The incendiary units are waste balls about 2.5 in. in diameter and having an average weight of 2.5 ounces, tied securely with strong twine. These are soaked in a special oil mixture. Carbon disulfide and crude turpentine, or carbon disulfide, benzene heads and crude kerosene gave satisfactory results. A later development attempted to replace the waste balls by solid oil, but the difficulties of manufacture and questions of transportation argued against its adoption.
These bombs were not used at the front. Nearly all of the American incendiary bombs proved too light on the nose and lower half, generally resulting in deformation upon impact and very poor results. New ones will be made stronger.
Incendiary Darts
The British early recognized the value of a small bomb, and consequently adopted their B.I.B. (Baby Incendiary Bomb), weighing about 6.5 ounces. These are capable of being dropped in lots of 100 or more and thus literally shower a given territory with fire. The intensity of fire at any given point is much less than that obtained with the larger bombs, but the increased area under bombardment more than counter balances this disadvantage. While the British aimed at the perfection of a universal bomb, the American service felt that two classes should be developed, one to be used against grain fields and forests, the other against buildings.
The first class was called the Mark I Dart. This consisted of an elongated 12-gauge shot gun shell, filled with incendiary material and provided with a firing mechanism to ignite the primer as the dart strikes the ground. The flash of the primer sets fire to the booster, which, in turn, ignites the main incendiary charge. The latter burns several minutes, with a long flame. A retarding stabilizer attached to the tail of the dart serves the two-fold purpose of insuring the functioning of the firing mechanism and, by retarding the final velocity of the dart, preventing the collapse of the dart body when dropped from very high altitudes.
The incendiary mixture is one which gives a long hot flame, burns for several minutes and leaves very little ash. In general it consists of an oxidizing agent (barium or sodium chlorate), a reducing agent (aluminum, or a mixture of iron, aluminum and magnesium), a filler (rosin, powdered asphaltum or naphthalene) and in some cases a binder (asphaltum, varnish or boiled linseed oil).
The Mark II Dart was developed to furnish a small size penetrating agent. It consists of a two-inch (diameter) zinc case filled with thermit and solid oil as the incendiary materials and provided with a cast iron nose for penetration. During the first half minute after firing, a pool of molten iron is formed by the thermit, which is very penetrating and affords a good combustible surface for the oil, which burns for an additional ten minutes.
It has an advantage over the Mark I dart in that it penetrates, and over the Mark II bomb in that it is smaller and lighter in weight.
Incendiary Shell
Incendiary shell have been successfully used against aircraft and to some extent in bombardments of inflammable ground targets. Anti-aircraft shell are of small caliber and are usually tracer-incendiary. Such shell are filled with pyrotechnic mixtures which ignite at the moment of firing, or by time fuse, and are effective against highly inflammable material. Shell filled with thermit which explode and scatter the molten iron have been used against aircraft and ground targets, but with rather poor results. Large shell, which burst upon impact and scatter units of burning materials, have been used with some success against ground targets.