ings, and hence the cautious aeronaut need seldom be, and, as a matter of fact seldom is, caught in so dangerous a situation. However, more than one disaster is attributable to just such winds as these—to aerial breakers.
The above eight types of atmospheric conditions may conveniently be divided into two groups with respect to the method by which they force an aeroplane to drop.
1. The Vertical Group.—All those conditions of the atmosphere, such as aerial fountains, cataracts, cascades, breakers and eddies (forward side), that, in spite of full speed ahead with reference to the air, make it difficult or impossible for an aeronaut to maintain his level, belong to a common class and depend for their effect upon a vertical component, up or down, in the motion of the atmosphere itself. Whenever the aeronaut, without change of the angle of attack and with a full wind in his face, finds his machine rapidly sinking, he may be sure that he has run into some sort of a down current. Ordinarily, however, assuming that he is not in the grasp of storm breakers, this condition, bad as it may seem, is of but little danger. The wind can not blow into the ground and therefore any down current, however vigorous, must somewhere become a horizontal current, in which the aeronaut may sail away or land as he chooses.
2. The Horizontal Group.—This group includes all those atmospheric conditions—wind layers, billows, eddies (central portion), torrents and the like—that, in spite of full speed ahead with reference to the ground, abruptly deprive an aeroplane of a portion at least of its dynamical support. When this loss of support, due to a running of the wind more or less with the machine, is small and the elevation sufficient there is but little danger, but, on the other hand, when the loss is relatively large, especially if near the ground, the chance of a fall is correspondingly great.
1. Holes in the air, in the sense of vacuous regions, do not exist.
2. Conditions in the atmosphere favorable to precipitous falls, such as would happen in holes, do exist, as follows:
a. Vertical Group
1. Aerial Fountains.—Uprushes of air, most numerous during warm clear weather and over barren soil, especially above conical hills, are disconcerting and dangerous to the novice, but do not greatly disturb an experienced aviator.
2. Aerial Cataracts.—Down rushes of air, like the up rushes with which they are associated in a vertical circulation, though less violent, must also be most frequent during warm weather when the ground is