Consequently it is possible, within
certain limits, to anticipate the route which an invading craft
will follow: the course is practically as obvious as if the
vessel were condemned to a narrow lane marked out by sign-posts.
Moreover, if approaching under cover of night or during thick
weather, it will metaphorically "hug the ground." To attempt to
complete its task at a great height is to court failure, as the
range of vision is necessarily so limited.
Under these circumstances the mining of the air could be carried
out upon the obvious approaches to a threatened area. The mines,
comprising large charges of high-explosive and combustible
material, would be attached to small captive balloons similar to
the "sounding balloons" which are so much used by meteorologists
in operations for sounding the upper strata of the atmosphere.
These pilot balloons would be captive, their thin wires being
wound upon winches planted at close intervals along the
coast-line. The balloon-mines themselves would be sent to
varying heights, ranging from 1,000 to 5,000 feet, and with
several attached to each cable, the disposition of the mines
in the air in such an irregular manner being in fact closely
similar to the practice adopted in the mining of a channel for
protection against submarines and hostile ships.
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