles to another small crater on the N.W. of Diophantus, and expanding at
a point due N. of the formation into a spindle-shaped marking. At
sunrise, the W. portion of the streak has all the appearance of a cleft,
with a branch about midway running to the S. side of Delisle. Under the
same phase a broad band of shadow extends from the N.E. wall to the
triangular mountain just mentioned, representing a very sudden drop in
the surface--resembling on