Arab slavers might discover them, beating along the coast in well-armed dhows.

Twice, in five days, latteen-sailed craft passed south, and one of these put in to investigate; but a tray of blanks from a machine-gun, at half a mile, turned the invader's blunt nose seaward again.
The greatest peril of all was that some news of the wreck might reach Rio de Oro and be wirelessed to civilization.

That would inevitably mean ruin.

Either it would bring an air-squadron swooping down, or battle-ships would arrive.
The Master labored doggedly to get his neutralizing apparatus effectively operating once more; and besides this, he spent hours locked in his cabin, working on other apparatus the nature of which he communicated to no one.

But the Legion knew that nothing could save them from long-range naval guns, if that kind of attack should develop.