48 
rOrULAR SCIENCE REVIEW. 
Accordingly, Zollner was able to estimate the anomalous 
illumination of various parts of Mars’ disc. He had already 
done this in the case of the moon, and had come to the conclu- 
sion that the anomalies in the lunar illumination (mean) are 
due to the existence of irregularities over the moon’s surface, 
and he estimated the mean angle of inclination of the slopes of 
the lunar mountains to be somewhat over fifty degrees. As- 
suming that the same explanation held in the case of the ano- 
malies of Martial illumination, he found that the surface of 
Mars must be covered with mountains having a slope of about 
seventy-six degrees. 
But this view is utterly untenable. We accept Zollner’s ex- 
planation in the case of the moon ; in fact we may almost say 
that it is obviously the true one. We can conceive no other 
cause available to produce the effect considered, and further we 
see that all over the moon there are mountains having very 
.steep sides. But in the case of Mars we cannot admit such an 
explanation because a large part of the surface of the planet 
appears to be covered with water, and because also, a slope of 
seventy degrees and upwards is outrageously steep. Mars ought 
to be covered all over with hills shaped like sugar-loaves to 
account for his anomalous illumination in the way suggested by 
Zollner. 
To us a far more natural way of explaining the difficulty 
seems to be the following. We have every reason for believing 
that clouds form over the surface of Mars as over that of the 
earth. Secchi, Dawes, Lockyer, and Browning, agree in de- 
scribing effects which can scarcely be due to any other cause. 
And besides we shall presently see that there is good reason for 
feeling absolutely certain that the vapour of water exists in 
large quantities in the atmosphere of Mars. Now, it would not 
be a very bold speculation to argue from the observed anomalies 
in the illumination of Mars, that clouds prevail much more 
towards morning and evening (Martial) than in the middle of 
the day. If this were so, it would, of course, follow that the 
parts of ^lars which as seen from the sun lie near the edge of 
the limb, would be much more brilliant than the rest. For 
they are the parts where it is morning or evening with the Mar- 
tialists ; therefore according to the as.sumption they are cloud- 
covered ; but clouds reflect much more light than the solid or 
lifiuid surface of Mars ; therefore these parts of the disc would 
seem proportionately more brilliant. 
But we are not even required to make such an assumption as 
tlii.«. For if clouds were pretty uniformly distributed over the 
whole surfiice of Mars there would still result a greater brilliancy 
of the limb. Consider fig. 5 for example. Here a fourth part 
of the circumference of iSIars is supposed to be illuminated by 
the sun on the left, and clouds are represented which are 
