80 LAWRENCE HARGRAVE. 
There 1s no doubt that the wind drawing into and striking the 
concave side of the sail is more powerful than the current impinging 
direct on to the forward part of the concave side, although the hollow 
surface is altogether masked by the rounded surface. Both the 
kite and the windmill experiments refer to moving air passing 
stationary bodies. 
When the kites E. and F. are discharged from the crossbow in 
calm air, they both have the same trajectory. It is difficult to 
imagine a more convincing or simpler proof that the laws govern- 
ing the motion of a body through still air are distinct from those 
that determine its action when moving through wind. Evidently 
a machine with curved surfaces flying against the wind would 
come to grief if the wind fell calm, unless provision had been 
made for either increasing the surface or the driving power. No 
experiments have been made here yet to determine what support 
disturbed air gives to surfaces travelling in the same direction 
and faster thanthe wind. Weare therefore on sure ground when 
we make our surfaces as flat as possible, and of sufficient size to 
support the machine in calm air; should the air become disturbed, 
the same horizontal speed by log could then be maintained by 
reducing the driving power and the angle of incidence. 
As to the solution of the soaring problem, the only fact observed 
is, that on a gusty day, kites u. and F. both shoot up nearly over- 
head and slack the string into a deep bight, then drift away to 
leeward until the string brings them upagain. ‘This wants careful - 
and undisturbed observation—the writer unfortunately had to 
experiment in public. Itis clear that the wind must be considered 
as volumes of air of different densities. 
Kite z. has four flat planes four inches by fifteen inches. The 
angle between each pair of planes is 108°. A similar one with 
curved sails was difficult to adjust ; both flew fairly well but they 
cannot be compared with the cellular form for steadiness ; and it — 
is certain that the numerous accidents that have happened to the 
india-rubber, and compressed air driven machines have been solely 
due to imperfections in the flat or V-shaped body planes. 
