262 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



only when the humidity at the interface is such that the slight differ- 

 ence in temperature between the crests and tlie 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 straightaway 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 consid- 

 erable distance, is absolutely inevitable, and a disastrous one highly 

 probable. To all intents and purposes a hole, a perfect vacuum, has 

 been run into. 



The reason for the fall will be understood when it is recalled 

 that, for all ordinary velocities, wind pressure is very nearl}^ propor- 

 tional to the square of the velocity of the wind with respect to the 

 thing against which it is pressing. Hence, for a given inclination of 

 the wings, the lift on an aeroplane is approximately proportional to 

 the square of the velocity of the machine with reference, not to the 

 ground, but to the air in which it happens to be at the instant under 

 consideration. If then it glides, with propellers at rest, into air that 

 is moving in the same horizontal direction and with the same velocity, 

 it is in exactly the condition it would be if dropped from the top of a 

 monument in still air. It must inevitably fall to ruin, unless, indeed, 

 rare skill in balancing, or, possibly, mere chance should bring about a 

 new glide after additional velocity had been acquired as the result of a 

 considerable fall. Warping of wings, turning of ailerons, dipping and 

 twisting of rudders, and all tlie other devices of this nature would be 

 utterly useless at first, totally without effect so long as wind and 

 machine have the same velocity, for, as already explained, there 

 would be no pressure on them in any position, and consequently 

 nothing that could be done with them would at first have any effect 

 on the behavior of the machine. However, as stated above, a skillful 

 pilot may secure a new glide with a proi)erly constructed machine, and, 

 finally, if high enough, make a safe landing. 



Of course, such an extreme case must be of rare occurrence, but 

 cases less extreme are met with frequently. On passing into a cur- 

 rent where the velocity of the wind is more nearly that of the aero- 

 plane, and in the same direction, more or less of the supporting force 

 is instantly lost, and a corresponding tlrop or dive becomes at once 



