The theory that these ring fortresses could hold out indefinitely was based upon the idea that the fort so far out from the fortress would keep the enemy’s guns too far away to damage the inner place of stores and garrison, and that the supporting fire of the various forts would prevent anyone getting between them. The three systems--first the stone wall, then the earthwork, then the ring fortress, are roughly expressed in the second diagram. [Illustration: DIAGRAM II. Mankind at war has always used devices whereby he has been able with a small number to detain the advance of a larger number. Some of these systems are roughly expressed above. 1. The old stone fortress or castle of the Middle Ages. 2. The wall round a town. 3. The earthworks of a fortress of the period 1620-1860. 4. The “Ring” Fortress (1860-1914)--a series of detached forts lying three or four miles outside the inner place of stores, barracks, etc., which it was desired to defend.] Well, the chief lesson, perhaps, of the present war is that these ring fortresses fall quickly to howitzer fire. Each of the individual forts can be easily reduced by howitzer fire. This is concentrated against certain of the forts, which quickly fall, and once their ring is broken the result is equivalent to the breach in the wall of a fortress, and the whole stronghold falls. That is because in quite recent years two new factors have come in: (1) the mobile heavy howitzer; (2) the highest kinds of explosives. [Illustration: DIAGRAM III. A howitzer is a gun with a shorter barrel than the ordinary gun, and designed not to shoot its projectile more or less straight across the earth, as an ordinary gun does, but to lob it high up so that it falls almost perpendicularly upon its target.] A howitzer is a gun with a shorter barrel than the ordinary gun (and therefore lighter in proportion to the width of the shell, and so to the amount of the explosive it can fire) and designed not to shoot its projectile more or less straight across the earth, as an ordinary gun does, but to lob it high up so that it falls more or less perpendicularly upon its target. Thus the German 11.2-inch howitzer, of which we have heard so much in this war, has a maximum range when it is elevated to 43 degrees, or very nearly half-way between pointing flat and pointing straight up--and howitzers can be fired, of course, at a much higher angle than that if necessary. [Illustration: DIAGRAM IV. You can hide a howitzer behind a hill. The gun, though it has a longer range than the howitzer, can only get at the howitzer indirectly by firing over the point where it supposes the howitzer to be, as at A. Secondly, the howitzer can drop its shell into a comparatively narrow trench which the projectile of the gun will probably miss.]