rance so long as the mechanical agencies of
man are able to cope with the influences of Nature. It can ascend to
a height which is out of all proportion to the depth to which the
submarine can descend in safety. It is a matter of current knowledge
that a submarine cannot sink to a depth of more than 250 feet: an aerial
vessel is able to ascend to 5,000, 8,000, or even 10,000 feet above the
earth, and the higher the altitude it attains the greater is its degree
of safety. The limit of ascension is governed merely by the physical
capacities of those who are responsible for the aerial vessel's
movement.

It is for this reason that the defensive measures which are practised in
the waters of the earth are inapplicable to the atmosphere. Movement
by, or in, water is governed by the depth of channels, and these may be
rendered impassable or dangerous to negotiate by the planting of mines.
A passing ship or submarine may circumvent these explosive obstructions,
but such a successful manoeuvre is generally a matter of good luck. So
far as submarines are concerned the fact must not be over looked
that movements in the sea are carried out under blind conditions: the
navigator is unable to see where he is going; the optic faculty is
rendered nugatory. Contrast the disability of the submarine with the
privileges of its consort in the air. The latter is able to profit from
vision. The aerial navigator is able to see every inch of his way, at
least during daylight. When darkness falls he is condemned to the same
helplessness as his confrere in the waters below.

A well-known British authority upon aviation suggested that advantage
should be taken of this disability, and that the air should be mined
during periods of darkness and fog to secure protection against
aerial invasion. At first sight the proposal appears to be absolutely
grotesque, but a little reflection will suffice to demonstrate its
possibilities when the area to be defended is comparatively limited. The
suggestion merely proposes to profit from one defect of the dirigible.
The latter, when bent upon a daring expedition, naturally prefers to
make a bee-line towards its objective: fuel considerations as a matter
of fact compel it to do so. 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, i