Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/232

Rh of volcanic action, therefore, as respects the supply of heat, the energy, of explosive or impulsive or seismic action, is a question of limits as regards the supply of water. If there be no water, there is no supply of steam; if the supply of water be unlimited or excessive, the temperature is greatly reduced, enormous volumes of vapour, may be evolved, but the tension is small. At some determinate and intermediate rate of supply, the volume and tension together, in a given time, must be a maximum, and at that rate of supply, the impulsive energy, whether at the open volcano's mouth, or closed over and covered, as in the earthquake, will be most violent.

We may, therefore, well conceive it probable, that a large reduction in the supply of rain, especially when falling over a highly porous, loose, and fissured country, such as South Italy is, may sensibly increase seismic action beneath, by permitting the temperature of the pyrigenous "couches," to rise, and the tension of the steam, whether pent up or escaping in regimen, at neighbouring volcanic vents, to rise in proportion.

The anterior duration of drought at the surface, necessary to produce this effect to its greatest extent, would depend largely upon local conditions, chiefly geognostic. Under given circumstances, of rock formation, &c., a particle of water penetrating at the surface, takes a determinate time to reach the depth of the igneous foci—greater as the depth is so—and as the filtration is performed through finer channels, and through deeper beds of material, surcharged with water. Such surcharged beds, are the immediate magazine of supply for evaporation, they measure the capacity of "the boiler"—and if the "feed-water" from the surface,