"I suppose it wouldn't be difficult to recognize a submarine?" she asked presently.
"No," said Rankine; "not if it were at the surface. One might, however, mistake a spar or batten, floating upright, for a periscope."
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.