ly two planetary
bodies we are in a position to compare with Mars. Yet even these do, I
think, afford us some hints towards an interpretation of the mysterious
lines. But as our knowledge of the internal structure and past history
even of our earth is still imperfect, that of the moon only conjectural,
and that of Mars a perfect blank, it is not perhaps surprising that the
surface-features of the latter do not correspond with those of either of
the others.

_Mr. Pickering's Suggestion._

The best clue to a natural interpretation of the strange features of the
surface of Mars is that suggested by the American astronomer Mr. W.H.
Pickering in _Popular Astronomy_ (1904). Briefly it is, that both the
'canals' of Mars and the rifts as well as the luminous streaks on the
moon are cracks in the volcanic crust, caused by internal stresses due
to the action of the heated interior. These cracks he considers to be
symmetrically arranged with regard to small 'craterlets' (Mr. Lowell's
'oases') because they have originated from them, just as the white
streaks on the moon radiate from the larger craters as centres. He
further supposes that water and carbon-dioxide issue from the interior
into these fissures, and, in conjunction with sunlight, promote the
growth of vegetation. Owing to the very rare atmosphere, the vapours, he
thinks, would not ascend but would roll down the outsides of the
craterlets and along the borders of the canals, thus irrigating the
immediate vicinity and serving to promote the growth of some form of
vegetation which renders the canals and oases visible.[13]

[Footnote 13: _Nature_, vol. 70, p. 536.]

This opinion is especially important because, next to Mr. Lowell, Mr.
Pickering is perhaps the astronomer who has given most attention to Mars
during the last fifteen years. He was for some time at Flagstaff with
Mr. Lowell, and it was he who discovered the oases or craterlets, and
who originated the idea that we did not see the 'canals' themselves but
only the vegetable growth on their borders. He also observed Mars in the
Southern Hemisphere at Arequipa; and he has since made an elaborate
study of the moon by means of a specially constructed telescope of 135
feet focal length, which produced a direct image on photographic plates
nearly 16 inches in diameter.[14]

[Footnote 14: _Nature_, vol. 70, May 5, p.xi, supplement.]

It is clear therefore that Mr. Lowell's views as to the artificial
nature of the 'c