HOLES IN THE AIR HUMPHREYS. 267 



(a) VERTICAL GROUP. 



1. Aerial JountaiTis. — 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 experiencetl aviator. 



2. Aerial cataracts. — Downrushes of the free air, like the uprushes 

 with which they are associated in a vertical circulation, though less 

 violent, must also be most frequent during warm weather when the 

 ground is strongly heated. They, too, however annoying to the 

 beginner, should not be dangerous to the experienced man, because 

 even when strong enough to carry the machine down for a distance 

 their descent necessarily becomes slower and their chief velocity 

 horizontal before the surface is reached. Downrushes of weighted 

 air over precipices, analogous to waterfalls, must be strictly avoided. 



3. Aerial cascades.— The lower wind, in following as it must surface 

 contours, sweeps down to the leeward of hills and mountains in 

 cascatle-like falls, and the stronger the wind the more rapid the 

 cascades. But they are of no danger to the aeronaut so long as he 

 takes the precaution to keep above the eddies and other surface 

 disturbances. 



4. Aerial hreaJcers. — The choppy, breaker-like winds of thunder- 

 storms that surge up and down and in all sorts of directions are as 

 nmch to be avoided by aerial craft as are ocean breakers by water 

 craft. Hence a flight should positively not be attempted under 

 any such circumstances. 



5. Wind eddies (forward side). — The air on the forward side of a 

 strong eddy has a rapid downward motion and therefore should be 

 avoided. If caught in the down current of an eddy the aeronaut 

 should head lengthwise of the hill or mountain to which the eddy is 

 due. By heading away from the mountain he might, to be sure, 

 get entirely out of the whirl, but the chances are just as great that 

 instead of getting out he would only get the deeper in and thus 

 encounter downward currents swifter and still more dangerous than 

 those he had sought to shun. 



(6) HORIZONTAL GROUPS. 



1. Wind layers. — The atmosphere is often made up of two or more 

 superimposed layers moving each with its own velocity and direction. 

 Such a condition is a source of danger to the aeronaut because transi- 

 tion from one of these layers to another more nearly coincident in 

 direction and velocity with his aeroplane is certain to result in at 

 least a sudden decrease in the magnitude of its supporting ])ressure 

 and in the effectiveness of the balancing devices. Under certain 

 extreme conditions this transition, even when the winds are parallel, 

 is well nigh inevitably disastrous. When the layers move more or 



