The first explode when struck, but, as they may be dangerous to friends as well as foes, should be applied with great caution. With the second, also called dormant mines, each mine or set of mines must be fired when the vessel passes over. The position of the enemy's vessel over the mine-field must therefore be accurately determined, and a careful watch kept both by day and night. With electro-contact mines the mine itself, or a smaller float above it, called a circuit-closer, indicates through the cable when contact is made with a passing ship. The mine can then be discharged under a hostile vessel, whilst a friendly one is allowed to pass over. In arranging a field of submarine mines the passage of friendly vessels should be hindered as little as possible. If it extends across an entrance, say, from 1,000 to 1,800 yards, observation mines might be used from 400 to 800 yards from the observing station, and thus afford an open channel for traffic; whilst the rest of the field could be guarded with electro-contact and mechanical mines. With regard to the position of a field of submarine mines, great depth of water is unsuitable; so is a broken, rocky bottom; tumultuous tide-rips, strong tides, and exposure to the unbroken force of ocean waves will disarrange the electric contact and in time damage the cables. It must be borne in mind that submarine mines can be removed or rendered harmless by the deliberate action of an enemy working with boats—in countermining, dragging them, or cutting the cables by which they are fired, and that they must therefore be protected by the fire of artillery on the boats and on the enemy's vessels covering their action. I will now briefly refer to the defences required at the principal harbours and towns. Auckland. I have already spoken of what may be called the northern cluster of harbours. Auckland is the naval centre from which all these must be defended. The town is situated on the south side of the harbour (see Plate I.), and upwards of 4,000 yards distant from the entrance—1,500 yards wide—between the North Head and the opposite shoal. There are two approaches to the harbour: one—the main channel—to the westward, the other to the southward of Rangitoto Island. These are alike lighted by a lighthouse on the Bean Rock, which is about 2,000 yards distant, and nearly to the eastward of the North Head. Both channels are well commanded from the North Head; and on that point, therefore, batteries should be established. It would