face of the plateau,
would rush on to the beach. Consequently, the level of the lake would
be greatly lowered, and the opening where the water escaped would be
exposed, which was their final aim.

Under the engineer's directions, Pencroft, armed with a pickaxe, which
he handled skillfully and vigorously, attacked the granite. The hole was
made on the point of the shore, slanting, so that it should meet a
much lower level than that of the water of the lake. In this way the
explosive force, by scattering the rock, would open a large place for
the water to rush out.

The work took some time, for the engineer, wishing to produce a great
effect, intended to devote not less than seven quarts of nitro-glycerine
to the operation. But Pencroft, relieved by Neb, did so well, that
towards four o'clock in the evening, the mine was finished.

Now the question of setting fire to the explosive substance was raised.
Generally, nitro-glycerine is ignited by caps of fulminate, which in
bursting cause the explosion. A shock is therefore needed to produce
the explosion, for, simply lighted, this substance would burn without
exploding.

Cyrus Harding could certainly have fabricated a percussion cap. In
default of fulminate, he could easily obtain a substance similar to
guncotton, since he had azotic acid at his disposal. This substance,
pressed in a cartridge, and introduced among the nitro-glycerine, would
burst by means of a fuse, and cause the explosion.

But Cyrus Harding knew that nitro-glycerine would explode by a shock.
He resolved to employ this means, and try another way, if this did not
succeed.

In fact, the blow of a hammer on a few drops of nitro-glycerine, spread
out on a hard surface, was enough to create an explosion. But the
operator could not be there to give the blow, without becoming a victim
to the operation. Harding, therefore, thought of suspending a mass of
iron, weighing several pounds, by means of a fiber, to an upright just
above the mine. Another long fiber, previously impregnated with sulphur,
was attached to the middle of the first, by one end, while the other lay
on the ground several feet distant from the mine. The second fiber being
set on fire, it would burn till it reached the first. This catching
fire in its turn, would break, and the mass of iron would fall on the
nitro-glycerine. This apparatus being then arranged, the engineer, after
having sent his companions to a distance, filled the hole, so