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.