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.

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.

Lilienthal recognised the position, of course, and saw there might be peril in the use of a biplane; but he was content, none the less, to rely upon his skill. In each glide he made he became more expert; instead of allowing his machine to slip to the ground when struck by a gust, he restored its equilibrium by an instantaneous movement of his body: he was, in fact, like a man who had learned to ride a bicycle—balancing himself without pausing to think what he should do. But Lilienthal, in his navigation of the air, was facing a danger the cyclist has not to fear. He was braving dangerous wind-gusts; and he did not know, and had no means of knowing, with just what strength these gusts would strike his craft. Also—and this too was a danger no man on earth need fear—he had empty air below him should he fall. The peril grew greater as his skill increased, because he soared higher, and left greater distances between him and the ground below. In another way, also, he courted greater risk; and this was through gliding in stronger winds. At first, when he was unskilled, he had cared only to brave a wind of a velocity, say, of 10 or 15 miles an hour. But soon, feeling his balancing power grow greater, he ventured into the air when there was a wind of from 20 to 25 miles an hour.

In regard to this question of the strength of the wind, uncertainty often exists. What, for example, is a “stiff breeze”? What is a “strong wind”? And at what velocity must the wind blow before it is called a gale? Such questions are often asked, and the table below should prove instructive:

Velocity of wind  
(miles per hour).		Description given
such strength.
————————————
0-5  Calm.
6-10  Light air.
11-15  Light breeze.
16-20  Gentle breeze.
21-25  Moderate breeze.
26-30  Fresh breeze.
31-36  Strong breeze.
37-44  Moderate gale.
45-53  Fresh gale.
54-60  Strong gale.
61-69  Whole gale.
70-80  Storm.
   80 upwards  Hurricane.