"It would have shone all round," said Andrew. "By coming ashore they got the crag for a screen and a high platform. The light could be seen farther off, but only from the sea."
"But what would they want to signal from a place like this, and whom would they signal to?"
"I don't pretend to know. It's a long distance from a main line, but a fast car would cover a good deal of ground in an hour or two."
Andrew stopped, and, taking a chart from a rack, pointed to the narrow channel between Scotland and Ireland.
"You see how close Fair Head is to Kintyre," he resumed. "Well, all the shipping from the Clyde and a good deal from Liverpool passes through that gap. You can imagine what would happen if it were filled with mines."
"The difficulty is that the mine-sowers would be seen. The lighthouses on Rathlin and Kintyre command the channel."
"It's hard to see a vessel that carries no lights, and a mine-sower wouldn't proclaim his intentions. There's a big fleet of trawlers working in the Irish Sea, and a stranger would excite no remark by slipping in among them. It wouldn't take long to paint on a registered number and copy the funnel of a steam fishing company."
"So Rankine has another duty besides taking soundings! A small survey vessel could cruise about among the shoals without attracting much notice. Her business would be obvious, but that needn't stop her crew from watching out."
"Well," said Andrew, "it isn't difficult to form a theory to fit the few things we know. However—"
"It would probably be all wrong when you'd made it," Dick broke in.