Why This Rule Prevails.
There is good reason for this precaution. The higher
the altitude the more rarefied (thinner) becomes the air,
and the less sustaining power it has. Consequently the
more difficult it becomes to keep in suspension a given
weight. When sailing within 30 feet of the ground sustentation
is comparatively easy and, should a fall occur,
the results are not likely to be serious. On the other
hand, sailing too near the ground is almost as objectionable
in many ways as getting up too high. If the craft
is navigated too close to the ground trees, shrubs, fences
and other obstructions are liable to be encountered.
There is also the handicap of contrary air currents
diverted by the obstructions referred to, and which will
be explained more fully further on.
