In military parlance a “gun” is a long-barreled piece that fires its projectiles with a flat trajectory, that is, the projectile is fired at a low angle and describes a long flat curve. A “mortar” is a short-ranged weapon which fires at a high angle so as to land its projectile over the walls of a fortification. The barrel of the mortar was formerly very short and had a smooth bore, but later the barrel was extended and the bore rifled so as to give a greater range, developing what is known as the “howitzer.” The famous 42-centimeter gun with which the Germans started the war was a howitzer which fired a shell a yard and a half long weighing 2,108 pounds. We have big guns to defend our coasts which fire a shell 16 inches in diameter, which is half an inch less than 42 centimeters, and the weight of the shell is 2,400 pounds. But the startling thing about the German big howitzers was that they were portable and could be brought up to smash fixed defenses. As the war proceeded enormous guns as well as howitzers were set on railroad mounts and moved about from time to time to avoid discovery by spying aviators.
The 76-mile gun which bombarded Paris fired projectiles of only 8.27-inch caliber. The projectile described a wide curve which carried it about twenty-four miles above the surface of the earth or about 3½ times as high as the greatest altitude ever reached by man in an aeroplane. Had it pursued a perfectly straight line from the forest of St. Gobain to Paris its course would have carried it 3,750 feet below the surface, because of the curvature of the earth. The range of the projectile was very materially increased by rising to such a great elevation because of the extreme tenuity of the atmosphere. The air resistance that a shell is obliged to overcome is not generally appreciated. In the denser strata of the lower atmosphere the resistance is very great, but as a shell mounts to higher levels the air resistance falls off considerably and at twenty miles it is practically nonexistent.
It was not until the summer after the armistice that details of this gun were disclosed. There were seven of these powerful guns that participated in the various bombardments. They wore out very quickly under the terrific strain to which they were subjected and were rebored to a diameter of 9.4 inches. In the last bombardment a number of shells of this size reached Paris. The guns were fired at an angle of 55 degrees from the horizontal so as to pass quickly through the denser layers of the air. The shell left the muzzle of the gun with the velocity of about 5,000 feet per second and arrived in Paris about three minutes later with a velocity of about half that amount. The enormous muzzle velocity was obtained by using a very long gun so that the powder could keep pushing the projectile for a comparatively long time. The guns were built out of worn-out 15-inch naval guns. These guns which were 56 feet long were rebored, fitted with a heavy tube and pieced out to a length of 118 feet. The last 20 feet of the bore was not rifled and served as a guide to keep the shell in perfect axial alignment when it emerged from the muzzle. A comparatively slow powder was used so as not to put too severe a strain on the gun at the breech, but gradually to accelerate the shell in its travel through the bore.