HOLES IN THE AIR — HUMPHREYS. 263 



inevitable. Ordinarily, liowever, this is a matter of small conse- 

 quence, for the new speed necessary to support the machine is soon 

 acquired, especially if the engine is in full operation. Occasionally, 

 though, the loss in support may be large and occur but a short dis- 

 tance above the ground, and therefore be distinctly dangerous. 



If the new wind layer is against and not with the machine, an 

 inwease instead of a decrease in the sustaining force is the result, 

 and but little occurs beyond a mere change in the horizontal speed 

 with reference to the ground, and a slowing up of the rate of descent. 



All the above discussion of the effect of wind layers on aeroplanes 

 is on the assumption that they flow in parallel directions. Ordi- 

 narily, however, they flow more or less across each other, as indicated 

 by figure 2, and therefore, as a rule, the aeronaut on passing out of 

 one of them into the other has to contend with more than a discon- 

 certingly abrupt change in the supporting force. That is to say, on 

 crossing the interface between wind sheets, in addition to sufl^ering 

 a partial loss of support, he usually has to contend with the turmoil 

 of a choppy aerial sea, in which his equilibrium is by no means 

 secure — in which ''holes" seem to abound ever3^vhere. 



Wind sheets, witlim ordinary flying levels, are most frequent dur- 

 ing weather changes, especially as fine weather is giving way to 

 stormy. Tliis, then, is a time to be on one's guard against the most 

 dangerous of all "holes in the air," even to the extent of maldng test 

 soundings for them with sniall pilot balloons. It is also well to avoid 

 making gr&at changes in altitude, smce wind sheets, of whatever 

 intensity, remain roughly parallel to the surface of the earth, and the 

 greater the change in altitude the greater the risk of runnmg into a 

 treacherous ''hole." Also, lest there might be a whid sheet near the 

 surface, and for other good reasons, landmgs should be made, if 

 possible, squarely in tlie face of the surface wind. 



WIND BILLOWS. 



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

 of different density billows are set up between them, as illustrated 

 by tlie cloud pictures. Of course, as already explained, the warning 

 clouds are comparatively seldom present, and therefore even the cau- 

 tious 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 enc^ounter one or jnore sudden changes in wuid 

 velocity and direction due, in part, to the eddy-like or rolling motion 

 within the biUows, with chances in each case of being suddenly de- 

 j)rived of a large portion of the requisite sustaining force — of encoun- 

 tering a "hole in tlie an*." There may be perfect safety in either 

 layer, but, unless headed just right, there necessarily is some risk in 

 going from one to the other, and therefore, since flying at the billow 



