tion; but, if a complete and universal level surface exists no
such system would be necessary._ For on a level surface--or on a
surface slightly inclined from the poles towards the equator, which
would be advantageous in either case--the melting water would of itself
spread over the ground and naturally irrigate as much of the surface as
it was possible for it to reach. If the surface were not level, but
consisted of slight elevations and expressions to the extent of a few
scores or a few hundreds of feet, then there would be no possible
advantage in cutting straight troughs through these elevations in
various directions with water flowing at the bottom of them. In neither
case, and in hardly any conceivable case, could these perfectly straight
canals, cutting across each other in every direction and at very varying
angles, be of any use, or be the work of an intelligent race, if any
such race could possibly have been developed under the adverse
conditions which exist in Mars.

_The Scanty Water-supply._

But further, if there were any superfluity of water derived from the
melting snow beyond what was sufficient to moisten the hollows indicated
by the darker portions of the surface, which at the time the water
reaches them acquire a green tint (a superfluity under the circumstances
highly improbable), that superfluity could be best utilised by widening,
however little, the borders to which natural overflow had carried it.
Any attempt to make that scanty surplus, by means of overflowing canals,
travel across the equator into the opposite hemisphere, through such a
terrible desert region and exposed to such a cloudless sky as Mr. Lowell
describes, would be the work of a body of madmen rather than of
intelligent beings. It may be safely asserted that not one drop of water
would escape evaporation or insoak at even a hundred miles from its
source. [5]

[Footnote 5: What the evaporation is likely to be in Mars may be
estimated by the fact, stated by Professor J.W. Gregory in his recent
volume on 'Australia' in _Stanford's Compendium_, that in North-West
Victoria evaporation is at the rate of ten feet per annum, while in
Central Australia it is very much more. The greatly diminished
atmospheric pressure in Mars will probably more than balance the loss of
sun-heat in producing rapid evaporation.]

_Miss Clerke on the Scanty Water-supply._

On this point I am supported by no less an authority than the historian
of modern