straight dark lines. Further still, in some photographs made there quite
lately, several canals are said to appear visibly double.

Following up the idea alluded to in Chapter XVI., that the moon may be
covered with a layer of ice, Mr. W.T. Lynn has recently suggested that
this may be the case on Mars; and that, at certain seasons, the water
may break through along definite lines, and even along lines parallel to
these. This, he maintains, would account for the canals becoming
gradually visible across the disc, without the necessity of Professor
Lowell's "pumping" theory.

And now for the views of Professor Lowell himself with regard to the
doubling of the canals. From his observations, he considers that no
pairs of railway lines could apparently be laid down with greater
parallelism. He draws attention to the fact that the doubling does not
take place by any means in every canal; indeed, out of 400 canals seen
at Flagstaff, only fifty-one--or, roughly, one-eighth--have at any time
been seen double. He lays great stress upon this, which he considers
points strongly against the duplication being an optical phenomenon. He
finds that the distance separating pairs of canals is much less in some
doubles than in others, and varies on the whole from 75 to 200 miles.
According to him, the double canals appear to be confined to within 40
degrees of the equator: or, to quote his own words, they are "an
equatorial feature of the planet, confined to the tropic and temperate
belts." Finally, he points out that they seem to _avoid_ the blue-green
areas. But, strangely enough, Professor Lowell does not so far attempt
to fit in the doubling with his body of theory. He makes the obvious
remark that they may be "channels and return channels," and with that he
leaves us.

The conclusions of Professor Lowell have recently been subjected to
strenuous criticism by Professor W.H. Pickering and Dr. Alfred Russel
Wallace. It was Professor Pickering who discovered the "oases," and who
originated the idea that we did not see the so-called "canals"
themselves, but only the growth of vegetation along their borders. He
holds that the oases are craterlets, and that the canals are cracks
which radiate from them, as do the rifts and streaks from craters upon
the moon. He goes on to suggest that vapours of water, or of carbonic
acid gas, escaping from the interior, find their way out through these
cracks, and promote the growth of a low form of vegeta