Fig. 3.

A third round is fired, and the shell explodes at F. In this
instance the explosion takes place below the balloon. Both the
observers and the artillery man concur in their deductions upon
the point at which the shell burst. But the shell must explode
above the balloon, and accordingly a fourth round is discharged
and the shell bursts at G.

This appears to be above the balloon, inasmuch as the lines of
sight of the two observers and B converge at this point. But
whether the explosion occurs immediately above the vessel as is
desired, it is impossible to say definitely, because it may
explode too far behind to be effective. Consequently, if this
shell should prove abortive, the practice is to decrease the
range gradually with each succeeding round until the explosion
occurs at the critical point, when, of course, the balloon is
destroyed. An interesting idea of the difficulty of picking up
the range of a captive balloon may be gathered from the fact that
some ten minutes are required to complete the operation.

But success is due more to luck than judgment. In the foregoing
explanation it is premised that the aerial vessel remains
stationary, which is an ex tremely unlikely contingency. While
those upon the ground are striving to pick up the range, the
observer is equally active in his efforts to baffle his
opponents. The observer follows each successive, round with keen
interest, and when the shells appear to be bursting at
uncomfortably close quarters naturally he intimates to his
colleagues below that he desires his position to be changed,