“These duplications are not an optical effect depending on increase of visual power, as happens in the observation of double stars, and it is not the canal itself splitting in two longitudinally.  Here is what is seen:  To the right or left of a pre-existing line, without any change in the course and position of that line, one sees another line produce itself, equal and parallel to the first, at a distance generally varying from six to twelve degrees—­i.e., from 350 to 700 kilometres (217 to 434 miles); even closer ones seem to be produced, but the telescope is not powerful enough to distinguish them with certainty.  Their tint appears to be a quite deep reddish brown.  The parallelism is sometimes rigorously exact.  There is nothing analogous in terrestrial geography.  Everything indicates that here there is an organization special to the planet Mars, probably connected with the course of its seasons."[1]

[Footnote 1:  L’Astronomie, vol. i, 1882, pp. 217 et seq.]

Schiaparelli adds that he took every precaution to avoid the least suspicion of illusion.  “I am absolutely sure,” he says, “of what I have observed.”

I have quoted his statement, especially about the duplication of the canals, at so much length, both on account of its intrinsic interest and because it has many times been argued that this particular phenomenon must be illusory even though the canals are real.

One of the most significant facts that came out in the early observations was the evident connection between the appearance of the canals and the seasonal changes on Mars.  It was about the time of the spring equinox, when the white polar caps had begun to melt, that Schiaparelli first noticed the phenomenon of duplication.  As the season advanced the doubling of the canals increased in frequency and the lines became more distinct.  In the meantime the polar caps were becoming smaller.  Broadly speaking, Schiaparelli’s observation showed that the doubling of the canals occurred principally a little after the spring equinox and a little before the autumn equinox; that the phenomenon disappeared in large part at the epoch of the winter solstice, and disappeared altogether at the epoch of the summer solstice.  Moreover, he observed that many of the canals, without regard to duplication, were invisible at times, and reappeared gradually; faint, scarcely visible lines and shadows, deepened and became more distinct until they were clearly and sharply defined, and these changes, likewise, were evidently seasonal.

The invariable connection of the canals at their terminations with the regions called seas, the fact that as the polar caps disappeared the sealike expanses surrounding the polar regions deepened in color, and other similar considerations soon led to the suggestion that there existed on Mars a wonderful system of water circulation, whereby the melting of the polar snows, as summer passed alternately from one hemisphere to the other,