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 almost every portion of the
planet's surface, whether dark or light, whether greenish, ochre, or
brown in colour." In some parts they are much closer together than in
others, "forming a perfect network of lines and spots, so that to
identify them all was a matter of extreme difficulty." Two such portions
are figured at pages 247 and 256 of Mr. Lowell's volume.

_The Oases._

The curious circular black spots which are seen at the intersections of