towards the paravane, where the mooring would be cut and the mine would
float to the surface.]

Among those devices which had for their object the minimising of the
result of a mine explosion may be mentioned the "Blister System" so
successfully employed in the construction of monitors and other big
ships, the idea being to surround the inner hull with an outer casing
which received the effect of the explosion of either a mine or torpedo
and left the inner or real hull of the ship water-tight. Its one weak
feature was that it reduced the speed of the ship and the ease with
which she could be manoeuvred. In future types of large and heavily
armed ships this drawback will undoubtedly be largely overcome by an
increase in engine-power made possible by the development of engineering
science.

The "blister," although outwardly forming a continuous structure round
the entire vessel, extending well above and below the water-line,
tapered off towards the bows and stern, and was subdivided into
different compartments. In this way an explosion against one section did
not necessarily damage any other part. The British monitors which so
successfully bombarded the Belgian coast and the fortifications of the
Dardanelles were fitted with blisters, and more than one of them owed
their salvation to this means.

CHAPTER XIII

THE MINE BARRAGE

WHAT undoubtedly forms the most effective counter to unrestricted
submarine warfare is the explosive mine barrage, as employed against the
German U-boats in the North Sea and the Straits of Dover.

The practicability of these barrage systems depends, however, very
largely upon the following factors:--(1) the geographical features of
the area of operations; (2) the hydrographical peculiarities of the seas
in which the mines have to be laid; (3) the number of properly equipped
mine-laying vessels available; (4) an adequate and highly trained
personnel; and (5) the mechanical skill and manufacturing power of the
nation employing the system.

There are several forms of mine barrage. One is simply an elongated
mine-field laid across a narrow sea to prevent the safe passage of
hostile surface craft. In this case the mines are laid in the ordinary
manner and at the ordinary depth below the surface. The anti-submarine
barrage, however, consists of an enormous number of mines, laid _at a
considerable depth below the surface_ and in such formation as to ensure
that a submarine attempting to