They were working along the foot of a bank with faces bent to the earth, when suddenly they were startled by a voice hailing them a few yards away. 'Hallo, there!' called someone. The boys glanced up, and at once straightened themselves and came to the salute. A tall man in khaki and putties stood on the top of the bank looking at them, a revolver in the holster strapped at his side. 'And who may you be, and what do you want here?' he asked pleasantly, and returned the salute. 'We're Boy Scouts,' replied Dick, 'and our patrols are out for a big scouting-run over the heath.' 'Ah, yes! Boy Scouts--I've heard of you,' said the big man, still smiling at them. 'Well, I'm in the same line myself. But you can't come any further this way, mateys. You'll have to scout back, if you don't mind.' 'Why must we do that, sergeant?' asked Dick, who had noted the chevrons on the big man's sleeve, and understood them. 'Well,' said the good-natured soldier, 'it's like this: We've got a lot of big, bad convicts at work over there,' and he jerked his head behind him, 'and we keep 'em strictly to themselves, you see. They're bad company for anybody but the men as looks after 'em, so we keep this corner of the country clear of other people.' 'At that rate,' laughed Dick, 'the track we want isn't likely to be laid your way?' 'Not it,' said the sergeant, 'else I should ha' spotted it on my round. No, mateys, you can cut right back. Ta-ta!' The boys gave him a farewell salute, and ran back towards the spot where they had left the rest of the patrol. 'That's a rum game, ain't it?' remarked Chippy--'a soldier a-walkin' round in a quiet place like that theer. Who's he a-tryin' to cop?' 'Perhaps watching to see that no convicts escape,' suggested Dick. 'You know, Chippy, they often try to cut and run if they see a chance.' 'Yus,' said the Raven. 'I've seed that in the papers. But wot do they want convicts for on the h'eth?' 'I know,' cried Dick--'I know. I heard my father talking about it at dinner the other day. It's the Horseshoe Fort at the mouth of the river. They're making it ever so much bigger and putting new guns there so as to be ready if ever some enemy should come to our country and try to sail up the river. The convicts are at work there, digging and building and doing all sorts of things.' 'I see,' nodded Chippy; 'that's 'ow they mek' 'em useful, I s'pose.' 'That's it,' said Dick, 'and that sergeant we saw was one of the men in charge of them.' 'He soon started us back,' murmured Chippy. 'Yes,' said Dick; 'I heard my father say that they are very strict about letting any stranger go near the place.' 'That was on'y gammon of his about them convicts,' remarked Chippy. 'Of course it was,' agreed Dick; 'he wouldn't let anyone go nearer the fort on any account.' 'How far are we off?' asked Chippy.