Efforts to make the bursts of fire from aircraft guns of maximum effectiveness have led to the development of three distinct types of ammunition—tracer, armor-piercing, and incendiary. The tracer type of ammunition was developed to assist the gunner in correcting his aim, and is equally useful by night or day, as the course of the bullet can be traced by a trail of white smoke in the daytime and by a bright spark at night. Armor-piercing ammunition has a projectile consisting of a hard steel core with a soft nickel casing. The object of this ammunition, as its name implies, is to pierce any of the metallic parts of an enemy plane, particularly the gasoline-tanks or the engine, the soft nickel casing acting as a lubricant and preventing the steel core from glancing off. Incendiary ammunition is loaded with yellow phosphorus. When the cartridge is fired the rifling in the barrel of the machine-gun opens a small hole in the case of the projectile, thus permitting the phosphorus to come in contact with the air, whereupon it immediately ignites and sets fire to any inflammable part of a plane which it may hit. It is customary to load the belts or pans of aircraft machine-guns with these three types of special ammunition in a certain sequence, depending upon the notions of the pilot himself. A sequence commonly used was, first, the tracer cartridge, which assisted the gunner in correcting his aim; next, two or three armor-piercing cartridges, in the hope that they would pierce the enemy’s gasoline-tank or damage his engine; and then one or two incendiary cartridges, which if the gasoline-tank was pierced would ignite the leaking gasoline and set fire to the machine. This sequence was continued throughout the loading of the belt or pan. Another branch of sky warfare which was being rapidly developed was aerial bombing. Though bombs of a sort were used by Italian aviators against the Arabs during the Libyan campaign, and by American soldiers of fortune serving with the Villista forces in northern Mexico, these attempts were so amateurish and ineffective as to merit no serious consideration. It may be said that the first bombs dropped from an aircraft in the history of warfare were those loosed from the German Zeppelin which raided Antwerp in August, 1914. I speak with a certain personal knowledge of my subject, for the first bomb dropped on the night in question exploded less than a hundred yards from the window in which I was sitting, demolishing a house and killing three persons.