Before we go any farther we must learn the meaning of the word "trajectory." No bullet or shell travels in a straight line. As soon as it leaves the muzzle of the gun, it begins to fall, and its course through the air is a vertical curve that brings it eventually down to the ground. This curve is called the "trajectory." No gun is pointed directly at a target, but above it, so as to allow for the pull of gravity. The faster the bullet travels, the flatter is this curve or trajectory, because there is less time for it to fall before it reaches its target. Modern rifles fire their missiles at so high a speed that the bullets have a very flat trajectory. But in trench warfare a flat trajectory was not desired. What was the use of a missile that traveled in a nearly straight line, when the object to be hit was hiding in the ground? Trench fighting called for a missile that had a very high trajectory, so that it would drop right into the enemy trench.
Trench warfare is really a close-quarters fight of fort against fort, and the soldiers who manned the forts had to revert to the ancient methods of fighting an enemy intrenched behind fortifications. Centuries ago, not long after the first use of gunpowder in war, small explosive missiles were invented which could be thrown by hand. These were originally known as "flying mortars." The missile was about the size of an orange or a pomegranate, and it was filled with powder and slugs. A small fuse, which was ignited just before the device was thrown, was timed to explode the missile when it reached the enemy. Because of its size and shape, and because the slugs it contained corresponded, in a manner, to the pulp-covered seeds with which a pomegranate is filled, the missile was called a "grenade."
Grenades had fallen out of use in modern warfare, although they had been revived to a small extent in the Russo-Japanese war, and had been used with some success by the Bulgarians and the Turks in the Balkan wars. And yet they had not been taken very seriously by the military powers of Europe, except Germany. Germany was always on the lookout for any device that might prove useful in war, and when the Germans dug themselves in after the First Battle of the Marne, they had large quantities of hand-grenades for their men to toss over into the trenches of the Allies. These missiles proved very destructive indeed. They took the place of artillery, and were virtually hand-thrown shrapnel.