ws of considerable size resemble clefts under low powers.
Still it seems probable that the greater number of these features are
immense furrows or cracks in the surface and nothing more; for the higher
the magnifying power employed in their examination, the less reason there
is to object to this description. Dr. Klein of Cologne believes that
rills of this class are due to the shrinkage of parts of the moon's
crust, and that they are not as a rule the result of volcanic causes,
though he admits that there may be some which have a seismic origin. No
good reason has as yet been given for the fact that they so frequently
cross small craters and other objects in their course, though it has been
suggested that the route followed by a rill from crater to crater in
these instances may be a line of least surface resistance, an explanation
far from being satisfactory.

Whether variations in the visibility of lunar details, when observed
under apparently similar conditions, actually occur from time to time
from some unknown cause, is one of those vexed questions which will only
be determined when the moon is systematically studied by experienced
observers using the finest instruments at exceptionally good stations;
but no one who examines existing records of observations of rills by
Gruithuisen, Lohrmann, Madler, Schmidt, and other observers, can well
avoid the conclusion that the anomalies brought to light therein point
strongly to the probability of the existence of some agency which
occasionally modifies their appearance or entirely conceals them from
view.

The following is one illustration out of many which might be quoted. At a
point in its course, nearly due north of the ring-plain Agrippa, the
great Ariadaeus cleft sends out a branch which runs into the well-known
Hyginus cleft, reminding one, as Dr. Klein remarks, of two rivers
connected in the shortest way by a canal. This uniting furrow was
detected by Gruithuisen, who observed it several times. On some occasions
it appeared perfectly straight, at others very irregular; but, what is
very remarkable, although two such accurate observers as Lohrmann and
Madler frequently scrutinised the region, neither of them saw a trace of
this object; and but for its rediscovery by Schmidt in 1862, its
existence would certainly have been ignored by selenographers as a mere
figment of Gruithuisen's too lively imagination. Dr. Klein has frequently
seen this rill with great distinctness