fine as only to be visible on special occasions of atmospheric clearness
and steadiness, which hardly ever occur at lowland stations, even with
the best instruments, and almost all are seen to be as perfectly
straight as if drawn with a ruler.

_The Double Canals._

Under exceptionally 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