"If the aviator is enshrouded in fog or tries to sail through a heavy bank of clouds, he is quite likely to lose all sense of direction," continued Paul.  "He will not know whether he is banking or traveling on an even keel.  Sometimes pilots have come out of a low cloud to find themselves dangerously close to the earth and in an awkward position, perhaps in a steep bank, a side-slip, or even in the terrifying nose-dive, and they have not had time to right themselves before crashing to earth.  So you see that before flying can become reasonably safe, some way must be found of keeping the machine automatically on a level keel.