616 
POPULAE SCIENCE EEVIEW. 
mountains tlirougli a splendid telescope by Wray, armed with 
a power of 1,000 (one wbicb would bring the moon witliiii 
a distance of 240 miles of the earth), and the quantity of detail 
in and about them w'as quite mar\mllous, and to attempt to 
delineate those minutise would puzzle a pre-Raphaelite. With 
a powerful telescope we can detect the stratification of the 
rocks, as if successive layers of lava had been deposited at 
various times. In the mountain Petavius the shower of lava 
which formed the wall appears to have been arrested for a 
time and then to have burst out afresh, as we find here the 
phenomenon of a double luall. The height of those circular 
walls is sometimes enormous : that of Newton is surrounded bv 
•/ 
walls, some of the peaks of which rise to a height of nearly 
24,000 feet. The great diameter of the volcanoes and the 
height of the walls may be explained by the small density of 
the matter which composes the moon, and also its weight in 
respect to that of the earth; for it would take eighty-eight 
moons to balanc-e our earth ; and whilst the lunar matter is 
only three times heavier than water, that of the earth is five 
and a half times its weight. Volcanic force, therefore, if it 
took place on the moon, would eject the lava and ashes to a far 
greater distance than on the earth, as a w^eight which would only 
be three ounces on the moon would be a pound on the earth, 
and the same force would throw a body six and a half times 
farther or higher on the former body. The great dimensions 
of these formations are best seen in the circular walled plains, 
the diameters of several of which range from 100 to 150 miles, 
though they may be as low as forty miles. Instead of a con- 
cave interior, as in the last-mentioned formation, w'e have 
here a flat floor, polished like marble in some cases, but in 
others it is exceedingly rough, with a number of rough 
blocks scattered confusedly over it. In the mountain Gassendi 
more than 100 such blocks are visible, whilst in others, as in 
the mountains Plato and Archimedes, the floor is perfectly 
smooth. In general, the circular- walled plains are not equally 
symmetrical with the concave formations ; but the two last- 
named mountains are exceptions to this, the walls being very 
regular and rising in terraces. But in others the wall is 
of irregular height and shape, and in some cases even one- 
third is missing, though occasionally the gap is partly filled up 
with broken fragments. They are furnished frequently with 
the usual central peak, but in many cases that point is only 
denoted by a confused assemblage of rocks at the centre. In 
others we have great mountain-chains passing through them, 
and hollow craters and immense boulders scattered over their 
surface, giving an idea of their being subjected to great 
ravages after their formation. The floor is, as in the case of 
