projected to a greater height than the water. To leeward of the vapor a
heavy shower of rain was seen to fall.[777]

[Illustration: Fig. 95.

Eruption of the New Geyser in 1810. (Mackenzie.)]

Among the different theories proposed to account for these phenomena, I
shall first mention one suggested by Sir. J. Herschel. An imitation of
these jets, he says, may be produced on a small scale, by heating red
hot the stem of a tobacco pipe, filling the bowl with water, and so
inclining the pipe as to let the water run through the stem. Its escape,
instead of taking place in a continued stream, is then performed by a
succession of violent explosions, at first of steam alone, then of water
mixed with steam; and, as the pipe cools, almost wholly of water. At
every such paroxysmal escape of the water, a portion is driven back,
accompanied with steam, into the bowl. The intervals between the
explosions depend on the heat, length, and inclination of the pipe;
their continuance, on its thickness and conducting power.[778] The
application of this experiment to the Geysers merely requires that a
subterranean stream, flowing through the pores and crevices of lava,
should suddenly reach a fissure in which the rock is red hot or nearly
so. Steam would immediately be formed, which, rushing up the fissure,
might force up water along with it to the surface, while, at the same
time, part of the steam might drive back the water of the supply for a
certain distance towards its source. And when, after the space of some
minutes, the steam was all condensed, the water would return, and a
repetition of the phenomena take place.

[Illustration: Fig. 96.

Supposed reservoir and pipe of a Geyser in Iceland.[779]]

There is, however, another mode of explaining the action of the Geyser,
perhaps more probable than that above described. Suppose water
percolating from the surface of the earth to penetrate into the
subterranean cavity A D (fig. 96) by the fissures F F, while, at the
same time, steam at an extremely high temperature, such as is commonly
given out from the rents of lava currents during congelation, emanates
from the fissures C. A portion of the steam is at first condensed into
water, while the temperature of the water is raised by the latent heat
thus evolved, till, at last, the lower part of the cavity is filled with
boiling water and the upper with steam under high pressure. The
expansive force of the steam becomes, at length, so gre