timba and the
San Pedro, on the 28th of September 1789, a whole tract of ground, from
three to four miles in extent, surged up like a foam-bubble, or the
swell of a wave, to a height of upwards of 500 feet. Flames, lurid and
crackling, broke forth over a surface of more than half a square league;
and the earth, as if softened by heat, was seen to rise and sink like
the rolling tide. Vast chasms opened in the earth, into which the two
rivers poured their waters headlong; reappearing afterwards at no great
distance from a cluster of _hornitos_, or small volcanic cones, which
sprang out of the mighty mud-torrent that gradually covered the entire
plain. Wonderful and awful as were these phenomena, they were surpassed
by the sudden opening of a chasm which vomited forth fire, and red-hot
stones and ashes, until they accumulated in a range of six large
mountain masses,--one of which, now known as the volcano of Jorullo,
attains an altitude of 1690 feet above the ancient level.

In like manner Sir John proceeds to describe an eruption of Mount
Tomboro, in the island of Sumbawa, the influence of which was felt to a
distance of 1000 miles from its centre, in strange tremulous motions of
the earth, and in the clash and clang of loud explosions. He says that
he had seen it computed that the quantity of ashes and lava ejected in
the course of this tremendous eruption would have formed three mountains
of the size of Mont Blanc.

As to the nature of the forces which operate to produce this astounding
result, Herschel puts forward a theory of singular simplicity and
directness.

"The origin," he says, "of such an enormous power thus occasionally
exerting itself, will no doubt seem very marvellous--little short,
indeed, of miraculous intervention; but the mystery, after all, is not
quite so great as at first it seems. We are permitted to look a little
way into these great secrets of Nature; not far enough, indeed, to clear
up every difficulty, but quite enough to penetrate us with admiration of
that wonderful system of counterbalances and compensations, that
adjustment of causes and consequences, by which, throughout all nature,
evils are made to work their own cure, life to spring out of death, and
renovation to tread in the steps and efface the vestiges of decay." And
he finds the clew to the secret, the key of the whole matter, in the
earth's vast central heat. This it is which produces the convulsions
that change the terrestrial