ction which cannot be obtained with any system of direct
laying over sights. This instrument has been applied on a high site to
control guns placed low, or where guns are so placed as to be liable to
obscuration by fog or mist the position-finder can be placed below the
fog-line. In either case direct laying is provided for as an
alternative. In some defences batteries equipped with old pattern 9-in.
muzzle-loading guns, mounted as howitzers for long-range firing, have
been placed in folds in the ground so as to be quite invisible from the
sea and therefore invulnerable. Such batteries are fought entirely by
the position-finder.

The next adjunct to coast defences is the submarine mine. In Great
Britain the first submarine mining company dates from 1873, and from
that date mining defences were gradually installed both at home and
abroad; but the modern system of mining, which for twenty years was
maintained at British ports, really started into full life under the
impetus of Sir A. Clarke, about the same year (1885) in which we have
dated the commencement of the modern coast defence system.

With the increased speed of warships, a method of attack on
fortifications was evolved by running past the main defences and either
taking them in reverse, or disregarding them and attacking the dockyard
or other objective at short range. This was made more possible at most
defended ports by the pushing forward of the defences which has been
already alluded to, and it is especially dangerous where dockyards or
towns are situated some way up a river or estuary, so that once the
defences are passed there is a large stretch of water (e.g. the Thames,
the Solent, and Cork harbour) in which the enemy can manoeuvre. In such
cases there are two possible forms of defence, first by arranging for
gun-fire behind the main gun position, usually called the defence of
inner waters, and secondly by placing in the entrance and under the fire
of the main gun defence some form of obstruction to detain ships under
fire. This obstruction can be _passive_ (booms, chains, rows of piles or
sunken ships) or _active_ (mines or torpedoes). Passive obstructions are
only effective against comparatively small craft, and at important ports
mines are the only efficient obstruction which can be used against large
vessels.

After some years of experiment, English engineers adopted two main
classes of mines, called "observation" and "contact" mines (see
SUBMARINE