As a matter of precaution he tied his handkerchief over his nose and mouth, an example that his companions hastened to copy. They realised that it was but a sorry protection--useless against the full strength of the deadly chlorine, but sufficient, perhaps, to ward off the effects of a "tail-end" of the poison-cloud. Weirdly fascinated the fugitives watched the proceedings. It seemed strange to witness the diabolical preparations for their intended execution. Dimly Barcroft wondered whether he would be conscious when he fell from the bough, or whether the gas would overcome him instantly. "The first whiff and I'll shoot," he thought grimly. "I'd like to shatter the nozzles of those pretty masks and let the brutes have a good sniff at their vile mixture." A faint hiss betokened the fact that the taps controlling the discharge tubes had been turned on. Clouds of black vapour, eddying and seething, issued from the nozzles and rose sullenly in the cold, damp air. CHAPTER XXIX GASSED SOMETHING fluttered past the flight-sub's ear. It was a dead leaf. Whisked by a sudden gust it disappeared. Simultaneously the wind moaned dismally betwixt the gaunt branches. Hitherto the air had been heavy and still. Now, almost miraculously, a stiff breeze had sprung up, blowing in the direction of the infernal gas cylinders, just as they liberated their poisonous contents. The rolling columns of vapour, forced back by the wind, literally enveloped the hideously masked operators. More, the deadly cloud, keeping close to the ground, travelled at prodigious speed towards the two Hun officers, who hitherto had been thoroughly enjoying the proceedings. Quickly their brutal hilarity changed to an attitude of terror, as the death-dealing gas, spreading from the right and left of them, bore down at a rate exceeding that of a trotting horse. For a brief moment Barcroft had a vision of two grey-coated forms, two pairs of heels in the air and two pairs of outstretched arms. Then the cloud hid them from sight. Already the operators, finding that the gas had been misdirected, had shut off the controlling valves. But the mischief was already done. When the cloud had drifted away before the now steady breeze the German officers could be discerned lying on the ground and beating a frantic tattoo with their elbows and heels as the poisonous vapour tore their lungs. Aghast the corporal watched his superiors' death agonies. While his men hastened to render aid--a useless task--the luckless non com., tearing away his mask and liberating the poisonous vapour, held his face close to the hissing nozzle. Then he, too, dropped, writhing on the ground in mortal pain.