ionally favourable conditions, many of the lines that have
been already seen single appear double--a pair of equally fine lines
exactly parallel throughout their whole length, and appearing, as Mr.
Lowell says, "clear cut upon the disc, its twin lines like the rails of
a railway track." Both Schiaparelli and Lowell were at first so
surprised at this phenomenon that they thought it must be an optical
illusion, and it was only after many observations in different years,
and by the application of every conceivable test, that they both became
convinced that they witnessed a real feature of the planet's surface.
Mr. Lowell says he has now seen them hundreds of times, and that his
first view of one was 'the most startlingly impressive' sight he has
ever witnessed.

_Dimensions of the Canals._

A few dimensions of these strange objects must be given in order that
readers may appreciate their full strangeness and inexplicability. Out
of more than four hundred canals seen and recorded by Mr. Lowell,
fifty-one, or about one eighth, are either constantly or occasionally
seen to be double, the appearance of duplicity being more or less
periodical. Of 'canals' generally, Mr. Lowell states that they vary in
length from a few hundred to a few thousand miles long, one of the
largest being the Phison, which he terms 'a typical double canal,' and
which is said to be 2250 miles long, while the distance between its two
constituents is about 130 miles.[3] The actual width of each canal is
from a minimum of about a mile up to several miles, in one case over
twenty. A great feature of the doubles is, that they are strictly
parallel throughout their whole course, and that in almost all cases
they are so truly straight as to form parts of a great circle of the
planet's sphere. A few however follow a gradual but very distinct curve,
and such of these as are double present the same strict parallelism as
those which are straight.

[Footnote 3: This is on the opposite side of Mars from that shown in the
frontispiece.]

_Canals extend across the Seas._

It was only after seventeen years of observation of the canals that it
was found that they extended also into and across the dark spots and
surfaces which by the earlier observers were termed seas, and which then
formed the only clearly distinguishable and permanent marks on the
planet's surface. At the present time, Professor Lowell states that this
"curious triangulation has been traced over