Upon re-embarking Captain Jones gave the order for half speed ahead, and the "Pride of Rhodesia," at a modest twenty-five miles an hour, flew over the course of the tributary at a height of less than five hundred feet.
Glasses were brought to bear upon the country beneath them, and every rock and shoal in the river was thus carefully examined. The "Vorwartz" had almost reached the limit for navigation, for at less than a mile above the spot where she had stranded the river shallowed considerably and was interrupted by a series of cataracts. In one of these Hythe spotted a brass-bound box wedged in between two rocks, and by the identification letters on its side Captain Restronguet knew that it was one from a liner's strong-room.
"They've been in difficulties already," he observed. "They've had to jettison some of their booty. Judging by the state of the river, I don't think they can have gone very far."
"There's a dense forest ahead," announced Captain Jones. "That makes our task more difficult. If the rascals have taken it into their heads to make a way through the trees, the thick foliage will screen them from view. However, I'll keep up-stream for some distance yet. Perhaps they haven't the sense of caution to destroy their boat or haul it up out of sight. However, we will see."
"No chance of a boat getting over that," exclaimed the sub, as after a short interval the airship came in view of a waterfall quite forty feet in height and extending from bank to bank. As the "Pride of Rhodesia" drew nearer the noise of the falling water almost outvoiced the purr of the airship's motors. One glance was sufficient to show that nothing afloat would be able to surmount that formidable barrier.
"There's their boat," bawled Captain Jones, into the ear of his successful rival.
He was right. Lying half-way up the steep bank, and only partially screened by the foliage, was a craft of native construction. Two or three paddles, a length of rope and a piece of canvas lay within it, while a jagged hole close to the water-line showed that the boat had come to grief against one of the sharp rocks with which the bed of the river abounded.
"See any tracks?" asked Captain Restronguet.
Hythe, to whom the question was addressed, was minutely studying the ground through a powerful pair of binoculars.
"Not much doubt about it, sir," he replied. "There's a well-defined track leading in a south-easterly direction. I can follow it as far as those trees."