260 ANNUAL REPORT SMITHSONIAN INSTITUTION^ 1912. 



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

 angle of attack, that is, 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 nev/ 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 oi^er adjustments by the aeronaut m his 

 hasty attempt to correct for the abrupt changes. Such an adjust- 

 ment 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 clays and over barren ground. Isolated hills, 

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

 daj^s, for on such occasions their sides are certain to be wa,rmer than 

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

 many chimneys in producing upd rafts. Rising au' 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 sunrise and at such times as the sky is wholly covered 

 with clouds. 



AERIAL CATARACTS. 



There are two kinds of aerial cataracts, the free-air cataract and the 

 surface cataract. The former is the counterpart of the aerial foun- 

 tain and is most likely to occur at the same time. It is seldom rapid 

 save in connection with thunderstorms, and such effect as it may 

 have is exactly similar to, but in the opposite direction from, that of 

 the rising column. 



The second or surface cataract is caused by the flow of a dense or, 

 what comes to the same thing, a heavily laden surface layer of air 

 up to and then over a precipice, much as a waterfall is formed. Such 

 cataracts are most frequent among the barren mountains of high 

 latitudes where the surface A\dnds catch up and become weighted 

 with great quantities of dry snow and, because of this extra weight, 

 often rush down the lee sides of steep mountains with the roar and 

 the force of a hurricane. 



But the violence of such wmds clearly is all on the lee side and of 

 shallow depth, and therefore where such conditions prevail the aero- 

 naut should keep well above the drifting snow or other aerial ballast, 

 and, if possible, strictly avoid any attempt to land within the cataract 

 itself. 



