46 
POPULAR SCIENCE REVIEW. 
merely random touches thrown in to indicate the general ap- | 
pearance of that part of Mars to which they belonged. But I j 
soon found that every one of these streaks was to be taken as 
the indication of a Martial marking which Mr. Dawes had actu- 
ally seen. The strange variations of figure which a spot on a 
globe undergoes when the globe is looked at in various directions, 
had prevented me at first from recognising the identity of 
several large markings. Mr. Dawes himself was not aware in 
some cases, that a spot which was presented with one figure in 
one drawing w^as in reality the same as one which appeared with 
a totally different figure in another drawing. But when due , 
account was taken of the effects of foreshortening, the almost i 
perfect correspondence between the different views, indicated at ; 
once the accuracy of Mr. Dawes’ drawing, and the permanence 
of the spots which mark the globe of Mars. 
The result was the construction of a chart of Mars containing 
a number of features which had not before appeared in works of 
the sort. In my Half-hours with the Telescope,” (Plate VI.) 
a small copy of the equidistant chart originally drawn by me i 
is presented. Fig. 4 (PI. XXXIX.) represents the same features 
on Mercator’s projection. If a series of tracings be taken 
from figs. 1, 2, and 3, and the features represented in fig. 4 be 
drawn in, according to the meridians and parallels of the re- | 
spective figures, the aspect of Mars as he will appear during the ^ 
present quarter will be deduced. Plate XL. represents eight views ! 
of Mars in opposition ; so that if Mars be observed on any night, ! 
say from February 3 to February 23, he will be found (if the ob- : 
server’s telescope be sufficiently powerful) to present an appear- 
ance resembling one or other of the views in this plate. 
A feature of the planet Mars which has recently attracted 
some attention has been incidentally noticed above. I refer to 
the whiteness of the disc near the limb. This phenomenon is : 
worthy of a careful examination ; and I believe that the true S 
explanation has not yet been put forward. 
In the first place it is to be remarked that this phenomenon 
is real and not merely apparent. The edge of Jupiter’s disc 
seems to be brighter than the central part, but is in reality 
darker. I believe ninety-nine observers out of a hundred 
woidd be deceived regarding this feature of Jupiter, if they 
trusted to the unaided eye. Why it should be so, is not perhaps 
Very easy to say. Perhaps the contrast between the dark back- 
ground of the sky jind the illuminated limb of the planet tends 
to give to the latter a brightness which does not belong to it. 
P)C this as it may, a series of observations which Mr. Browning 
has lately made on Jupiter, with the express object of deter- 
mining tliis (juestion, has resulted in placing the greater darkness 
of the planet’s limb, as compared with the central part of the 
