The bomb case consists of two parts: a body and a nose. The body is a tapering zinc shell which carries the firing mechanism and stabilizing tail fin at the small end and at the large end a threaded ring which screws into the nose. The nose is of drawn steel of such shape as to have low end-on resistance and is sufficiently strong to penetrate frame structures. [Illustration: FIG. 110.—Loading Bombs with “Solid” Oil.] The incendiary effect is produced by a thermit charge carried in the nose of the bomb. This charge is ignited by a booster of “Thermit Igniter” fired by black powder. The latter is ignited by a flash from the discharge of a standard 0.30 caliber service cartridge contained in the body of the bomb, and exploded by a firing mechanism of the impact type. This method of firing has proven wholly unsatisfactory and will be superseded by some more direct-acting mechanism. The body of the bomb is filled with solidified oil. The molten thermit burns through the case of the bomb and liberates the oil which has been partially liquefied by the heat of the thermit reaction. Additional incendiary effect is afforded by the sodium nitrate contained in the nose below the thermit, and by two sheet lead cylinders filled with sodium and imbedded in the solid oil. The sodium increases the difficulty of extinguishing the fire with water. =Mark III Bomb.= This bomb is simply a larger size of the Mark II bomb, its weight being approximately 100 pounds as compared with 40 pounds for the Mark II bomb. It is designed to be dropped from an aeroplane and is intended for use against buildings when marked penetrating effect is desired. The method of functioning is the same as the Mark II bomb and it has the same defects in the firing mechanism. =Mark I Bomb (Scatter Type).= The Mark I incendiary drop bomb is also designed to be dropped from aeroplanes and is intended for use against grain fields, ammunition dumps, light structures or similar objectives when only a low degree of ignition is required. It is of the so-called scatter type, due to the action of the exploding charge which casts out incendiary material within a radius of 20 feet from the point of contact. The incendiary action is due to the ejection of the various incendiary units in the bomb by the explosion of the black powder in the nose. The flash of this explosion serves to ignite the units. A powder charge in the rear of the bomb acts simultaneously with the nose charge, opening the bomb casing, and aiding materially in the scatter of the units. The bomb is so arranged as to function close to the ground, which is a further factor in the scatter of the units.