In the upper of the two flights shown, profiting by a day when there were rising currents in the wind, Lilienthal had allowed himself to be lifted, for a moment or so, to a point in the air actually higher than that from which he started. Then, in order to obtain forward speed, he dived, only to incline his wings more steeply again, and allow the wind to bear him upward. In this way, by exercising skill in the balancing of his machine, he was able to prolong a glide, and under favourable conditions to traverse, before touching ground, a distance through the air of nearly 1000 feet. [Illustration: FIG. 25.--Lilienthal’s artificial hill.] In the second of the glides shown in Fig. 25, Lilienthal is making a swift, low flight--one of those during which he was never far from the ground, and with which he contented himself in early tests. On glancing again at this sketch it will be noted that the upper of the craft shown has two main sustaining wings, placed one over the other in the girder construction already described. It is in fact a biplane, whereas the lower machine is a monoplane--such a craft as was illustrated in Fig. 22. It was when Lilienthal became expert at balancing himself in the air that he built a machine on the biplane principle. His reason for doing so was that he required more surface in the sustaining wings, so that they might carry him farther through the air. What he wanted to do in each of his glides was to remain in the air as long as possible, and thus gain a maximum of experience. The difficulty, as explained, was to obtain enough practice. In five years, for instance, although assiduous in his experiments, Lilienthal was not more than five hours in the air. [Illustration: FIG. 26.--Lilienthal’s Biplane Glider.] Thus it was that he built a biplane, each of the wings being 18 feet in span, and containing 100 square feet of surface. The craft is shown in Fig. 26. Lilienthal did with it what he expected he would be able to do; he increased materially the length of his glides. But there were drawbacks in the use of this machine, and one introduced an element of danger. With the monoplane, so soon as he made balancing movements by instinct, Lilienthal found he could control his craft quite well; it was small, and responded quickly when he threw the weight of his body from side to side. But the biplane, being considerably larger, and having more surface upon which the wind could act, was sluggish in its response to his controlling movements. In the case of both monoplane and biplane, Lilienthal relied merely upon the weight of his body to counteract falling movements. In the biplane, therefore, although it required a greater leverage to restore its balance, he was unable to increase the correcting influence. This difficulty, in the use of a large machine, was faced subsequently by the Wrights, and how they solved it will be shown.