When an upward movement of wind rises from barren land or conical hills, it is called an aerial fountain. Sometimes this air rises at a velocity of twenty-five feet per second. Sometimes an aeroplane when caught in one of these fountains will rise like a cork on the top of a water-spout, or the wing will be tilted if it is hit by this column of hot air. An aerial cataract is caused by descending cold air, and has the opposite effect on an aeroplane flying through the air to that of the fountain. These are encountered in flying over very broken ground. Aerial cascades are encountered often in flying over narrow valleys or steep hills. The contours of the land cause the air to follow down into the valleys suddenly, thus often making it dangerous for fliers to attempt to land on rivers enclosed in steep banks, unless of course they fly up or down the river. With aerial torrents the same principle applies, except that the area of disturbance is broader and more powerful. Great velocity is attained near open valleys, due to the cold air rushing to replace the hot air moving upward. A cross, choppy wind will cause choppy air surfaces and bad eddies, and can be discerned on a cloudy day by rips in the surface of clouds. Over the crests of hills vertical eddies are encountered. They are usually called pockets by fliers. Often the machine drops straight down, and the pilot should immediately head his machine into the current. Sometimes winds will be found blowing in different directions and passing in layers above one another. These have a tendency to turn the ship about, and is one of the reasons why the aviators prefer to get altitude before doing any stunt flying. Except close to the ground these contrary winds are not dangerous. So just as a vessel is safest far from a coast in a storm, so an aeroplane is safest at a reasonable altitude where the wind is not so bumpy. Clouds and mist are two of the worst enemies of the aerial navigator; first because it shuts off the observer’s vision of the terrain, preventing him from knowing exactly where he is, and because it makes it difficult for him to locate his landing-field. Directional wireless and the wireless telephone do help a great deal in giving information about the lay of the land beneath the clouds or mist, but of course it cannot visualize the ground on which the aeroplane is to land for the pilot to see exactly where he should set the wheels down. For that reason a knowledge of clouds is essential to piloting aircraft. There are many different kinds of clouds, but they are all formed by condensation when an ascending volume of moist air mingles with another mass of a different temperature, or when a mass of arising vapor condenses. With a knowledge of the direction clouds are moving in it will reveal certain facts about the weather to the pilot. Clouds take almost every conceivable shape.