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 contrivance—a compound of mechanism and galvanism—which, when the buoys are bumped, close the electric circuit and cause the mine to explode. Thus when a ship-of-war sails against one of these circuit-closers, she is immediately blown up.”

“Is not that rather a sneaking way of killing one’s enemies?” asked my mother.

Young Firebrand laughed, and admitted that it was, but pleaded that everything was fair in love and war.

In actual warfare the circuit-closers are placed just over the mines which they are designed to explode, but for safety on this occasion they were placed at a safe distance 

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