put them there herself, she had at least taught the honest Germans to put
them there. And often Madam How turns her worn-out craters into
beautiful lakes. There are many such crater-lakes in Italy, as you will
see if ever you go there; as you may see in English galleries painted by
Wilson, a famous artist who died before you were born. You recollect
Lord Macaulay's ballad, "The Battle of the Lake Regillus"? Then that
Lake Regillus (if I recollect right) is one of these round crater lakes.
Many such deep clear blue lakes have I seen in the Eifel, in Germany; and
many a curious plant have I picked on their shores, where once the steam
blasted, and the earthquake roared, and the ash-clouds rushed up high
into the heaven, and buried all the land around in dust, which is now
fertile soil. And long did I puzzle to find out why the water stood in
some craters, while others, within a mile of them perhaps, were perfectly
dry. That I never found out for myself. But learned men tell me that
the ashes which fall back into the crater, if the bottom of it be wet
from rain, will sometimes "set" (as it is called) into a hard cement; and
so make the bottom of the great bowl waterproof, as if it were made of
earthenware.

But what gives the craters this cup-shape at first?

Think--While the steam and stones are being blown out, the crater is an
open funnel, with more or less upright walls inside. As the steam grows
weaker, fewer and fewer stones fall outside, and more and more fall back
again inside. At last they quite choke up the bottom of the great round
hole. Perhaps, too, the lava or melted rock underneath cools and grows
hard, and that chokes up the hole lower down. Then, down from the round
edge of the crater the stones and cinders roll inward more and more. The
rains wash them down, the wind blows them down. They roll to the middle,
and meet each other, and stop. And so gradually the steep funnel becomes