To the
system of the Mare Acidalium undoubtedly belong the temporary lake
called Lacus Hyperboreus and the Lacus Niliacus. This last is ordinarily
separated from the Mare Acidalium by means of an isthmus or regular dam,
of which the continuity was only seen to be broken once for a short time
in 1888. Other smaller dark spots are found here and there in the
continental area which we may designate as lakes, but they are certainly
not permanent lakes like ours, but are variable in appearance and size
according to the seasons, to the point of wholly disappearing under
certain circumstances. Ismenius Lacus, Lunae Lacus, Trivium Charontis,
and Propontis are the most conspicuous and durable ones. There are also
smaller ones, such as Lacus Moeris and Fons Juventae, which at their
maximum size do not exceed 100 to 150 kilometers (60 to 90 miles) in
diameter, and are among the most difficult objects upon the planet.
All the vast extent of the continents is furrowed upon every side by a
network of numerous lines or fine stripes of a more or less pronounced
dark color, whose aspect is very variable. These traverse the planet for
long distances in regular lines that do not at all resemble the winding
courses of our streams. Some of the shorter ones do not reach 500
kilometers (300 miles), others, on the other hand, extend for many
thousands, occupying a quarter or sometimes even a third of a
circumference of the planet. Some of these are very easy to see,
especially that one which is near the extreme left-hand limit of our map
and is designated by the name of Nilosyrtis.
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