Page:Popular Science Monthly Volume 81.djvu/59

Rh 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 frequently 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.

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 connection 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.

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