Well, gunpowder is strong sometimes: but not always. You may carry it in
a flask, or in your hand, and then it is weak enough. It only becomes
strong by being turned into gas and steam. But steam is always strong.
And if you look at a railway engine, still more if you had ever
seen--which God forbid you should--a boiler explosion, you would agree
with me, that the strongest thing we know of in the world is steam.

Now I think that we can explain almost, if not quite, all that we know
about earthquakes, if we believe that on the whole they are caused by
steam and other gases expanding, that is, spreading out, with wonderful
quickness and strength. Of course there must be something to make them
expand, and that is _heat_. But we will not talk of that yet.

Now do you remember that riddle which I put to you the other day?--"What
had the rattling of the lid of the kettle to do with Hartford Bridge Flat
being lifted out of the ancient sea?"

The answer to the riddle, I believe, is--Steam has done both. The lid of
the kettle rattles, because the expanding steam escapes in little jets,
and so causes a _lid-quake_. Now suppose that there was steam under the
earth trying to escape, and the earth in one place was loose and yet
hard, as the lid of the kettle is loose and yet hard, with cracks in it,
it may be, like the crack between the edge of the lid and the edge of the
kettle itself: might not the steam try to escape through the cracks, and
rattle the surface of the earth, and so cause an _earthquake_?

So the steam would escape generally easily, and would only make a passing
rattle, like the earthquake of which the famous jester Charles Selwyn
said that it was quite a young one, so tame that you might have stroked