52 THE POPULAR SCIENCE MONTHLY 



who walk the earth and those who fly the air, do not exist. We will now 

 consider some of the things that do exist and produce effects such as 

 actual holes and half holes would produce — sudden drops, and occa- 

 sional disastrous falls. 



Aerial Fountains 



A mass of air rises or falls according as its density is less or greater 

 respectively than that of the surrounding atmosphere, just as, and for 

 the same reason that a cork bobs up in water and a stone goes down. 

 Hence warm and therefore expanded and light air is buoyed up when- 

 ever the surrounding air at the same level is colder; and as the atmos- 

 phere is heated mainly through contact with the surface of the earth, 

 which in turn has been heated by sunshine, it follows that these convec- 

 tion currents, or vertical uprushes of the atmosphere, are most numerous 

 during warm clear weather. 



The turbulence of some of these rising columns is evident 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 separated from the sur- 

 rounding quiescent atmosphere, as is evident from the closely adhering 

 long columns of smoke occasionally seen to rise from chimneys. 



The velocity of ascent of such fountains of air is, at times, sur- 

 prisingly 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 soaring 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 fountains 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 more or 

 less disconcerting to the aeronaut. The trouble may occur: 



1. On grazing the column, with one wing in the rising 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 increasing 

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



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



