ch, in favourable weather, can be towed by a lorry and can
be moved frequently without loss of efficiency.

AEROPLANES are of the greatest value for reconnaissance and
inter-communication, and not only obtain, and return to their base
with, information of the highest value, but facilitate personal
reconnaissance of the battlefield by commanders and staff officers.
Their offensive and defensive action is also very great and the moral
effect of their offensive action is of the highest value. Although
aeroplane squadrons are mobile units they lose efficiency if the units
are moved too frequently. The action of aircraft in various phases of
fighting is dealt with throughout the Lectures.

KITE BALLOONS carry two observers, who can remain in telephonic
communication with the ground up to a {176} height of 5,000 feet.
Inflated balloons can be moved in favourable weather at a maximum speed
of 8 miles an hour while at a height of about 500 feet. Their extreme
vulnerability to artillery fire prevents their use close to the battle
front.

GAS

"The advisability of employing gas as a military weapon is a matter for
consideration by the authorities concerned before a campaign begins.
Once authorised, however, and assuming that weather conditions are
favourable, gas may be expected to play a part in every action. . . .
The different methods in which gas can be employed make it a weapon
which can be used by all arms, thus _Artillery_ deal with gas shells,
_Infantry_ with light mortar gas bombs, _Aircraft_ with aerial gas
bombs, and _Engineers_ with all methods of use that call for special
manipulation" ("Field Service Regulations," vol. ii. (1921)).

Gas was introduced by the Germans during the _Second Battle of Ypres_
(April 22-May 18, 1915), and the numerous experiments and trials
necessary before gas can be used, and the great preparations which have
to be made for its manufacture, show that its employment was not the
result of a desperate decision, but had been prepared for deliberately.
During the _First Battle of the Somme_ (September 1-November 18, 1916)
"the employment by the enemy of gas and liquid flame as weapons of
offence compelled us not only to discover ways to protect our troops
from their effects, but also to devise means to make use of the same
instruments of destruction. . . . Since we have been compelled, in
self-defence, to use similar methods, it is satisfactory to be able to
record, on the evidence