54 THE POPULAR SCIENCE MONTHLY 



hole has been encountered, when of course, there is nothing of the kind. 

 Indeed such cascades should be entirely harmless so long as the aeronaut 

 keeps his machine well above the surface and therefore out of the 

 treacherous eddies, presently to be discussed. 



Wind Layers 



It is a common thing to see two or more layers of clouds moving 

 in different directions and at different velocities. Judgment of both 

 the actual and the relative velocities of the cloud layers may be badly 

 in error — the lower seems to be moving faster, and the higher slower, 

 than is actually the case. Accurate measurements, however, are possible 

 and have often been made. 



These differences in direction and velocity of the winds are not 

 confined to cloud layers, nor even to cloudy weather, as both pilot and 

 manned balloons have often shown- Occasionally balloons float for 

 long intervals with a wind in the basket, showing that the top and the 

 bottom of the balloon are in currents of different velocities. Another 

 evidence of wind layers moving with different velocities is the waves or 

 billows so often seen in a cloud layer. A beautiful example of the?e 

 cloud waves, both regular, when the directions are the same but the 

 velocities different, and irregular, when the winds are more or less 

 crossed, is shown in the accompanying picture, taken by Professor A. J. 

 Henry, of the U. S. Weather Bureau, and kindly lent for this illus- 

 tration. 



It was explained by Helmholtz as far back as 1889 that layers of 

 air differing in density are of frequent occurrence, and that they glide, 

 sharply divided and with but little intermingling, the one over another 

 in much the same manner that air flows over water, and with the same 

 general wave-producing effect. These air waves are " seen " only when 

 the humidity at the interface is such that the slight difference in tem- 

 perature between the crests and the troughs is sufficient to keep the 

 one cloud-capped and the other free from condensation. In short, the 

 humidity condition must be just right, and therefore, though such clouds 

 are often seen, air billows must be of far more frequent occurrence. 



Consider now the effect on an aeroplane as it passes from one such 

 layer into another. For the sake of illustration let the case be an 

 extreme one. Let the propeller be at rest and the machine be making a 

 straight away glide to earth, and let it suddenly pass into a lower layer 

 of air moving in the same horizontal direction as the machine and with 

 the same velocity. This of course is an extreme case, but it is by no 

 means an impossible one. Instantly on entering the lower layer, under 

 the conditions just described, all dynamical support must cease and with 

 it all power of guidance. A fall, for at least a considerable distance, is 

 absolutely inevitable, and a disastrous one highly probable. To all 

 intents and purposes a hole, a perfect vacuum, has been run into. 



