The reason for the fall will be understood when it is recalled that, for all ordinary velocities, wind pressure is very nearly proportional to the square of the velocity of the wind with respect to the tiling 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 the 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 properly 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 current where the velocity of the wind is more nearly that of the aeroplane, and in the same direction, more or less of the supporting force is instantly lost, and a corresponding drop or dive inevitable. Ordinarily, however, this is a matter of small consequence, 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 distance above the ground, and therefore be distinctly dangerous.
If the new wind layer is against and not with the machine an increase instead of a decrease in the sustaining force is the result, and but little occurs beyond a mere change in the horizontal speed of the machine with reference to the ground, and a slowing up of its rate of descent.
"Wind sheets, within ordinary flying levels, are most frequent during weather changes, especially as fine weather is giving way to stormy. This then is a time to be on one's guard against the most dangerous of-all "holes in the air." It is also well to avoid making great changes in altitude since 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 running into a treacherous "hole." Also,
