in the early stages of the war. This was the dart, a heavy bullet p]
steel sharpened to a point at one end. These darts, released in
showers, were intended to be effective against personnel.
(C. F. A.)
AIR DEFENCE. Even before aerial navigation and aviation
had been developed to a practical point, the employment of
aircraft in war for the attack of vulnerable places was discussed
from time to time in a speculative way, and in the seven or
eight years preceding the World War the types and characteristics
of aircraft became so far definite that technical study could be
brought to bear on the problem of defence, especially that of
artillery defence. But this period of seven or eight years was
short; military history could give no lead; practical experiments
were almost impossible. Moreover, in the existing state of
international law, liability to air attack was understood to
depend not on whether a place was vulnerable, in the sense that
its destruction would impair the capacity of the nation for
carrying on the war, but on whether it was " defended," i.e.
fortified in the conventional sense of the word, or at any rate
held by a ground garrison against ground attack. Attention was
therefore directed chiefly to the question of air attack on what
according to the prevailing ideas were " military " objectives,
and in view of the small numbers of aircraft then available such
attacks were regarded as unlikely to affect the course of operations
seriously.
In these conditions, and especially in the absence of all data
based on practical experience, it is not surprising that defence
against air attack was everywhere in a rudimentary state at the
outset of the World War. In the war itself, on the contrary,
experience, data and methods crowded upon one another from
first to last, and through the clearer definition of the problems to
be faced on the one hand and the constant process of trial and
error on the other, it has become possible to formulate the main
principles of air defence with some approach to precision.
/. General Considerations.
Air defence, as discussed in the present article, deals with
the arrangements which deny to enemy aircraft access to vulner-
able points. By " access " is implied the gaining of a position
either directly over the objective or sufficiently close to it for the
success of the attack. Amongst " vulnerable points " are in-
cluded bodies of troops and their materiel in the field, centres of
population, large magazines, arsenals, harbours, ports, dock-
yards, ships and convoys at sea, big industrial centres, and the
like. As with a fleet, the primary duty of the air forces is to seek
out the enemy air forces and destroy them; but the problem in
the air is far more difficult than on the sea, as a third dimension
has to be taken into account, i.e. that of height. When we
remember what difficulties have been introduced into naval war
by the introduction of the submersible warship, slow as this is
and small as is its up-and-down range, and when we realize
further that, in the air, opposing craft can pass at great heights
both above and below each other, move at speeds that are enor-
mous relatively to any rate of movement on or under the sea and
have to cope with extraordinary difficulties in detecting one
another's proximity, it will be seen that the task in front of the
air forces of any nation is the most difficult of all.
There is no " command of the air " while the enemy disputes
it. Therefore, against attacks by air, it is a logical necessity to
provide ground defences, and to limit the radius of the air
units allotted to cooperate with them. But though defence
against attack by air, as on the ground, may be active or passive,
yet to be effective it must be both. Further, the conditions
under which aircraft move, by day and by night respectively,
are so widely different, that the conditions of the defence must
be correspondingly varied to meet them.
It is not intended here to deal with operations launched over
long distances against enemy aerodromes and depots, such being
the role of the air forces alone, but solely with the local defence
of areas occupied by " vulnerable points " of the kind which have
been enumerated above.
The instruments of air defence are: (a) the machine-gun
on the ground; (6) the machine-gun in the aeroplane; (c) the
heavier guns on the ground; (d) the searchlight, the sound locator,
the observer post; (e) the aerial obstacle; (/) local protection on
the ground, i.e. against bomb splinters and machine-gun fire, and
camouflage.
Each of these weapons supplements a deficiency in one or more
of the others; none is complete without one or more of the
others acting in conjunction with it. From this it follows that, in
any anti-aircraft organization, cooperation in effort can only be
effected by organizing units of the air force, artillery and engineers
under a single command, as in a formation of all arms in ground
warfare.
The following are some of the forms which attacks by air may
take: bombing by airships, aeroplanes or seaplanes, singly or
in formation; the harassing of troops on the ground or sea with
machine-gun fire by aeroplanes or seaplanes, singly or in pairs;
torpedoing ships at anchor, by seaplanes, probably in pairs or
escorted by " scout " (i.e. air fighting) machines. To these may
be added, though they only indirectly affect the problems here
discussed: photographic or visual reconnaissance, by aeroplanes
or seaplanes, singly; and aerial engagements, by aeroplanes in
formation on hostile patrol, i.e. ready to engage air-fighting
groups of the enemy, or by aeroplanes, singly or in groups,
attacking machines which are engaged in observation duties
(especially artillery observation) in connexion with ground
operations.
Anti-aircraft units are concerned primarily with hostile
attacks the objectives of which are on the ground or sea; the
defence of objectives in the air is a secondary matter, but never-
theless important when air-force units are not at hand to under-
take the duty themselves.
It may not be amiss at this stage to mention a few of the
peculiarities of sound and light. Although these properties are
generally known, their full importance in relation to air defence is
seldom recognized by those who have little experience in anti j
aircraft work. Sound travels at a certain known rate, namely
about 1,100 ft. per second. Aircraft are generally first detected
by ear. By the time the sound of a machine reaches the ear, the
machine making it will have moved some distance away from the
spot where it made the sound. A path of sound is deflected
by the different velocities of the various air currents through
which it passes on its way to the ground. A machine in the air is
only visible to an observer on the ground by reason of light rays
reflected from the surfaces of the machine, reaching his eye in
sufficient intensity to enable him to detect it. Thus a machine
flying " straight into the sun " is invisible to an observer also
facing the sun. A spherical or cylindrical surface reflects light
chiefly in the direction from which the illumination comes;
hence, in the case of a balloon or airship, the observer sees the
target best when he and a searchlight are on the same side of it.
A flat surface reflects light towards an observer further from the
source of illumination than its own position; thus an aeroplane
flying steadily in a searchlight beam is generally seen best when
it is between the observer and a searchlight. It does not follow
that because aircraft are invisible to observers on the ground
the ground is invisible to an observer in the machine, and
nice versa. The greatest difficulty is frequently experienced in
gaining the correct focus for the eye, and, having gained it, to
maintain that focus. This is a difficulty common to the observer
both on the ground and in the air, but whilst the former has only
to look upwards, the latter has in addition to look all round and
below his machine. Neither has the assistance of intermediate
objects by means of which the focus of the eye can be altered
and held at definite stages. By day the observer in the air is
deaf, by night he is deaf and blind. A searchlight shining into
the sky is only visible by reason of the particles of " dust " or
moisture in the path of the beam. In a perfectly clear or clean
atmosphere the beam is invisible. 1
Unusually clear atmospheric conditions with a few high clouds
were responsible for popular rumours prevalent during the war in 1914-8, to the effect that a new invisible searchlight had been dis- covered which simply threw a disc of light far away up in the sky, and was otherwise quite invisible.
