ker system. A cleft traverses the N. side of the
floor of Rhaeticus, and extends across the plain on the E. as far as the
N. side of Reaumur.

TRIESNECKER.--Apart from being the centre of one of the most remarkable
rill-systems on the moon, this ring-plain, though only about 14 miles in
diameter, is an object especially worthy of examination under every
phase. At sunrise, and for some time afterwards, owing to the superior
altitude of the N.W. section of the wall, a considerable portion of the
border on the N. and N.E. is masked by its shadow, which thus appears to
destroy its continuity. On more than one occasion, friends, to whom I
have shown this object under these conditions, have likened it to a
breached volcanic cone, a comparison which at a later stage is seen to be
very inappropriate. The rampart is terraced within, and exhibits many
spurs and buttresses without, especially on the N.W. The central mountain
is small and not conspicuous. The rill-system is far too complicated to
be intelligibly described in words. It lies on the W. side of the
meridian passing through the formation, and extends from the N. side of
Rhaeticus to the mountain-land lying between Ukert and Hyginus on the N.
Birt likened these rills to "an inverted river system," a comparison
which will commend itself to most observers who have seen them on a good
night, for in many instances they appear to become wider and deeper as
they approach higher ground. Published maps are all more or less
defective in their representations of them, especially as regards that
portion of the system lying N. of Triesnecker.

HYGINUS.--A deep depression, rather less than 4 miles across, with a low
rim of varying altitude, having a crater on its N. edge. This formation
is remarkable for the great cleft which traverses it, discovered by
Schroter in 1788. The coarser parts of this object are easily visible in
small telescopes, and may be glimpsed under suitable conditions with a 2
inch achromatic. Commencing a little W. of a small crater N. of Agrippa,
it crosses, as a very delicate object, a plain abounding in low ridges
and shallow valleys, and runs nearly parallel to the eastern extension of
the Ariadaeus rill. As it approaches Hyginus it becomes gradually
coarser, and exhibits many expansions and contractions, the former in
many cases evidently representing craters. When the phase is favourable,
it can be followed across the floor of Hyginus, and I have frequently