FRENCH avion de chasse darted swiftly past, close on the trail of a Boche Albatross. The planes twisted and turned, vrille following vrille in quick succession, and now and then long luminous wires of fire and smoke shot out, brilliant even in the sunshine, followed by the slower vibrations of the sound-waves -the sharp rat-tat-tat-tat of the mitrailleuse. Less apparent to the eye but infinitely more sinister than the luminous tracer-bullets were the quartets of invisible leaden missiles which spat forth between each of the gleaming silver wires. And now we watched a lumbering old bus climbing in slow, effortful spirals from her aerodrome and at last, like an overfed

vulture, steering heavily off toward the Boche lines. Two tiny fighting-machines unravelled themselves from wisps of drifting cloud high overhead and circled casually in the direction of the big plane beneath. Instinctively one thought of a pair of dainty warblers keeping guard over the first flight of their great changeling cowbird, to which their unsuspecting parent instincts had been sacrificed. On and on went the low-swinging plane, now climbing, now descending, as if on some gigantic aerial roller-coaster track. Occasional puffs of blue-white smoke, wide of the mark, showed the advantage of this constant shifting of altitude. When almost vanished in the distance we saw her descend a thousand feet, braving the shrapnel, which now were mere noiseless

specks flecking the sky about her. Twice she circled, then, at the approach of a small squadron of Boche fighters, seemed to spread her wings still wider and with all the power of her revolving blades streaked for home. No aged hen, threatened by some yapping cur, ever more completely cast dignity aside and fled toward barnyard safety than this unlovely biplane, signalling frantically for help to her attendant guardian angels, and barging with all speed toward her canvas hangar. After watching the approach and manoeuvring of the dainty fighting-planes, dancing on the invisible air, answering every touch of their joy-sticks, one could scarce but laugh at the aged hen, whose progress had been so slow and fearful, and after a moment's hesitation over enemy lines, had cowardly turned tail and fled. Yet she was not an atavism, nor an error on the part of the aviation commandant, but a most valuable cog in that great aerial machine which forever filled the heavens with trails, with whirlpools of impalpable wakes, with utilization of a myriad forms of invisible vibrations, manifested in heat, in light, and photography; as evanescent as the music of an orchestra, but as effective as projectiles from heavy artillery.

The old lumbering bus held in her heart a sensitive bit of glass which, fertilized by an instant of sunlight, would soon develop into a weapon more potent than a score of the mocking, dancing avions de chasse which so superciliously deigned to lend her their protection.

For days these little dragon-flies had sped here and there over a certain section of the Boche lines and had watched a mysterious swarming. They dared not slacken speed, for the least delay drew a burst of shrapnel, and they could but guess what was going on.

So, in despair, the old bus was trundled out, with a big ugly affair like a baby's coffin or a misshapen dwarf cannon clamped to her hip. At the bottom was a great staring eye, a huge bulging lens of glass, eight inches across. A second box, heavy with a weight more precious than any of powder and shot, was inserted into this impossible instrument, and the great aeroplane-camera was loaded for its deadly work.

Rocking and jolting, the big plane taxied across the muddy field, slowly lifted, and when three thousand feet up turned toward Bocheland. Trusting to the little guardian planes, bumping against the cloud ceiling a mile higher, the operator focussed all his attention on the big glass finder with its crossed hairlines. In front of him was a map with a big red blotch on the spot he was to watch for. The familiar spider-web of rear-trench and road-lines became recognizable and slipped behind; the première ligne Française was passed, and after two short directions to the pilot there crept into the glass field a whitish patch at the edge of the forest. Slowly, like the weird shadow of an eclipse, came the strange mussed-up area, and then the observer pulled hard on a handle. There was a click, a sound of something falling into a metal groove. The handle was pushed back-the photograph was taken and another plate ready in position. A flurry of cloud swept past and dimmed the lens with drops of moisture, and before it cleared a quartet of black-crossed aviatiks came into view full tilt. Without a moment's delay the big plane scuttled into a low-drifting cloud-bank and for a halfmile beetled by compass alone toward home. Now and then she poked her nose out above the clouds to give the acrobatic guardian angels a hint of her whereabouts, and when the clear sunshine began to dry the isinglass guards, a whole bevy of Spads came dancing on their way to welcome her, hopeful of a scrap with the pursuing Boche. A quick, bumpy landing and the little box was passed overboard to an orderly. He delivered it in turn to a yellow-fingered photographer, and the most secret details of the Boche mystery were unfolded one by one before his eyes.

Many weeks before, a body of German peasants and Belgian prisoners had been brought up with great secrecy, and set to work building a road-bed across ploughed fields, miles back of the front-line trenches. The road led to the edge of a dense wood and here concrete-workers and mechanics prepared for the support and erection of a big gun. For weeks the labor went on with the usual accompaniment of accidents and quarrels. Once an