268 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1912. 



less across each other, as they iisually do, the turmoil of the lesult- 

 ing short and choppy billows, by rendering equilibrium difficult, add 

 an additional danger all their own. 



Dangerous wind laj^ers are most frequent at flying levels during 

 the transition of fair to foul weather. 



2. Wind hillows. — 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 result- 

 ing 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 cross- 

 ing it oftener than is absolutely necessary. 



3. Wind gusts. — ^The horizontal velocity of the wind near the 

 surface of the earth is exceedingl}'^ irregular and fluctuates from 

 second to second at times by as much as the whole of the average 

 velocity. In such a wind, if at all swift, the support to an aero- 

 plane is exceedingly erratic and both its launching and its landing 

 full of danger. 



Obviously, too, the stronger the wind the higher, because of these 

 surface disturbances, one should fly, if at all. 



4. 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 liills and mountains. When the wind 

 is strong a landing should not be attempted in any such place. If 

 forced to land in a place of this Idnd, the machine should be headed 

 along and not at right angles to the direction of the hill. 



5. Aerial torrents. — Steep barren valleys, especially of clear still 

 nights and when the upper reaches are snow covered, are the beds of 

 aerial drainage rivers that at times amount to veritable torrents. 

 Therefore, however quiet the upper atmosphere and however smooth 

 its sailing, it would be extremely dangerous to attempt to land an 

 aeroplane at such a place and such a time. 



NOTE. 



All the above sources of danger, whether near the surface like 

 the breakers, the torrents, and the eddies, or well up like the billows 

 and the wind sheets, are less and less effective as the speed of the 

 aeroplane is increased. But this does not mean that the swiftest 

 machine necessarily is the safest; there are numerous other factors 

 to be considered, and the problem of minimum danger or maximum 

 safety, if the aeronaut insists, can only be solved by a proper com- 

 bination of theory and practice, of sound reasoning and intelligent 

 experimentation. 



