It was a happy thought of Dick to use his patrol whistle upon reaching the strip of country where they had seen the sergeant. The latter heard the very first shrill note. He was haunting that stretch of the heath for a purpose, eyes and ears wide open. He ran towards the sound, and came plump on the boys as they raced round a bend in the way, for the two scouts were now following the heath-track where they had last seen the prints of the soldier's ammunition boots. 'Hooray!' yelled Chippy, who was a little in front. ''Ere he is. Hooray!' and Dick joined in the cheer. 'You two again!' cried the astonished sergeant. 'What on earth are you nippers up to?' 'We've discovered a spy, sergeant,' panted Dick. 'He's running after us. He'll be up in a minute.' At the word 'spy' the sergeant's face underwent an extraordinary change. It filled with wonder, and then a grim alertness sprang to life all over him. He dropped his hand to his holster, and whipped out a big regulation 455 revolver, blue and sombre. The boys formed behind as under cover of a tower of strength, and the spy dashed round the bend. 'Hands up!' bellowed the sergeant, and the spy knew better than to disobey with that grim dark muzzle laid full on his body. 'Heavenly powers!' murmured the sergeant, 'I was right. As sure as my name's John Lake I was right. Didn't I see you on the heath just about here last Thursday?' he demanded of the spy. The latter made no reply. He stood, drawn up to his full height, his hands above his head, and in one of them was a long-bladed hunting-knife of the sort which folds into small compass. Now it was fully opened, and looked a very dreadful weapon. The man was white as death, and gasping fiercely from his run and this frightful surprise. 'Drop that knife,' commanded the sergeant, 'or I'll put a bullet through your wrist.' The spy's wild eyes were fixed on the English soldier's grim face. He knew when a man meant what he said, and he dropped the knife. 'Step two yards back,' went on the sergeant. The spy did so. 'One o' you boys pick up that knife,' murmured the sergeant; and Dick ran and fetched it. 'Now, I'm in the dark yet,' went on the sergeant quietly; 'all this looks very suspicious, but how do you boys come to reckon you've nabbed a spy?' Dick began with the boot and the papers hidden in it. 'That's enough, my lad,' said the sergeant. 'We'll lose no time. There's plenty o' reason, I can see, to take him in on suspicion, and after hearing that I'd shoot him at once if he tried to escape. Now you,' he went on to the spy, 'turn right round and march ahead as I tell you. And remember I'm a yard behind you with a cocked revolver. March!' The spy turned, and went as he was bidden. 'Come on, boys; you must come with me,' said the sergeant and the little party went across the heath, the prisoner turning as the sergeant bade him, and taking as direct a line as possible to the Horseshoe Fort.