58 THE POPULAR SCIENCE MONTHLY 



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. 



Classification 



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 atmos- 

 pheric conditions — wind layers, billows, eddies (central portion), tor- 

 rents 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 suffi- 

 cient 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. 



Conclusions 



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 Avould 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 



