The first 3 in Stokes mortars appeared in the summer of 1915, firing high-explosive cylindrical projectiles. About simultaneously something like 20 similar mortars of a higher caliber are ordered and delivered for use with 'special' ammunitions, that is to say with chemical rounds. This is how the '4 in Stokes Mortar' is introduced, also called 'Chemical Mortar'.

At the end of 1915, this weapon was the standard British device to launch smoke or incendiary projectiles, then tear gas in 1916, and finally toxic gas in 1917. Its first official use has been recorded at Loos on sept 25, 1915.

Thos mortar was produced in less quantities than the very popular 3-in mortar, and was gradually replaced by the arrival of the Livens Projectors and the chemical shells of the conventionnal artillery.

Main characteristics :

• Weight in firing position : 80 kg
• Calibre : 107 mm (4.2 in)
• Tube length : 1330 mm
• Projectile weight : 8.16 kg (cylindric) including 3.18 kg of chemical agent
• Initial velocity : unknown
• Maximum range : 987 m (cylindric projectile)
• Direction range : 18 degrees
• Elevation range : 45 to 7à degrees