[Illustration: DIAGRAM V. If you want to make your shell fall into a trench of a fortification, A, or come down exactly on the top of the shelter in a fort, B, it is obvious that your howitzer, firing from H, and lobbing a projectile along the high-angle trajectory M, will have a much better chance of hitting it than your gun G, sending a projectile further indeed but along the flatter trajectory N.] The advantage of the howitzer is two-fold. In the first place, you can hide it behind a hill or any other form of obstacle or screen, as it shoots right up in the air. A gun which fires more or less flat along the earth cannot get at it. 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 in Diagram IV, and so timing the fuse that the shell bursts exactly there. Now, that is a difficult operation, both because it is difficult to spot a machine which you cannot see, and though modern time fuses are very accurate, they cannot, of course, be accurate to a yard. Secondly, the howitzer can drop its shell into a comparatively narrow trench, which the projectile of a gun with its flat trajectory will probably miss. If you want to make your shell fall into a trench of a fortification or come down exactly on the top of the shelter in a fort, as at A, the trench in the fifth diagram, or at B, the shelter, it is obvious that your howitzer firing from H, and lobbing a projectile along the high-angle trajectory M, will have a much better chance of hitting it than your gun G, sending a projectile further, indeed, but along the flatter trajectory N. Of course, another howitzer within the fortifications could, in theory, lob a shell of its own over the hill and hit the besieging howitzer, but in practice it is very easy for the besieging howitzer to find out exactly where the vulnerable points of the fortress are--its trenches and its shelter and magazine--and very difficult for the people in the fortress to find out where the howitzer outside is. Its place is marked upon no map, and it can move about, whereas the fortress is fixed. [Illustration: DIAGRAM VI. The fort on an elevation at A, and confined within a narrow space, is a target for howitzers placed anywhere behind hills at, say, four miles off--as at B-B, C-C, D-D. It is difficult enough for the fort to find out where the howitzer fires from in any case; furthermore, the howitzer can shift its position anywhere along the lines B-B, C-C, and D-D.] Look, for instance, at Diagram VI. The fort on an elevation at A, and confined within a narrow space, is a target for howitzers placed anywhere behind hills at, say, four miles off, as at B-B, C-C, D-D. It is difficult enough for the fort to find out where the howitzer fires from in any case, and even when it has spotted this the howitzer can move anywhere along the lines B-B, C-C, or D-D, and shift its position.