But no such impediments are encountered in the ether. The craft
of the air is virtually a free age in the three dimensions. It
can go whither it will without let or hindrance 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.
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