s, then, in broad outline, was the task which confronted this section
of the naval service, and its successful accomplishment forged a big
link in the steel chain encompassing the glorious victory.

Before passing on to describe the ships and the appliances used it is
first necessary to give a more detailed account of the operations
generally included under the heading of minesweeping. As it was
impossible to tell exactly where mines would be laid from day to day, an
immense area of sea had to be covered by what was known as _exploratory
sweeping_. This consisted of many units of ships emerging from the
different anti-submarine bases almost every day throughout the year and
proceeding to allotted areas of water, where they commenced sweeping
north, south, east or west, in an endeavour to discover if the areas in
question were safe for mercantile traffic. If no mines were discovered
that particular area would be reported safe, but if only one of these
weapons was cut from its mooring by a sweep-wire the area would be
closed to merchant ships until the sea around was definitely cleared of
the hidden danger. This system of open and closed areas entailed an
enormous amount of efficient administrative staff work apart from the
actual sweeping, and its success was partly dependent upon the vigilance
of the patrols employed to divert shipping from dangerous patches of
sea.

When a mine-field was discovered which interfered with the free movement
of a large number of ships a big concentration of sweepers from all the
adjacent bases was ordered by telegraph and wireless. The area was
isolated by patrols and the mines swept up. In one field no less than
300-400 mines were known to have been laid. Finally a further
exploratory sweep was made, and if nothing further was discovered the
area was again opened to traffic, and the sweepers turned their
attention either to routine duties or to the clearance of another field.

The entrance to every important harbour was swept once or twice a day,
and all convoys had sweepers ahead of them when they left or entered
such confined waters. The seas adjacent to harbours and naval bases were
searched at low water for mines which might be showing above the
surface. Around the anchorage of the Grand Fleet in Scapa Flow a wide
belt of sea was kept clear of mines so that at any moment the fleet
could reach blue water without risk from these weapons. The same
precautions were taken off the Firth o