HOLES IN THE AIR 57 



explained, of treacherous " holes in the air." The whirl is best avoided 

 by landing in an open place some distance from bluffs and large ob- 

 structions, or, if the obstruction is a hill, on the top of the hill itself. 

 If, however, a landing to one side is necessary and the aeronaut has 

 choice of sides, he should, other things being equal, take the windward 

 and not the lee side. Finally, if a landing close to the lee side is com- 

 pulsory he should, if possible, head along the hill, and not toward or 

 from it; along the axis of the eddy and not across it. Such a landing 

 would be safe, unless made in the down draft, since it would keep the 

 machine in winds of nearly constant (zero) velocity with reference to 

 its direction, whatever the side drift, provided the hill was of uniform 

 height and slope and free from irregularities. But as hills seldom 

 fulfill these conditions lee side landings of all kinds should be avoided. 



Aerial Torrents 



Just as water torrents are due to drainage down steep slopes, so too 

 aerial torrents owe their origin to drainage down steep narrow valleys. 

 Whenever the surface of the earth begins to cool through radiation or 

 otherwise the air in contact with it becomes correspondingly chilled 

 and, because of its increased density, flows away to the lowest leveL 

 Hence of clear still nights there is certain to be air drainage down al- 

 most any steep valley. When several such valleys run into a common 

 one, like so many tributaries to a river, and especially when the upper 

 reaches contain snow and the whole section is devoid of forest, the 

 aerial river is likely to become torrential in nature along the lower 

 reaches of the drainage channel. 



A flying machine attempting to land in the mouth of such a valley 

 after the air drainage is well begun is in danger of going from rela- 

 tively quiet air into an atmosphere that is moving with considerable 

 velocity — at times amounting almost to a gale. If one must land at 

 such a place he should head up the valley so as to face the wind. If he 

 heads down the valley and therefore runs with the wind he will, on 

 passing into the swift air, lose his support, or much of it, for reasons 

 already explained, and fall as though he had suddenly gotten into an 

 actual " hole in the air." 



Aerial Breakers 



The term " aerial breakers " is used here in analogy with water 

 breakers as a general name for the rolling, dashing and choppy winds 

 that accompany thunder-storm conditions. They often are of such 

 violence, up, down and sideways in any and every direction that an 

 aeroplane in their grasp is likely to have as uncontrolled and disastrous 

 a landing as would be the case in an actual hole of the worst kind. 



Fortunately aerial breakers usually give abundant and noisy warn- 



