wing surfaces. So far as could be done, Lilienthal tabulated
the amount of air resistance offered to a bird's wing,
ascertaining that the curve is necessary to flight, as offering
far more resistance than a flat surface. Cayley, and others,
had already stated this, but to Lilienthal belongs the honour of
being first to put the statement to effective proof--he made
over 2,000 gliding flights between 1891 and the regrettable end
of his experiments; his practical conclusions are still regarded
as part of the accepted theory of students of flight. In 1889
he published a work on the subject of gliding flight which
stands as data for investigators, and, on the conclusions
embodied in this work, he began to build his gliders and
practice what he had preached, turning from experiment with
models to wings that he could use.

It was in the summer of 1891 that he built his first glider of
rods of peeled willow, over which was stretched strong cotton
fabric; with this, which had a supporting surface of about 100
square feet, Otto Lilienthal launched himself in the air from a
spring board, making glides which, at first of only a few feet,
gradually lengthened. As his experience of the supporting
qualities of the air progressed he gradually altered his designs
until, when Pilcher visited him in the spring of 1895, he
experimented with a glider, roughly made of peeled willow rods
and cotton fabric, having an area of 150 square feet and
weighing half a hundredweight. By this time Lilienthal had
moved from his springboard to a conical artificial hill which he
had had thrown up on level ground at Grosse Lichterfelde, near
Berlin. This hill was made with earth taken from the
excavations incurred in constructing a canal, and had a cave