AEROKAUTICS. 



215 



balloon and all forms of fixed aeroplanes. In nature, small 

 wings driven at a high speed produce the same result as large 

 wings driven at a slow speed (compare fig. 58, p. 125, with 

 fig. 57, p. 124). In flight a certain space must be covered 

 either by large wings spread out as a solid (fig. 57, p. 124), or 

 by small wings vibrating rapidly (figs. 64, 65, and 66, p. 139). 



Fig. 111. — Cayley's Flying Apparatus. 



The Aerial Screw. — Our countryman. Sir George Cayley, 

 gave the first practical illustration of the efiicacy of the screw 

 as applied to the air in 1796. In that year he constructed a 

 small machine, consisting of two screws made of quill feathers 

 (fig. 111). Sir George writes as under : — 



" As it may be an amusement to some of your readers to 

 see a machine rise in the air by mechanical m.eans, I will con- 



