le time--at least 10 minutes--after it has ceased. It is, in
fact, a common practice for the enemy infantry to retire to the second
and third line of trench whilst gas is being discharged. There is,
therefore, no object in opening an intense S.O.S. barrage of artillery
on "No man's land" during the actual gas cloud and it is advisable
that the warning to the artillery of a gas attack should be a signal
differing from the ordinary S.O.S. signal, as the latter may have to
be sent later if an infantry attack develops.

It must be remembered that smoke may be used by the enemy at the same
time as, or alternately with, the gas and that under cover of a smoke
cloud he may send out assaulting or raiding parties. A careful
look-out must, therefore, be kept; hostile patrols or raiders may be
frustrated by cross-fire of rifles and machine guns and should an
assault develop the ordinary S.O.S. procedure should be carried out.

I. Precautions Against Gas Shells:

Owing to the small explosion which occurs with these shells, they are
liable to be mistaken for blinds, and even when the gas is smelt men
may not realize its possibly dangerous character at once and so may
delay putting on respirators or helmets until too late. Men sleeping
in dug-outs may be seriously affected unless they are roused. Men in
the open air are unlikely to be seriously affected by poison gas
shells, provided they put on respirators or helmets on first
experiencing the gas. The following points should therefore be
attended to:

(i) All shells which explode with a small detonation or appear to be
blind should be regarded with particular attention; the respirator or
helmet should be put on at the first indication of gas and blanket
protection of shelters adjusted.

(ii) Arrangements must be made for giving a _Local_ alarm in the event
of a sudden and intense bombardment with poison gas shells, but care
must be taken that this alarm is not confused with the main alarm.
Strombos horns must on no account be used to give warning of a gas
shell bombardment.

(iii) All shelters in the vicinity of an area bombarded with poison
gas shells must be visited and any sleeping men roused.

(iv.) Box respirators or helmets should continue to be worn throughout
the area bombarded with poison gas shells until the order is given by
the local unit Commander for their removal.

Lachrymatory or "tear" shells are frequently used by the enemy for the
purpose of hindering