152 PICKERING. — LUNAR AND HAWAIIAN PHYSICAL FEATURES COMPARED. 
The force of gravitation at the surface of the Moon is but one-sixth as great as it is 
upon the Earth, and this difference is usually given as the cause of the great compara- 
tive size of the lunar formations. On the old theory that the lunar craters were due 
to explosions of steam, like our explosive volcanoes, it was evident that matter 
expelled from a crater vent could be thrown six times as far as upon the Earth. 
Although this theory is now practically abandoned, gravitation would still have an 
influence on the relative size, since a cliff or pinnacle upon the Moon could be six 
times the height of one upon the Earth, and yet exert no greater crushing force on 
the material beneath it. Still it is very evident that this explanation alone is inade- 
quate to account for the great difference in size actually observed. 
The facts seem to be that we are really trying to compare objects formed under 
entirely different conditions. The larger craters on the Moon came into existence 
when the thin, solid crust covering the molten interior was, owing to the solidifica- 
tion and contraction of the crust, much too small to contain the liquid material. 
The craters were therefore formed by the lava bursting through the crust, and 
so relieving the pressure. 
Later, after this relief had been found, and the crust had thickened, the interior 
regions by cooling shrank away from the solid shell which was now too large, and 
being insufficiently supported caved in, permitting the great fissure eruptions which 
produced the maria. These extensive outflows of lava dissolved the original solid 
shell wherever they came in contact with it much as they do at the present day in 
Hawaii. Had the Moon been much smaller, these extensive eruptions would not 
have attained such relatively great size, or might even not have occurred at all. On 
the other hand, had the Moon been larger, their relative size would have been greater, 
since the volume of the sphere would have been larger in proportion to its surface and 
would therefore have shrunk more in proportion. This was precisely what took place 
upon the Earth in all probability : our original gigantic craters were destroyed by the 
outflow of the earlier archaic rocks, which completely submerged and dissolved them. 
Our present Hawaiian craters must therefore be compared, not with the primary 
formations still left upon our Moon, but rather with the secondary ones formed 
later upon the surface of its maria. Of these Bessel, twelve miles in diameter, 
is a large and well known example. From this size down countless craterlets are 
known. 
Three craters are found upon the Earth measuring about fifteen miles in diameter. 
They occur in Kamchatka, in Japan, and in the Philippines, but are all of the explo- 
sive type, and therefore not comparable to those found on the Moon. It is possible 
