44 
POPULAR SCIENCE REVIEW. 
to Venus, as is sometimes done, a rotation-period carried to the 
second decimal place of seconds. Thus Di Vico’s period, 23 h. 
21 m. 23*93 s., is often spoken of as if it might be trusted to the 
last figure, and as a veritable triumph of astronomical accuracy 
of observation. But, as a matter of fact, this determination, 
founded as it is on a comparison of Di Vico’s observations in 
1840-2 with those of Bianchini in 1726-7 (a period of 116 
years), could not be depended upon to the last figure, even if it 
were certain that the exact number of rotations taking place in 
the interval is known. But this cannot be the case, since 
observations of Venus have not been made often enough in the 
interval to enable us to carry back our estimate over gradually- 
increasing periods, as we can in the case of Mars ; and, without 
this precaution, it is quite impossible to be certain that no 
rotation is missed in the long interval. Even in a short period 
of two years. Sir W. Herschel dropped a rotation in the case of 
Mars ; and Venus is so much more difficult an object, and the 
spots upon her are so little recognisable (especially where tele- 
scopes of different power have been made use of by the observers 
whose views are to be compared), that there is much more 
likelihood of a similar mistake being made in her case. 
My former paper on Mars was accompanied by a plate pre- 
senting eight ideal views of the planet as it was to be seen 
during the opposition then approaching. These views were 
formed from the study of eight drawings of Mars by Mr. Dawes 
in 1864-5. I believe the attempt was the first that had ever 
been made to forecast the physical aspect of a planet in this 
way. But I was not wholly satisfied with the charting of the 
planet, and iis I had it in view at that time to draw up a mono- 
graph on Mars, I ventured to apply to Mr. Dawes for tracings 
of a few* drawings taken when the planet was presented in other 
ways to us. With the kindness for which he was so remarkable, 
and which endeared him so much to all who became acquainted 
with liim, he immediately sent me ten or twelve drawings 
and afterw'ards searched through his note-books for others. In 
all, if I remember rightly, he sent me tw*enty-one drawings, 
taken in 1852 (a most valuable series in this year), in 1856, in 
1860, and in 1862. 
The task of charting Mars from these drawings was not so 
easy a one as might at first sight have been supposed. Mr. 
Dawes had taken them at various hours, and there were no ready 
means of determining the position of the planet’s axis in each 
case. A tentative ]>rocess had to be gone through — for I was 
anxious that tlie charting of Mars should be independent of all 
previous efforts in that direction. 
Having calculated the presentation of Mars for the date of 
each drawing, I drew on tracing-paper the meridians and 
