THE MOON. 
619 
from Tycho, Kepler, Copernicus, and Aristarchus, whicli are 
best seen when the moon is full. The first of these has the 
most extensive streams_, passing as they do to an average 
distance of four or five hundred miles over mountains^ craters, 
and plains ; but the latter are the brightest, and the mountain 
itself the most brilliant part of the lunar disc. Those round 
Others and Byrginus are less favourably seen. It can scarcely 
be imagined that they are streams of lava proceeding from 
the central mountain, for one of those passing from Tycho can 
be traced to a distance of nearly two thousand miles, and the 
more natural idea would appear to be that they were immense 
fissures caused by the eruption of the central mountain, through 
which the lava oozed. It has been proved that they do not 
rise above the surface, and their breadth ranges from two to 
twenty miles. 
It curiously enough happens that we were acquainted with 
the height of the lunar mountains before those on our own 
earth, or before the barometer was invented wherewith to 
measure them. Galileo estimated the height of some of the 
mountains to be nearly 29,000 feet high, and in recent times 
it has been found from actual measurement that one named 
Doerfel, at the southern part of the moon, is nearly 25,000 
feet high. Upwards of twenty are higher than Mont Blanc. 
The highest peak on the earth is 28,180 feet; but if we 
compare the size of the earth and moon, we shall find that 
the proportion of the diameter of the latter to the height of 
its most elevated mountain is as 1 to 454, whilst the same 
ratio on the earth is as 1 to 1,481. Some of the highest of the 
mountains on the moon, as those of Neioton and Tycho, are of 
the cmcular type. 
The infinite variety of the scenery on the lunar surface can 
scarcely be described, and must be seen in order to be appre- 
ciated. It fortunately happens, however, that a telescope of 
moderate dimensions suffices to show the most striking fea- 
tures, although the higher the power made use of, the more 
detail is brought to view. The engraving accompanying -this 
is copied from a photograph taken by the writer with the great 
telescope at the Cambridge Observatory, and may serve to 
give an idea of its rocky surface. 
The existence of a lunar atmosphere is a qucestio vexata even 
at the present dajq though it has been satisfactorily proved 
that if one does exist it must be of extreme tenuity. It may 
be granted that when volcanoes and fire were in full action on 
its surface, the moon was surrounded by air ; at the present 
day, however, there is no sign of its existence. This has 
repeatedly been proved by the occultations of stars, which, 
taking the measured diameter of the moon only into account, 
