64 SCIENCE

you will hatfe to-morrow. But we do know almost:
certainly what the weather will be to-morrow,
next week, next month, on such and such a region
on Mars: if we do not wait till the winter comes,
we know that it will be fine. Hardly do we see
a cloud between the vernal equinox and the
autumnal equinox, neither in the equatorial nor
the temperate regions, and hardly even in the
circumpolar regions. If we are unable to make
a drawing of the telescopic image of the planet,
the difficulty hardly ever arises from the Martian
atmosphere, but from our own, which is so often
overcast or turbid. All the geographical configura-
tions, seas, rivers, plains, covered with vegetation
according to the moisture available, water-courses
varying with the seasons, canals and oases, are
mapped out with precision; we know in advance
which country will pass across the field of our
telescope; and the period of rotation, as already
mentioned, is known to the rooth of a second. It
is 24 hours 37 minutes 22.58 seconds. We also
know that the Martian year contains 59,355,041
seconds, i. e. 686 terrestrial days 23 hours 30
minutes 41 seconds. But since that planet turns
on its axis a little more slowly than the Earth,
there are only 668 Martian days in the Martian
year. In fact the Martian calendar is composed
of two successive years of 668 days and a leap
year of 669 days. As in our case, there is no
exact number of days in the Martian year. Per-