Bob steered the machine along the middle of the valley, keeping low in order to avoid the wind. On either side rose the lofty mountain barrier, here overhanging the river, there receding; at some spots an almost perpendicular wall, at others broken into peaks and parapets, with deep hollows in which a scanty vegetation struggled for existence on a thin soil. Now the valley narrowed so that there seemed barely space for the aeroplane to pass: now it widened into a series of rocky terraces, seamed by fissures in every direction. The track on the right bank followed the winding course of the river for many miles, then dipped to a ford and reappeared on the left. Some miles farther on it recrossed the river by a crazy bridge of rope, and continued along the right bank past the foothills and out into an extensive plateau. Bob as usual acted as pilot. By flying pretty close upon the river he not only avoided danger from gusts, but enabled his companions to keep a sharp observation upon the ground. Here and there, where this wound behind the rocks between the bank and the hillside, he left the river, planed a little higher, and steered a course exactly over the track. The recent invention of planes which could be lengthened or shortened at will rendered it possible to travel at more varied speed than had formerly been the case, and when he was several miles from the mine he reduced speed to the minimum. Even then, however, the aeroplane moved so swiftly that there was some danger of the watchers passing their quarry without perceiving them. This was not likely where the track was closely hemmed in between river and hillside. The risk was greatest where the latter receded from the course of the stream, leaving large areas of rough country, sometimes covered with bush, in which the fugitives could without much difficulty hide out of sight of any one not passing immediately above them. The ford at which the track crossed the river was about twenty miles from the mine. Coming to that point without having seen the fugitives, Bob followed the track along the left bank. Here the open spaces became more frequent, and it would have been impossible to examine the ground thoroughly without circling. For the present Bob hesitated to do this, feeling that it was more important to keep to the track for so long a distance as the fugitives might have covered had they started at the earliest likely moment, twelve hours before. Another twelve miles brought him to the rope bridge, where he again crossed the river, and so continued until, ten miles farther, the foothills were reached, and the country began to open out.