No one followed up his explanation, but the party seemed to find the pause trying.

"They burn gasolene, don't they?" Madge asked.

"Either that or oil when they're running on the surface. The engines are driven by electricity when they're submerged."

"Can they carry much gasolene?"

"Not very much," Rankine answered, guardedly.

"Can they carry enough to take them from Germany and back?" Madge persisted.

"I believe some can do so; but they wouldn't have much to spare, and they'd run a serious risk if they remained any time as far away from their base as the North Channel."

"You must see that the point's important," said Elsie.

"Its importance is obvious," Rankine agreed.

"If the boat couldn't carry enough fuel, she'd have to get some while she was out on a trip?"

There was another long pause and then Mrs. Woodhouse spoke.

"You must mean somewhere in Scotland," she said.

"Disagreeable conclusion, isn't it? But we don't know yet that it was a submarine," Rankine answered.

"But suppose there _should_ be an enemy submarine in the North Channel that hadn't much fuel left, how could she renew her supply?" Staffer asked.

Rankine seemed unwilling to talk about the subject, but he smiled.

"Oh," he said, "it's hard to tell. One could form plausible theories, but they'd probably be wrong. Perhaps we'd better leave the matter to the people whose business it is."

He began to talk about something else, and the curious tension that all had felt gradually slackened. Soon afterward, a servant announced that Mackellar had arrived. Staffer had been expecting him, and when he left the room Madge and d.i.c.k went to the drawing-room with Elsie and Mrs. Woodhouse, and Rankine found himself alone with Andrew and Whitney.

"Have you made any progress with your investigations?" he asked.

"No," Andrew answered quietly; "nothing very marked."

"And you are still resolved to keep them in your own hands, after the news we got to-night?"

"Do you know that the loss of the cargo boat has any connection with the matter?"

"No; but it looks suspicious," Rankine answered, with a touch of grimness. "If I did know, my course would be clear."

"So would mine," said Andrew. "We found some matches and a candle on board the wreck, and followed a man across Criffell to the beach abreast of her--or rather we followed his tracks. Then we saw another fellow on the sands at night; but that's all I have to tell."

"Could you see either of the men clearly?"

"No. I didn't see the first at all; and the other was some distance off, and a thick fog was coming on."

"That means it was impossible for you to recognize him."

"Quite," Andrew said firmly. "Besides, I didn't expect to recognize him; there was nothing to indicate it was anybody I'd ever met. Have you learned anything?"

Rankine smiled.

"I've examined the wreck and dug up the sand, besides watching the flats for several nights. The place might be used for a wireless installation, but, lying in a hollow, with hills on both sides, it's not particularly suitable." He paused and looked at Andrew. "That had some influence with me."

Andrew thought Rankine meant that if he felt certain that messages were sent from the wreck, he would have brought some pressure to bear on him.

"How did you get there?" he asked.

"We ran in behind Ross Island when it was too rough for surveying, and afterward brought up near Abbey Head. You get some shelter there so long as the wind's not south."

"But it's a long way from Abbey Head to the wreck," Whitney interposed.

"I shipped a steam launch at Belfast."

"And went to the wreck and back at night? Wasn't it blowing hard?"

"Hard enough," smiled Rankine. "We had some trouble to keep the fire from being swamped, but she's a powerful boat and has a good big pump.

Then we traveled most of the distance shortly before and after low-water, when the sea was not so bad; but I'll confess that I couldn't have found my way among the shoals except for Mr. Johnstone's directions. We made three trips and got back before daylight without noting anything suspicious."

They looked at him in surprise. A steam launch voyage along that dangerous coast on a wild winter night was a bold undertaking, particularly when one must cross surf-swept sands with only a few feet of water under the boat. And Rankine had safely accomplished it thrice.

"What about the digging?" Whitney asked. "Mightn't it alarm our man?"

"The surf would level the sand in a tide," Andrew said; and turned to Rankine. "What do you think of doing now?"

"I don't know, but I'm afraid I can't stay here as long as I expected.

The steamer's in Loch Ryan. We went in to make some repairs after a hammering we got. Now, perhaps we had better join the others."

Andrew left them in the drawing-room and found Mackellar alone in the library.

"I'll have finished with these in a few minutes," he remarked, indicating the papers before him. "Mr. Staffer's accounts don't give much trouble. He's a man o' parts."

"Yes," agreed Andrew; "the estate is managed well."

"We must give him all the credit he deserves, but there's another matter I'm anxious about. We have not got to the bottom o' your cousin's debts."

Andrew frowned.

"Do you mean that Williamson has got hold of him again?"

"No; I'm thinking he's out of the game, and the borrowed money's none o' his. But d.i.c.k has incurred some fresh liabilities. Here's a bit statement; ye can study it."

Andrew felt disturbed, but he waited until Mackellar put the papers into his pocket.

"I can't see how d.i.c.k has spent so much money; but how did he get it?"