"Most certainly it's S.O.S.," said Armstrong, after counting the recurring glows.

"I shouldn't wonder if Pratt is right after all, and it's Molly Rod signalling. He was right about the organ pipe."

"Doesn't it occur to you that the light may come from the tower?"

"But if the forgers are at work there, why should any one signal?"

"Can't we discover whether it's from the tower or the house?"

"We can't take any bearings in the dark. Stay, though. If we move back from the window, and go to the side of the room, perhaps we'll find a spot where the light just becomes invisible. I'll mark that on the floor, and in daylight there'd be no difficulty."

Acting on this suggestion, they were not long in discovering the required spot. Warrender scratched a pencil mark on the floor; then they descended to the cellar, cautiously lifted the flagstone, and groped their way through the tunnel until they came to the chamber at the end. Nothing was altered there, except that the opened bale of paper had been removed. They had intended to enter the archway on the farther side, and lift the flagstone which, they suspected, closed the entrance to another cellar; but from above there came dully a succession of regular thuds which proved that somebody was about, and active.

"I dare say that's the press at work," said Warrender in a whisper, after they had listened for a few minutes.

"Doing overtime," said Armstrong. "I suppose, not knowing exactly when Mr. Pratt will return, they want to make the most of their opportunity.

Who knows how many thousands of pounds of spurious money are getting into circulation? No doubt Gradoff had his trunk full of notes that morning we saw him driving off in the car."

They seated themselves on the unopened bales, hoping that work would presently cease, and the man would leave the tower. But the thuds continued with monotonous regularity.

"Every thud means a forged note," said Armstrong. "They may be going on all night. How long can you stick it?"

"We'll wait till eleven; then if they're still at it, we'll go back and reconnoitre the outside."

"Perhaps they have a sentry."

"Perhaps; but I fancy they'll feel pretty safe now that they've chevied us from the island."

At eleven o'clock the work was still going on. The boys retraced their course to the ruins, regained the pram, and allowed it to drift on the current down channel to the south of the island. There they lay to for a few minutes, listening, peering through the darkness. There was no moon; the starlight scarcely revealed the outlines of the trees.

Presently, with careful, soundless movements of the sculls, they rowed across to the left bank, and, pulling the craft out of sight, landed a little below the island, and laboriously pushed their way through the thicket, guiding themselves by the compa.s.s. Some fifty yards from the bank the vegetation thinned, and they found themselves in a wood of taller trees. Here the going was easier, though once or twice they stumbled over trunks that had been felled and stripped ready for carting. Emerging from the wood into park-like ground, where there were large trees only at intervals, they progressed still more rapidly, and at last caught sight, on their left, of the dim, square shape of the tower. Behind a broad elm they stood for a minute or two, watching.

There was no light in the tower. Its base was surrounded by a ma.s.s of low-growing shrubs. The doorway, no doubt, was on the farther side from them. The walls were covered with ivy, except at the window openings, where the recent boarding was visible as faint grey patches.

"Now for it," whispered Warrender.

They stole forward over the long gra.s.s. As they drew nearer to the tower they heard the dull regular thudding; there was no other sound.

Armstrong posted himself at one corner, while Warrender gently pushed a way through the shrubs to the wall. He examined the boarded window, apparently an old embrasure much widened. The boards were on the inside; the outside was protected by cross bars of iron. He went round the building. There was only one other window opening on the ground floor.

At the north-eastern angle he halted, looking out for a possible sentry, then crept along until he reached the entrance, a low iron-studded door flush with the wall. Putting his ear against the wood, he heard more clearly the metallic thuds, and men's voices. A footstep approached.

He slipped back to the corner, and crouched in the shelter of a shrub.

The door opened outwards, creaking on its hinges, and letting out a stream of light. A short, stout figure emerged from the tower, carrying a number of cans which rattled as he walked.

