264 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1912. 



level would necessitate frequent transitions of this dangerous nature, 

 it should be strictly avoided. 



WIND GUSTS. 



Near the surface of the earth the wind is always in a turmoil, 

 owing to friction and to obstacles of all kinds that interfere with the 

 free flow of the lower layers of the atmosphere and thereby allow the 

 next higher layers to plunge forward in irregular fits, swirls, and gusts, 

 with all sorts of irregular velocities and in every which direction. 

 Indeed, the actual velocity of the wmd near the surface of the earth 

 often and abruptly varies from second to second by more than the 

 full average value, and the greater the average velocity, the greater, 

 in approximately the same ratio, are the irregularities or differences 

 in the successive momentary velocities, as is well shown by pressure 

 tube traces, of which plate 3, figure 1 , copied from Reports and Memo- 

 randa, No. 9 (1909), of the British Advisory Committee for Aero- 

 nautics, is a fine example. 



Clearly in such wind, if at all violent, the support to an aeroplane 

 will be correspondingly erratic and vary between such wide limits 

 that the aeronaut will find himself in a veritable nest of ''holes" out 

 of which it is difficult to rise and dangerous to try. However, as the 

 turmoil due to horizontal winds rapidly decreases with increase of 

 elevation, and as the aeronaut's safety depends upon steady condi- 

 tions, or upon the velocity of his machine with reference to the atmos- 

 phere and not with reference to the ground, it is obvious that the 

 windier it is the higher in general he should fly. 



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 bot- 

 toms 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 

 dii-ection it is actually moving and with unchanged velocity, together 

 with its viscosity, necessitate these whirls with which nearly aU 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 sides of cuts, cliffs, and steep jnountams, but occur also, to a less 

 extent, on the windward sides of such places. In other words the 

 general distribution and direction of the wind currents on the sides 

 of and above large obstructions are somewhat as schematically 

 represented in plate 3, figure 2. 



