Page:Popular Science Monthly Volume 2.djvu/249

Rh be ascertained; but, as the present question is only about what we know, that is, the parts of the city which are visible, or examined already, I repeat that not the hundredth part of a square yard of lava can be found at Herculaneum, and that ashes are the only thing there is there.

The problem to be solved is, how so huge a mass of ashes was ever piled up above the unfortunate city, and, since water played so fearful a part in the catastrophe, whence that enormous quantity of water came. It is clear, in the first place, that these ashes were thrown out by the volcano. Judging from the character of the region, and from the vents formed at the mouth of the crater, the pumice-stones were all hurled toward Pompeii and Stabiæ, while the ashes drifted toward Herculaneum. Perhaps some allowance must be made for the wind which separated these substances, and the convulsions which ejected them irregularly. Then we must recollect that every very violent eruption is attended by steam produced by the sudden contact of fire with underground sheets of water. The origin of these sheets of water and the effect of their sudden gush into the furnace of eruption have been already explained. These vapors, exceeding the power of calculation in their volume and expansive force, condense at once on contact with the atmosphere; they cool, and fall again in torrents of rain. If M. Fougiré could demonstrate that in 1865, during an eruption by no means extraordinary, there fell on the mountain, in 24 hours, 22,000 cubic metres of water, the number must be multiplied by five, or even by ten, to represent that explosion of Vesuvius, 79, whose fury has never been equalled. Without adopting the hypothesis of mud-discharges from the crater, or citing the example of the volcanoes in Java, which eject mire instead of water-spouts, we may affirm that such volumes of water, mingling with the ashes and pulverized substances thrown out by other vents, suddenly produced a liquid compost, either in the air or on the ground they fell upon. The Neapolitans are familiar with a phenomenon of this sort, occurring more than once, though under limited conditions. They call it "muddy lava," and their use of the substantive would be correct if they always added the adjective, in saying that Herculaneum was buried under lava. Herculaneum, in fact, was buried by muddy lava, or, in simpler terms, by torrents of mud.

Moreover, these sudden rains, or, rather, deluges, pouring down from the sky at each outburst of steam, swept along all the ashes that had fallen on the slopes of the mountain, and carried them down upon the plain; an ash-avalanche rolled over Herculaneum. At the same time, the rivers, which ran to the right and left of the city, ceased to flow down to the sea. It has been explained how the coast was elevated, and Pliny's ships kept off, by sudden new shoals preventing access to the port of Resina. The effect of this lifting was to raise the mouths of the two rivers, and throw back their waters on the city, and this overflow added its share of mud, ashes, and vegetable matter. Nor must we omit the canals filled up, the sewers choked, the aqueducts