56 THE POPULAR SCIENCE MONTHLY 



iest there might be a wind sheet near the surface, and for other good 

 reasons, landings should be made, if possible, squarely in the face of 

 the surface wind. 



Wind Billows 

 It was stated above that when one layer of air runs over another of 

 different density billows are set up between them, as illustrated by the 

 cloud picture. Of course, as above explained, the warning clouds are 

 comparatively seldom present, and therefore even the cautious aeronaut 

 may, with no evidence of danger before him, take the very level of the 

 billows themselves, and before getting safely above or below them en- 

 counter one or more sudden changes in wind velocity and direction due, 

 in part, to the eddy-like or rolling motion within the billows, with 

 chances in each case of being suddenly deprived of a large part of the 

 requisite sustaining force — of encountering a " hole in the air." There 

 may be perfect safety in either layer, but, unless headed just right, 

 there necessarily is some risk in going from the one to the other, and 

 therefore, since flying at the billow level would necessitate frequent 

 transitions of this dangerous nature, it should be strictly avoided. 



Wind Eddies 



Eddies and whirls exist in every stream of water, from tiny rills 

 to the great rivers and even the ocean currents, wherever the banks are 

 such as greatly to change the direction of flow and wherever there is a 

 pocket of considerable depth and extent on either side. Similar eddies, 

 but with horizontal instead of vertical axes, occur at the bottoms of 

 streams where they flow over ledges that produce abrupt changes in the 

 levels of the beds. 



The inertia of the stream of water, its tendency to keep on in the 

 direction it is actually moving and with unchanged velocity, together 

 with its viscosity, necessitate these whirls with which nearly all are 

 familiar. Similarly, and for the same general reasons, horizontal eddies 

 occur in the atmosphere, and the stronger the wind the more rapid 

 the rotation of the eddy. They are most pronounced on the lee side of 

 cuts, cliffs and steep mountains, but occur also, to a less extent, on the 

 windward side of such places. 



The air at the top and bottom of these whirls is moving in dia- 

 metrically opposite directions, at the top with the wind, at the bottom 

 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 the downward motion is greatest. 



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

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

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



