can not be made to ascend slowly. Once progressing on their road with the tremendous speed adequate to their weight and wing surface, the kite machines can not be stopped or propelled at a lesser rate without at once descending from the level attained. 
Fig. 3.—Lilienthal's Flying Apparatus. The landing. The contrivances applied to counteract these disadvantages have not proved efficient to overcome them. Nevertheless, the experiments and constructions made by Stringfellow, Moy, Tatin, by Kress in Vienna, Lilienthal in Berlin, Koch in Munich, Philipps, Langley, Edison in America, Maxim in England, and by Hargrave in Australia, all represent so many stages of constant progress. Mr. O. Lilienthal has just succeeded in floating down at a moderate rate from a height of two hundred metres; his personal skill in the handling of the apparatus adds considerably to the advantages derived from its judicious construction (Figs. 2, 3, and 4). Some of the most remarkable experiments in the field of aërodynamics are those devised and carried out by Prof. Langley.
The essence of Prof. Wellner's innovation is his invention of the sail-wheel (Fig. 5). It consists of a horizontally placed axis with spokes and arched aëroplanes attached to them in a cylindrical form. While revolving round the axis the latter take a slightly slanting position, which causes the forward edges of these surfaces to be inclined, and consequently to compress the air in the way of a sail or a kite, calling into play the vertical force. Three ribs running across each lifting surface and made in the form of 
Fig. 4.—Lillienthal's Flying Apparatus. In flight. a screw at the same time serve to strengthen the aëroplanes and to add to the horizontal force.
These sail-wheels set in pairs can be placed, according to the size of airship aimed at, in one or more groups of two wheels, revolving in opposite directions, behind or beside each other. The cigar-shaped car, furnished with a motor and carrying the aëronauts, is attached horizontally under the center of the wheels, so that the whole construction will resemble a colossal bird, propelled, instead of by wings, by revolving wheels, the lifting surfaces of which are consecutively and constantly developing vertical and horizontal power. The bird's movements in flying and the speedy headway motion necessary to the kite-
