HOLES IN THE AIR — HUMPHREYS. 265 



The air at the top and bottom of these whu"ls is moving in dia- 

 metrically opposite directions, at the top with the wind, at the bot- 

 tom against it, and since they are close to the earth they may therefore, 

 as explained under ''wind layers," be the source of decided danger 

 to aeronauts. There may be danger also at the forward side of the 

 eddy where tlie downward motion is greatest. 



When the wind is blowing strongly landings should not be made, if 

 at all avoidable, on the lee sides of and close to steep mountains, hills, 

 bluffs, or even large buildings; for tliese are the favorite haunts, as 

 just explained, of treaclierous "holes in the air." The whirl is best 

 avoided by landing in an open place sonie distance from bluffs and 

 large obstructions, or, if the obstruction is a liill, on the top of the hill 

 itself. If, liowever, a landmg to one side is necessary and the aero- 

 naut has choice of sides, he should, other things beuig'^equal, take the 

 windward and not the lee side. Finally, if a landing close to the lee 

 side is compulsory 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 tlie machine in winds of nearly constant (zero) velocity 

 with reference to its direction, whatever the side drift, provided the 

 hill was of uniforni height and slope and free froxn irregularities. 

 But as hills seldom fulfill these conditions lee side landings of all 

 kinds should be avoided. 



AERIAL TORRENTS. 



Just as water toiTents 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 eartli begins to cool through radiation or 

 otherwise tlio air m contact with it becomes correspondingly dulled 

 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 

 almost any steep valley. "When several such valleys run into a com- 

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

 upper reaches contam snow and the whole section is devoid of forest, 

 the aerial river is likely to become torrential m nature along the 

 lower reaches of the drainage channel. 



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

 after the air dramage is well begun is in danger of gouig 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 lie will, 

 on passing into the swift air, lose his support, or much of it, for rea- 

 sons already explained, and fall as though he had suddenly gotten 

 into an actual "hole in the air." 



