Far up in the sky, which just then was as clear as crystal, a duel was in progress between two planes.  It was evident at a glance that both of the rival aviators were masters of their profession.  They circled deftly about each other like giant falcons, jockeying for position, each trying to get the weather gauge on the other where he could rake his opponent with his machine gun without exposing himself to his enemy's fire in return.