19] It has, indeed, been suggested that
the whiteness may not after all be due to this cause, but to carbonic
acid gas (carbon dioxide), which is known to freeze at a _very low_
temperature. The suggestion is plainly based upon the assumption that,
as Mars is so much further from the sun than we are, it would receive
much less heat, and that the little thus received would be quickly
radiated away into space through lack of atmosphere to bottle it in.

We now come to those well-known markings, popularly known as the
"canals" of Mars, which have been the subject of so much discussion
since their discovery thirty years ago.

It was, in fact, in the year 1877, when Mars was in opposition, and thus
at its nearest to us, that the famous Italian astronomer, Schiaparelli,
announced to the world that he had found that the ruddy areas, thought
to be continents, were intersected by a network of straight dark lines.
These lines, he reported, appeared in many cases to be of great length,
so long, indeed, as several thousands of miles, and from about twenty to
sixty miles in width. He christened the lines _channels_, the Italian
word for which, "canali," was unfortunately translated into English as
"canals." The analogy, thus accidentally suggested, gave rise to the
idea that they might be actual waterways.[20]

In the winter of 1881-1882, when Mars was again in opposition,
Schiaparelli further announced that he had found some of these lines
doubled; that is to say, certain of them were accompanied by similar
lines running exactly parallel at no great distance away. There was at
first a good deal of scepticism on the subject of Schiaparelli's
discoveries, but gradually other observers found themselves seeing both
the lines and their doublings. We have in this a good example of a
curious circumstance in astronomical observation, namely, the fact that
when fine detail has once been noted by a competent observer, it is not
long before other observers see the same detail with ease.

An immense amount of close attention has been paid to the planet Mars
during recent years by the American observer, Professor Percival Lowell,
at his famous observatory, 7300 feet above the sea, near the town of
Flagstaff, Arizona, U.S.A. His observations have not, like those of most
astronomers, been confined merely to "oppositions," but he has
systematically kept the planet in view, so far as possible, since the
year 1894.

The instrumental equipment o