A.—4.

10

with each other —who simultaneously watch the vessels approaching the mines, and by means of electrical appliances are able to fire the particular mine over which any vessel may be passing ; or, in the case of very narrow channels, each row may be fired by one observer on shore, as the vessel passes over it. I^lectro-contact mines are self-acting, when in an active state, and are intended to explode when struck by the hostile vessel in passing over them. They are, however, completely under the control of the observer on shore, who can render them passive at any time to permit of friendly ships entering the port. This self-acting apparatus may be applied either to ground or to buoyant mines. Mechanical mines are divided into two classes : —the electromechanical, the action of which depends upon self-contained electrical apparatus; and the mechanical, which are fired by mechanism. Both these methods of ignition are enclosed within the torpedo, and do not require connection with the shore as in the case of observation and electro-contact torpedoes, to which submarine wires are attached to connect them with the electrical instruments on land. The mechanical mines are consequently far less costly. The application of the several kinds of mines depends upon various circumstances. For a well defined and narrow channel, where there are suitable sites for the observers on shore, observation mines are preferred. In the case of a wide channel, the width may be narrowed by means of electro-contact mines ; and the gap left for the navigation of friendly ships may be defended by observation mines. The mechanical mines, designed up to the present time, are dangerous equally to friend and foe, when once rendered active by the removal of the safety apparatus which is employed in laying them ; for that reason they can only be used to close unfrequented channels, and after they are placed in position, it is a service of danger to remove them. They are not, therefore, applicable at present to the New Zealand harbors. Owing to the great improvements effected in the application of torpedoes, submarine cables and charges of gun-cotton, with their appurtenances, can, in favorable situations, be laid permanently on the actual spots where they are required, and arrangements can be made for firing the mines with facility. There need, therefore, now be no fear that torpedoes will not be in their places when required, and will not be effective against ships attempting to force the entrance to a port, provided always that they are protected by artillery fire, so as to prevent their removal by the enemy. It should be observed, however, that they entail the employment of delicate instruments, requiring skilled hands and steady, cool, heads for their manipulation ; otherwise they may become a source of considerable risk not only to those who use them, but to the friendly vessels which should pass over them in safety. Although the entrances to the principal harbors of the colony generally present favorable conditions for this species of defence, such as a slight rise and fall of tide

Mechanical mines,

Application of mines.

Efficiency of submarine mines.

Eeasons why they are not at present applicable to New Zealand harbors.