Knowing to an almost absolute certainty that the aircraft's crew had them under clear observation by means of powerful binoculars, Uncle Brian carefully avoided looking up. A sudden alteration of helm would be a false move. He kept steadily in his course for a few minutes before putting the helm to starboard and making for the Rio Tinto. "That's awkward," remarked Uncle Brian. "They won't leave us alone. We'll have to make a feint of ascending the Rio Tinto. It will mean a day's delay and a night dash for the Rio del Morte." "Perhaps when they see we're well on our way up the Rio Tinto they'll clear off," hazarded Peter. Apparently his surmise was correct, for after circling overhead for three hours, the flying-boat disappeared in the direction of Tepecicoa, without having made any attempt to molest or even communicate with the Englishmen. During the heat of the day the fugitives rested, anchoring their little craft by means of a big stone and a rope in the shade of an overhanging tree. Here they were screened from aerial observation, but no buzz of propellers disturbed their rest. The flying-boat had, in fact, returned to the base with the news that the mad Englishmen had really taken the Rio Tinto course. It was not until four in the afternoon that the outboard motor was restarted and the course retraced to the Rio Guaya. Not without considerable trepidation, Brian steered across the broad river and made the narrow entrance to the sinister Rio del Morte. Here the banks were lofty and precipitous, the stream flowing at the rate of four miles an hour through a bottle-necked gorge of less than a hundred yards in width and nearly a mile in length. Progress was in consequence tediously slow, an hour elapsing before the boat joined the wider expanse above the defile. Ahead the river broadened still more into a fairly large lake which the map had entirely ignored. At the farthermost end the flat shores were broken by a number of rocky pinnacles, but whether they were small islands or merely parts of the mainland, it was as yet impossible to determine. The forests had now been left behind, the shores of the lake being treeless and bare, save for occasional patches of pampas grass and cacti. "Think we'll fetch the other end before dark, Uncle?" asked Peter. "It's quite five miles off." "Might," replied Brian unconcernedly. "It doesn't matter much if we don't. We're carrying on at night. I think we decided upon that?" "Yes," agreed his nephew, "we did. But we didn't reckon on having to navigate a lake. We don't know where the inlet is. It might be between any of those projections we can see ahead; and it will be no joke barging about on a dark night trying to find a way out." "We'll do it, never you fear," rejoined his uncle, with one of those bursts of sublime optimism that characterized his mercurial spirits.