THE MOON’S FACE. 
245 
m't* 
comparatively rare, and it disappears altogether before the 
maximum size is reached. Increase in size is also accom¬ 
panied by atrophy of the wreath, 
but to this rule there is a conspicu¬ 
ous exception in that the greatest 
of all the craters preserves the 
third part of its wreath. In the 
absence of the wreath there is no 
sharp line of demarcation between 
the craters and the maria, and sev¬ 
eral large plains of oval outline, 
originally named as maria, are 
now classed with the craters. 
The inner plain is a constant 
feature with craters of maximum 
and medium size, but disappears 
as diameter diminishes. It is rare 
with diameters of less than five 
miles. The central hill also is ob¬ 
solescent downward, but persists 
farther than the inner plain. To 
my eye the interiors of most era-, 
ters under four miles and of all 
under two miles appear as simple 
cups.* The wreath of small cra¬ 
ters is correspondingly small and 
is apparently a simple ridge, but 
it does not disappear. The depth 
of crater varies with the width, 
but less rapidly, so that the small 
have less absolute but greater rela¬ 
tive depth. 
The craters are more abundant in some regions than in 
others, and there are comparatively few upon the maria. 
Fig. 4.—Varieties of lunar craters 
as related to size. The uppermost 
sketch represents the form of the 
smallest craters, the lowermost the 
form of the largest. 
* I am conscious that as the limit of telescopic vision is approached, 
the details of craters must disappear before the craters themselves are 
lost, and am therefore anxious to have this observation verified by those 
who are able to use higher powers than I could. 
