244 HUMPHREYS: HOLES IN THE AIR 



is, the inexperienced aeronaut may be seriously disturbed by 

 running suddenly into or out of the rising column. It is proba- 

 ble however that the chief danger is not in the rising column 

 itself but rather at its top where, as we see by the motions of 

 the heads of cumulus clouds, there is great turbulence and a 

 confusion of currents. 



2. Aerial cataracts. Downrushes of air, like the uprushes with 

 which they are associated, must also be most frequent during 

 warm weather when the ground is strongly heated. But though 

 annoying to the beginner they should not be dangerous to the 

 experienced aviator, because, however strong their descent at 

 considerable elevations, they necessarily become horizontal before 

 the surface is reached. 



3. Aerial cascades. The swift downward sweep of the wind 

 on the lee side of steep mountains tends to carry the aviator 

 with it to lower levels, but this in not a source of danger unless 

 by flying low he gets caught in an eddy. 



4. Wind layers. As fair weather is giving way to foul sharply 

 defined layers of air often slide over each other in different direc- 

 tions and with different velocities, and by so doing produce one 

 of the most serious dangers the aeronaut has to confront. Sup- 

 pose, for illustration, that in making a straight away glide with 

 the engine at rest the aeronaut should suddenly enter a wind 

 layer moving in the same direction and with the same velocity. 

 Under these extreme but possible conditions all dynamical support 

 and all power of adjustment are instantly lost and a drop, either 

 to the earth or until a considerable velocity with reference to the 

 air is again obtained, is inevitable. Such an extreme case of 

 course is unusual, but less extreme cases are frequent, and since 

 the support is proportional to the square of the velocity of the 

 machine with reference to the air even a comparatively small 

 decrease in this velocity may cause a considerable drop, which, 

 if begun near the surface of the earth, may well be dangerous. 



5. Wind billows. Wind layers, as they glide over each other, 

 are thrown into billows quite like unto water billows under the 

 influence of strong winds ; and since they mark the boundary of 

 currents of different velocity or direction or both, it obviously 



