d I can remember when it was learnedly
demonstrated that it would be quite impossible to construct a canal
across the Isthmus of Suez! How small the prophets must have felt when
the work was accomplished!

"I am afraid it is usual to take a very limited view of all such
matters, and we judge of them entirely from what we know ourselves,
never looking ahead, as it is considered unscientific to go beyond our
own knowledge. Because something may be quite impossible to us, it does
not follow that it is impossible to more advanced people.

"Think how many great scientific facts which are quite commonplace at
the present time were unknown and undreamed of even so recently as our
grandfathers' time! Who then can forecast what may be possible five
hundred years, or even a century hence; and who will be bold enough to
fix a limit to the possibilities of science! I freely admit I am an
optimist in these matters."

"I think, Professor," said John, "that your view is really the more
scientific of the two. While it may not be possible accurately to
forecast all the facts, intelligent anticipations may logically be
formed from a survey of our own past history."

Proceeding, I then remarked, that "Another discovery made at Flagstaff
Observatory was that at the ends of certain canals, where they joined
the dark areas, were small V-shaped dark markings which Professor Lowell
has termed carets. From their occurrence in these positions only, and
from his observations of the peculiar and extremely systematic manner in
which the canals, especially the double ones, run into the carets, he
has concluded that they must serve some special and important purpose.

"We have been told upon high authority that the carets are illusions,
and could not possibly be seen, as the planet is so distant from us. But
the fact remains that they have frequently been seen and drawn; they
always appear the same, and are never seen except in connection with
canals which join dark areas. These dark areas, I may say, are believed
to be the beds of ancient seas, from which the water has long since
departed.

"In connection with all these disputed lines and markings it has often
been urged that though they are seen through comparatively small
telescopes they are not seen when a very large instrument is used; and
it has also been said that observers, knowing what they wished to see,
simply imagined they did see it. We have, however, abundant proof that
both thes