Four thousand five hundred--he glanced at the hostile biplane to his left and saw that it hung larger in the sky. Even in the moment for which he watched it it dilated. It was approaching at top speed. He was discovered, pursued. Instantly he turned off to his right and raced across the battlefield in the direction of the threatening flank. As he did so, he perceived another aeroplane rising from the enemy's lines. It climbed swiftly in bold swoops and then shot off towards him in a great upward slant. Two! Where was the third? He failed to discover it and held on his course. His direction was at an angle across the battlefield which took him towards the enemy's left flank rather than to their own right. As he sped over it, he looked down upon a broad miles-long belt of yellow-grey dust that rose raggedly into the air, and was spotted with an innumerable multitude of white puffs that renewed themselves as fast as they were dissipated. In many places these puffs congregated thickly and, as they broke, linked themselves with others until they floated like little narrow clouds in the air below him. As he looked down into the great concavity of the earth he seemed to be over some enormous smoking fissure in a crater whose circumference was the horizon. The rumble and roar which ascended from it assisted the illusion. Tiny sparks of flame darted and flickered in the fumes of that inferno, and here and there flashed a number of glittering points, the reflection of the sun from advancing bayonets. To distinguish men was impossible, but in occasional rifts in the dust curtain he could make out brown patches of varying size, and, over to his left, on the enemy's side, similar though darker patches. He could permit himself no sustained scrutiny of the scene below him for the management of the machine began to claim all his attention. Even at that great height above the battle, the air on that windless day, shaken and riven by the unceasing concussions of the massed artillery of two armies, was full of flaws. The needle of the barograph flickered, oscillated violently in leaps to and fro. The monoplane, tilted dangerously, now on one side, now on the other, in eddies of the tortured atmosphere, slid downward dizzily ere it could be brought up to climb a bank of air. It needed strong arms at the controls, a quick brain and nerves of perfect tone to keep her upon the appointed course. Glancing back, the aviator saw that the flight of the nearer of the two hostile machines, the one which had risen from the enemy's lines and was now approaching him on his left, was similarly erratic.