xxx. New Zealand Institute. 



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 o£ mines 

 must be fired when the vessel passes over. The position of the 

 enemy's vessel over the mine-field must therefore be accurately deter- 

 mined, 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 ; tumul- 

 tuous 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 cover- 

 ing 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 



