The advantage of height was half the battle, and Barry wanted to snatch every inch of it he could gain. For that reason he passed a signal back to Spotty to open fire again, and Spotty obediently began to rip out a series of short bursts. The two men had flown so long together that each knew the other's dodges and ideas to an extent precious beyond words, and had a code of brief signals in head-noddings and jerkings and hand motions that saved much waste of time and breath in shutting off engine to shout messages or yelling through the communicating 'phone. Spotty figured now just the plan Barry had in mind, a plan to hustle the enemy into making his attempt before he was at the closest effective range for a diving attack. The plan succeeded too. His bullets must have been going somewhere close, for Spotty saw the nearest machine swerve ever so slightly, as if her pilot had flinched or ducked instinctively. Then Spotty saw her nose dip slightly until it was pointed straight at the "Marah," the machine-gun firing through her propeller broke out in a long rapid burst of fire, and the "tracer" bullets[3] came flashing and streaming past in thin pencils of flame and smoke. What followed takes a good deal longer in the telling than it did in the happening. All three machines were travelling, remember, at a speed of anything round a hundred knots, a speed that rose at times as they dipped and dived to nearer perhaps a hundred and thirty and forty. While they were flying on the same course with little difference in speed each airman could see the other closely and in detail, could watch each little movement, look over at leisure small items about each other's machines. Mere groundlings cannot get nearer to the sensation than to imagine or remember sitting at the window of a carriage on the slow lumbering sixty-mile-an-hour express, watching the almost equally slow mail rushing over the rails at sixty-five miles on a parallel line, and seeing the passengers at her windows scanning deliberately the shape of your hat or colour of your hair. [3] Tracer bullets emit smoke and flame to allow the shooter to follow their flight. In just such fashion Spotty saw the pilot of the leading machine rise slightly and glance astern at his companion, saw him settle himself in his seat, saw him raise a hand and motion downward. Instantly he jerked the cord fast to Barry's shoulder, signalling "look out," and with swift clockwork motions snatched the almost empty drum of his machine-gun, and replaced it with the full one he held ready clutched between his knees.