the tide. This
countermine consisted of an iron cylinder, containing 300 pounds of
powder, and was electrically connected with the _Nettle_. A small
charge of gun-cotton was fixed to the suspender that held the torpedo to
its casks. When at a safe distance from the ship, this charge was
fired. It cut the suspender and let the torpedo sink to the bottom.
There it was exploded with terrific violence, as was quickly shown by
the mighty fountain of mud, water, and smoke that instantly shot up into
the air. It has been proved by experiment that 500 pounds of gun-cotton
exploded below water, will destroy all the torpedoes that lie within a
radius of 120 yards. It is obvious, therefore, that a warship could
advance into the space thus cleared and then send a second countermine
ahead of her in the same way. If neither tide, current, nor wind will
serve to drift the casks, the operation might be accomplished by a small
boat, which could back out of danger after laying each torpedo, and
thus, step by step, or shot by shot, the advance could be made in safety
through the enemy's defences.

After this, twelve small charges of gun-cotton were sunk in various
directions, each representing a countermine of 500 pounds. These were
discharged simultaneously, to demonstrate the possibility of extending
the operations over a wide area. These miniature charges were sent down
in small nets, and were quite unprotected from the water, so that the
gun-cotton was wet when fired.

This fact caught the attention of my mother at once.

"How can it go off when _wet_?" she exclaimed, turning her bright little
eyes in astonishment on her young companion.

"Ha, that is one of the strange peculiarities of gun-cotton," replied
Firebrand, with an amused look; "you don't require to keep it dry like
powder. It is only necessary that there should be one small lump of dry
gun-cotton inside the wet stuff, with a detonator in its heart. A
detonator, you must know--"

"Oh, I know what a detonator is," said my mother, quickly.

"Well then," continued Firebrand, "the exploding of the detonator and
the dry disc causes the wet gun-cotton also to go off, as you have seen.
Now they are going to exhibit one of the modes of defending harbours.
They have sunk four mines, of 300 pounds of gunpowder each, not far from
where you see yon black specks floating on the water. The black specks
are buoys, called _circuit-closers_, because they contain a delicate