The portable Flammenwerfer consisted of a sheet steel cylinder of two compartments, one to hold compressed nitrogen, the other to hold the oil. The nitrogen furnished the pressure which forces the oil out through the flexible tube. Air cannot be used, because the oxygen would form an explosive mixture with the vapors of the oil, and any heating on compression, or back flash from the flame or fuse might make things very unpleasant for the operator. A pressure of about 23 atmospheres is reached when the cylinder is charged. The nitrogen appeared to be carried on the field in large containers and the flame projectors actually charged in the trenches. The oil used in the flame projectors varied from time to time, but always contained a mixture of light or easily volatile and heavy and less volatile fractions of petroleum or mineral oil, very carefully mixed. In some cases even ordinary commercial ether has been found in the cylinders. [Illustration: FIG. 114.—Small Flammenwerfer.] The most interesting part about the flame projector is the lighting device. This is so made that the oil ignites spontaneously the minute the jet is turned on, and is kept alight by a fiercely burning mixture which lasts throughout the discharge. This mixture is composed of barium nitrate, potassium nitrate, metallic magnesium and charcoal, with some resinous material. The priming consists of black powder and metallic magnesium. When the oil rushes out of the jet, it forces up the plunger of a friction lighter and ignites this core of fiercely burning mixture. The range of these small projectors is from 14 to 17 meters (17 to 20 yards) but the duration of the flame is rather less than a minute. In a later pattern, it was designed that one nozzle should be issued to three reservoirs. After the discharge of one, the jet is attached to the others in succession. This is called the “Wx” Flammenwerfer (interchangeable). In this way a squad of three men could carry 58 pints of inflammable oil. It is a question, though, whether the third man would live to use his reservoir. [Illustration: FIG. 115.—Boyd Flame Projector.] The fact that the trenches were often very close together during the early part of the war, made possible the use of large or stationary Flammenwerfer. These consisted of a steel reservoir 3⅓ feet in height and 1⅔ feet in diameter, weighing about 250 pounds, which could be connected to two steel cylinders, containing nitrogen under pressure. These carried 180 liters (40 gallons) of liquid and operated under a pressure of 15 atmospheres. The discharge nozzle was at the end of a metal tube three feet long, and its orifice was about ⁵/₁₆ of an inch in diameter. The range of this apparatus was from 33 to 40 yards and the duration of the flame from one to two minutes. Because of the comparatively short range of these guns and the ease with which they could be destroyed, if located by the enemy, their use was very limited.