Let us note some of the more striking facts about the canals which
Schiaparelli has described. We can not do better than quote his own
words:
"There are on this planet, traversing the continents, long dark lines
which may be designated as _canals_, although we do not yet know what
they are. These lines run from one to another of the somber spots that
are regarded as seas, and form, over the lighter, or continental,
regions a well-defined network. Their arrangement appears to be
invariable and permanent; at least, as far as I can judge from four and
a half years of observation. Nevertheless, their aspect and their degree
of visibility are not always the same, and depend upon circumstances
which the present state of our knowledge does not yet permit us to
explain with certainty. In 1879 a great number were seen which were not
visible in 1877, and in 1882 all those which had been seen at former
oppositions were found again, together with new ones. Sometimes these
canals present themselves in the form of shadowy and vague lines, while
on other occasions they are clear and precise, like a trace drawn with a
pen. In general they are traced upon the sphere like the lines of great
circles; a few show a sensible lateral curvature. They cross one another
obliquely, or at right angles. They have a breadth of two degrees, or
120 kilometres [74 miles], and several extend over a length of eighty
degrees, or 4,800 kilometres [nearly 3,000 miles].
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