HOLES IN THE AIR 53 



sudden decrease in the angle of attack and also abruptly losing the 

 supporting force of the rising mass of air. 



That flying with one wing in the column and the other out must 

 interfere with lateral stability and possibly cause a fall as though a 

 hole had been encountered, is obvious, but the effects of plunging 

 squarely into or out of the column require a little further consideration. 



Let an aeroplane that is flying horizontally pass from quiescent air 

 squarely into a rising column. The front of the machine will be lifted, 

 as it enters the column, a little faster than the rear, and the angle of 

 attack, that is to say, the angle at which the wing is inclined to the 

 horizon, will be slightly increased. This, together with the rising air, 

 will rapidly carry the machine to higher levels, which, of itself, is not 

 important. If, however, the angle of attack is so changed by the pilot 

 as to keep the machine, while in the rising column, at a constant level, 

 and if, with this new adjustment, the rising column is abruptly left, a 

 rapid descent must begin — the half hole is met. But even this is not 

 necessarily harmful. Probably the real danger under such circumstances 

 arises from over adjustments by the aeronaut in his hasty attempt to 

 correct for the abrupt changes. Such an adjustment might well cause 

 a fall so sudden as strongly to suggest an actual hole in the air. 



Rising columns, of the nature just described occur most frequently 

 during clear summer days and over barren ground. Isolated hills, 

 especially short or conical ones, should be avoided during warm still 

 days, for on such occasions their sides are certain to be warmer than the 

 adjacent atmosphere at the same level, and hence to act like so many 

 chimneys in producing updrafts. Eising air columns occur less fre- 

 quently and are less vigorous over water, and over level green vegetation, 

 than elsewhere. They are also less frequent during the early forenoon 

 than in the hotter portion of the day, and practically absent before sun 

 rise, and at such times as the sky is wholly covered with clouds. 



Aerial Cataracts 

 The aerial cataract is the counterpart of the aerial fountain, and js 

 most likely to occur at the same time. It is seldom rapid, save in con- 

 nection with thunder storms, and such effect as it may have is exactly 

 similar to, but in the opposite direction from, that of the rising column. 



Aerial Cascades 



The term " aerial cascade " may, with some propriety, be applied to 

 the wind as it sweeps down the lee side of a hill or mountain. It is 

 most pronounced when the wind is at right angles to the direction of the 

 ridge, and when the mountain is rather high and steep. The swift 

 downward sweep of the air when the wind is strong may carry the aero- 

 plane with it and lead observers, if not the pilot, to fancy that another 



