HOLES IN THE AIR 59 



strongly heated. They too, however annoying to the beginner, should 

 not be dangerous to the experienced man, because even when strong 

 enough to carry the machine down for a distance their descent neces- 

 sarily becomes slow and their chief velocity horizontal before the sur- 

 face is reached. 



3. Aerial Cascades. — Eapid falls of air are found to the lee sides of 

 hills and mountains, and the stronger the wind the more rapid the cas- 

 cade. But they are of no danger to the aeronaut so long as he takes the 

 precaution to keep above the eddies and other surface disturbances. 



4. Aerial Breakers. — The choppy, breaker-like winds of fchund r 

 storms that surge up and down and in all sorts of directions are as 

 much to be avoided by aerial craft as are ocean breakers by water craft. 

 Hence a flight should positively not be attempted under any such cir- 

 cumstances. 



5. Wind Eddies {Forward Side). — The air on the forward side of 

 a strong eddy has a rapid downward motion and therefore should be 

 avoided. If caught in the down current of an eddy the aeronaut should 

 head lengthwise of the hill or mountain to which the eddy is due. By 

 heading away from the mountain he might, to be sure, get entirely out 

 of the whirl, but the chances are just as great that instead of getting 

 out he would only get the deeper in and encounter downward currents 

 of higher speed. 



0. Horizontal Group 



1. Wind Layers. — The atmosphere is often made up of two or more 

 superimposed layers moving each with its own velocity and direction. 

 Such a condition is a source of danger to the aeronaut because transi- 

 tion from one of these layers to another more nearly coincident in 

 direction and velocity with his aeroplane is certain to result in a sud- 

 den decrease in the magnitude of its supporting pressure and in the 

 effectiveness of the balancing devices. Under certain extreme condi- 

 tions this transition is well nigh inevitably disastrous. 



Dangerous wind layers are most frequent at flying levels during the 

 transition of fair to foul weather. 



2. Wind Billows. — Wind waves analogous to water waves are set 

 up at the interface between two layers that are moving with different 

 velocities. If both layers are moving in the same direction the result- 

 ing waves are long and regular ; if in different directions they are short 

 and choppy. Therefore, other things being equal, it obviously is advis 

 able to keep within the lower layer, or at least to get away from the 

 billowy interface, either above or below, and to avoid crossing it oftener 

 than is absolutely necessary. 



3. Wind Eddies (Central Portion). — Eddies or horizontal rolls in 

 the atmosphere are found on both the windward and lee sides, espe- 

 cially the latter, of cliffs and steep hills and mountains. When the 