"_Fermez la porte!_"

The words, in a savage, half-suppressed shout, sounded from some little distance away in the direction of the house. The man addressed hastily closed the door behind him, and went on. Warrender saw another man meet him. They stopped and exchanged a few words. Rod continued his way to the house, his progress faintly marked by the rattling cans. The other man came towards the tower. He opened the door quickly, slipped inside, and shut it. In the one second during which the light shone out, Warrender recognised the pale face of Paul Gradoff.

He hurried round to the spot where Armstrong had remained on guard.

"All right!" he whispered. "No sentry. Rod has just gone to the house; Gradoff has gone in."

"Well," returned Armstrong, "what can we do?"

"We'll try the door first of all. Come on!"

They moved with slow, careful steps round the tower, came to the door, and gently tried the handle. There was no yielding; the door was fastened. They went on to the western face of the tower. Here also there was a window opening on the ground floor, as securely boarded up as the other. At equal intervals above it were two other embrasures, similarly blocked.

"No way of getting in," murmured Armstrong.

The sound of the door creaking sent them scurrying to cover in the undergrowth. When all was silent again, Warrender whispered--

"Come among the trees. We can talk more freely there."

They crept over the ground, and took post under a tall, thick-leaved beech nearly a hundred yards away.

"I don't see any chance of getting in," said Warrender, "and that's a pity. I wanted to see them actually turning out their forged notes."

"I suppose it was Gradoff going out again we heard just now," said Armstrong. "If he and Rod are both away, there can't be more than four others in the tower, probably not so many. They'll take turns at night-work."

"That doesn't matter. Any forcible entry is quite out of the question, if that's what you're thinking of. I say, isn't that a light up the tower?"

More than half-way up the wall a faint streak of light was visible.

"Evidently there's some one in the top room," said Warrender. "Some one sleeps there, I suppose. The machine is on the ground floor. Where light gets out, we should be able to see in. You've done some climbing already to-night; are you game to clamber up the ivy? There's no other way."

"I weigh eleven stone," said Armstrong, dubiously.

"But ivy's pretty tough. It may support you. You may find foothold in the wall."

"Hanged if I don't try. You'll stand underneath and break my fall if I tumble. I reckon it's about thirty feet up; plenty high enough to break one's neck or leg."

They hastened to the foot of the tower. With Warrender's help, Armstrong got a footing in the lower embrasure. Then, taking firm hold of the stout main stem of the ivy, he began to swarm up, seeking support for his feet in the thick, spreading tendrils and in notches of the stone-work. Warrender watched him hopefully. Slowly, inch by inch, he ascended. He gained the second embrasure, rested there a few moments, then climbed again, and was almost half-way to his goal, when he felt the ivy above him yield slightly. Digging his feet into the wall, he hung on, but at the first attempt to ascend he felt that the attenuated stem would no longer support his weight, and began slowly to lower himself.

At this moment Warrender heard the door creak, and threw up a warning whisper. Armstrong stopped, effacing himself as well as he could amongst the ivy, to which he clung with the disagreeable sensation that he was dragging it from its supports above. Voices were heard; heavy footsteps. After a few moments they ceased. Were the men turning to come back? Had they heard anything? Then came the scratching of a match. Warrender drew relieved breath; some one had halted, only, it appeared, to light his pipe or cigarette. The footsteps sounded again, gradually receding, and finally died away.

"All safe!" whispered Warrender.

Armstrong let himself down, and stood beside his friend.

"A quivery job," he murmured. "My arms ache frightfully. It's not to be done, Phil. Another foot up and I should have dragged down the whole lot, possibly a stone or two as well. We're fairly beaten."

"The sound inside has stopped. They've apparently knocked off work; it's past midnight. I wonder if any one's left inside?"

"Why should there be?"

"Well, there was some one up above. Is the light showing still?"

They walked some distance away from the tower, and looked up. The thin streak of light, so faint that it might have escaped casual observation, still showed at the level of the topmost room. They went to the door and again gently tried it. It was shut fast.

"We had better get back," said Warrender. "There's nothing to be done."

"Unless we try the tunnel again, now that all is quiet inside."