y be represented by the
formula 365-_x_, in which _x_ is as yet an unknown quantity, though it
is no doubt a quantity which will diminish as the art of aviation is
developed. The other is that there is as yet no known method of
navigating an aircraft with accuracy and precision out of sight of land.
The air-currents by which it is affected are imperceptible to those
embarked, variable and indeterminate in their force and direction, and
quite incapable of being charted beforehand. In these conditions an
airman who sought to steer by compass alone, say, from Bermuda to New
York, might perchance find himself either at Halifax, on the one hand,
or at Charleston on the other.

In my chapter on "Invasion" no mention is made of those subsidiary forms
of military enterprise across the sea which are known as raids. I have
treated invasion as an enterprise having for its object the subjugation
of the country invaded, or at least the subjection of its people and
their rulers to the enemy's will. As such it requires a force
commensurate in numbers with the object to be attained, and it stands to
reason that this force must needs be so large that its chances of
evading the vigilance of an enemy who is in effective command of the sea
must always be infinitesimal. A raid, on the other hand, is an
enterprise of much lesser magnitude and much smaller moment. Its method
is to elude the enemy's naval guard at this or that point of his
territory; and, having done so, its purpose is to land troops at some
vulnerable point of the territory assailed, there to create alarm and
confusion and to do as much harm as they can--which may be considerable
before their sea communications are severed by the defending naval force
assumed to be still in effective command of the sea affected. If that
command is maintained, the troops engaged in the raid must inevitably be
reduced sooner or later to the condition of a forlorn hope which has
failed. If, on the other hand, that command is overthrown, then the
troops aforesaid may prove to be the advanced guard of an invasion to
follow. Thus, although a successful raid may sometimes be carried out in
the teeth of an adverse command of the sea, yet it cannot be converted
into an invasion until that adverse command has been assailed and
overthrown. It is thus essentially fugitive in character, possibly very
effective as a diversion, certain to be mortifying to the belligerent
assailed, and not at all unlikely to