42 
POPULAR SCIENCE REVIEW. 
that the darkened crescent .which indicates the gibbosity of the 
planet does not, either in fig, 1 or in fig. 3, correspond with the 
position of the planet’s polar axis. Since, in fig. 1, the outline 
of the dark hemisphere clearly passes nearer to the planet’s pole 
than the outline of the hemisphere visible to us, we are to 
infer that the polar axis of the planet is less bowed towards the 
sun than it is towards ourselves at the period to which this 
figure corresponds. On the contrary, at the period correspond- 
ing to fig. 3, the planet is less bowed towards the earth than 
towards the sun. 
Since the last opposition of Mars a good deal has been added 
to our knowledge of the planet. I will begin with less impor- 
tant, but not, I think, uninteresting considerations. 
It may be remembered that in dealing in these pages with 
the rotation -period of Mars, I spoke of the values assigned to 
this element by Madler and Kaiser — the former giving 24 h. 
37 m. 23*8 s. and the latter 24 h. 37 m. 22*6 s. But, some time 
after my paper was written, I had occasion to examine a large 
number of pictures of the planet for the purpose of combining 
the information they afforded, and so forming a chart of Mars. 
This work had already been done by Sir W. Herschel, then by 
^Messrs. Beer and Madler, and more recently by Professor Phil- 
lips; but it seemed to me that Mr. Dawes’ admirable drawings 
of the planet promised to afford a chart of greater completeness 
than any of these, and, as the views which he had taken ex- 
tended over upwards of twelve years, it was necessary, in order 
that they might be fairly compared inter se, and the identity of 
the various features of the planet fully made out, that the 
rotation-period of Mars should be determined with great ac- 
curacy. The discrepancy between the determinations obtained 
by Madler and Kaiser, and the uncertainty I was in respecting 
the relative accuracy of these two astronomers, led me to test 
their estimates. I found that, for intervals of several years, 
either value corresponded very closely with observed appear- 
ances. In fact, it will be seen at once that a difference of a 
second in a Martial day would correspond to a difference of less 
than 3G5 seconds, or six minutes, in a terrestrial year. 
But when longer periods were taken, Kaiser’s value quickly 
showed a marked superiority, and I was led, as I had antici- 
pated (Kaiser’s being the latest estimate), to find a very close 
agreement between this value and the observed appearance of 
the planet at intervals of fifteen, twenty, or even thirty years. 
This was, in fact, all I wanted, since, as I have said, Dawes’ 
drawings covered only a period of twelve or thirteen years. But 
having gone thus far, I thought it would be well to enquire how 
far Kaiw-r’s period availed to account for the rotation of Mars 
(luring longer intervals, especially as there were several pictures 
