II. Another German theory had maintained that modern high explosives
fired from howitzers and the accuracy of their aim controlled by
aircraft would rapidly and promptly dominate permanent fortification.

This theory requires explanation. Its partial success in practice was
the most startling discovery and the most unpleasant one to the Allies
of the early part of the war.

In the old days, say up to ten years ago or less, permanent
fortification mounting heavy guns was impregnable to direct assault if
it were properly held and properly munitioned. It could hold out for
months. Its heavy guns had a range superior to any movable guns that
could be brought against it--indeed, so very heavily superior that
movable guns, even if they were howitzers, would be smashed or their
crews destroyed long before the fortress was seriously damaged by
them.

A howitzer is but a form of mortar, and all such pieces are designed
to lob a projectile instead of throwing it. The advantage of using
these instruments when you are besieging permanent works is that you
can hide them behind an obstacle, such as a hill, and that the heavy
gun in the fortress cannot get its shell on to them because that shell
has a flatter trajectory. The disadvantage is that the howitzer has a
very much shorter range than the gun size for size.

[Illustration: Sketch 19.]

Here is a diagram showing how necessarily true this is. The howitzer,
lobbing its shell with a comparatively small charge, has the advantage
of being able to hide behind a steep bit of ground, but on such a