the gas cloud approaches depends entirely on the
wind velocity. Gas attacks have been made with wind velocities varying
from 3 to 20 miles per hour, _i.e._, from 1-1/2 to 10 yards per
second. In a 9-mile wind, the gas would reach trenches 100 yards
distant in 20 seconds.

Gas attacks have been made on fronts varying from 1 to 5 miles; their
effects at points up to 8 miles behind the front trenches have been
sufficiently severe to make it necessary to wear helmets.

(2) GAS PROJECTILES:

The use of these is not entirely dependent on the direction of the
wind. In gas projectiles such as shells, hand grenades, and trench
mortar bombs, a part or the whole of the explosive charge is replaced
by a liquid which is converted into gas by the explosion. The
explosive force and noise of detonation of these projectiles is less
than that of the ordinary kind, and a large number of them are usually
discharged into a comparatively small space. After the explosion, the
irritant chemicals form a small gas cloud, though some may sink to the
ground and remain active for a considerable time.

For using gas shells, the best condition is calm, or a wind of low
velocity.

Gas projectiles can be used in all types of country. Woods, bushes,
corn fields and clumps of buildings may hold the gas active for a
considerable time.

Two kinds of shell gases are used by the enemy, viz., lachrymators,
which mainly affect the eyes, and poison gases, which may affect the
eyes and are just as deadly as the gases used in the form of clouds.

(1) TEAR, OR LACHRYMATORY SHELLS:

These shells on explosion drive the liquid chemical which they contain
into the air as a mist. They cause the eyes to water strongly and
thereby gradually put men out of action.

Their actual smell may be slight. Large concentrations of lachrymators
begin to affect the lungs and cause sickness, coughing and general
irritation.

(2) POISON SHELLS:

Besides the comparatively harmless lachrymators the enemy also uses
projectiles which contain a gas, the action of which is very similar
to that of phosgene. Because of their slight detonation, these shells
are liable to be mistaken for blinds, but they emit large quantities
of a gas which attacks the lungs strongly and is very dangerous, and
even in slight cases may cause serious after effects.

(3) SMOKE:

The enemy may make use of smoke, either in the form of a cloud or
emitted from shells and bombs. Smoke may be used with