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 necessarily becomes slow and their chief velocity horizontal before the surface is reached.
3. Aerial Cascades.—Rapid falls of air are found to the lee sides of hills and mountains, and the stronger the wind the more rapid the cascade. 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 thunder 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 circumstances.
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.
b. 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 transition from one of these layers to another more nearly coincident in direction and velocity with his aeroplane is certain to result in a sudden decrease in the magnitude of its supporting pressure and in the effectiveness of the balancing devices. Under certain extreme conditions 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 resulting waves are long and regular; if in different directions they are short and choppy. Therefore, other things being equal, it obviously is advisable 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, especially the latter, of cliffs and steep hills and mountains. When the
