HOLES IK THE AIR— HUMPHREYS. 259 



the atmosphere is lieated mainly tlirongli contact with the surface of 

 the earth, which in turn has l)een heated by sunshine, it follows that 

 these convection currents, or vertical uprushes of the atmosphere, 

 are most numerous during warm clear weather. 



The turbulence of some of these rising columns is c-\ddent from the 

 numerous rolls and billows of the large cumulus clouds they produce, 

 and it is obvious that the same sort of turbulence, probably on a 

 smaller scale, occurs near the tops of those columns that do not rise 

 to the cloud level. Further, it is quite possible, when the air is 

 exceptionally quiet, for a rising column to be rather sharply sepa- 

 rated from the surrounding quiescent atmosphere, as is evident from 

 the closely adhering long columns of smoke occasionally seen to rise 

 from chimne^^s. 



The velocity of ascent of such fountains of air is at times surpris- 

 ingly great. Measurements on pilot balloons and measurements 

 taken in manned balloons have shown vertical velocities, both up 

 and down, of as much as 10 feet per second. The soarmg of large 

 birds is a further proof of an upward velocity of the same order of 

 magnitude, while the fact that in cumulus clouds water drops and 

 hailstones often are not only temporarily supported, but even carried 

 to higher levels, shows that uprushes of 25 to 30 feet per second not 

 merely may but actually do occur. 



There are, then, aerial fountams of considerable vertical velocity 

 whose sides at times and places may be almost as sharply separated 

 from the surrounding air as are the sides of a fountain of water, and 

 it is altogether possible for the swiftest of these to produce effects on 

 an aeroplane more or less disconcerting to the pilot. The trouble 

 may occur : 



1 . On grazing the column, with one wing of the machine in the ris- 

 ing and the other in the stationary air; a condition that interferes 

 with lateral stability and produces a sudden shock both on entering 

 the column and on leaving it. 



2. On plunging squarely into the column; thus suddenly increasmg 

 the angle of attack, the pressure on the wings, and the angle of 

 ascent. 



3. On abruptly emerging from the column; thereby causing a 

 sudden decrease in the angle of attack and also abruptly losing the 

 supporting force of the rising mass of air. 



That flying vriih one whig 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 

 sciuarely into or out of the column require a little further considera- 

 tion. 



Let an aeroplane that is flying horizontally pass from quiescent air 

 squarely into a rising column. The front of the machine will be 



