Others in turn are
extremely difficult, and resemble the finest thread of spider's web
drawn across the disk. They are subject also to great variations in
their breadth, which may reach 200 or even 300 kilometers (120 to 180
miles) for the Nilosyrtis, whilst some are scarcely 30 kilometers (18
miles) broad.
These lines or stripes are the famous canals of Mars, of which so much
has been said. As far as we have been able to observe them hitherto,
they are certainly fixed configurations upon the planet. The Nilosyrtis
has been seen in that place for nearly one hundred years, and some of
the others for at least thirty years. Their length and arrangement are
constant, or vary only between very narrow limits. Each of them always
begins and ends between the same regions. But their appearance and their
degree of visibility vary greatly, for all of them, from one opposition
to another, and even from one week to another, and these variations do
not take place simultaneously and according to the same laws for all,
but in most cases happen apparently capriciously, or at least according
to laws not sufficiently simple for us to be able to unravel. Often one
or more become indistinct, or even wholly invisible, whilst others in
their vicinity increase to the point of becoming conspicuous even in
telescopes of moderate power. The first of our maps shows all those that
have been seen in a long series of observations. This does not at all
correspond to the appearance of Mars at any given period, because
generally only a few are visible at once.
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