smoke. Gunner D
likewise observes whether there is any deviation to right or left of
the target and corrects in a similar manner. From the sum of the three
rounds the observer corrects the altitude, completes his calculations,
and communicates his instructions for correction to the three gunners,
who now merely train their weapons for altitude. The objective is to
induce the shells hurled from the three corners of the triangle to burst
at a common point 4, which is considered to be the most critical spot
for the aviator. The fire is then practically concentrated from the
three weapons upon the apex of a triangular cone which is held to bring
the machine within the danger zone.

This method of finding the range is carried out quickly--two or three
seconds being occupied in the task. In the early days of the war the
German anti-aircraft artillerymen proved sadly deficient in this work,
but practice improved their fire to a marvellous degree, with the result
that at the moment it is dangerous for an aviator to essay his task
within an altitude of 6,000 feet, which is the range of the average
anti-aircraft gun.

The country occupied by a belligerent is divided up in this manner
into a series of triangles. For instance, a machine entering hostile
territory from the east, enters the triangle A-B-C, and consequently
comes within the range of the guns posted at the comers of the triangle.
Directly he crosses the line B-C and enters the adjacent triangle he
passes beyond the range of gun A but comes within the range of the gun
posted at D, and while within the triangular area is under fire from the
guns B-C-D. He turns and crosses the line A-C, but in so doing enters
another triangle A-C-E, and comes range of the gun posted at E.

The accompanying diagram represents an area of country divided up into
such triangle and the position of the guns, while the circle round the
latter indicate the training arc of the weapons, each of which is a
complete circle, in the horizontal plane. The dotted line represents
the aviator's line of flight, and it will be seen that no matter how he
twists and turns he is always within the danger zone while flying over
hostile territory. The moment he outdistances one gun he comes within
range of another.

The safety of the aviator under these circumstances depends upon his
maintaining an altitude exceeding the range of the guns below, the most
powerful of which have a range of 8,000 to 10,000 feet,