258 ANNUAL REPOKT SMITHSONIAN INSTITUTION, 1912. 



or whirl of the atmosphere, such that the "centrifugal force" would 

 be sufficient to equal the difference in pressure, at the same level, 

 between the regions of liigh and low density. 



Appropriate equations can be written to express the balance 

 between presMire gradient and deflective force in any sort of winds, 

 and at any part of the world (it depends slightly upon latitude). 

 Therefore it is possible with certain conditions given to compute the 

 wind velocity, or -wdth other conditions given to compute the pres- 

 sure gradient. But in the present case numerical calculations are 

 not necessary. We know that an ascent of half a mile, easily made 

 by an aeroplane, produces roughly a 10 per cent decrease in pressure, 

 and we know too that a greater pressure difference than this seldom 

 exists even between center and circumference of ^aolent tornadoes. 

 Hence a drop in density, or pressure, to which the density is directly 

 proportional, sufficient to cause an aeroplane to fall, would require a 

 tronadic whirl of the most destructive \dolence. Now there were no 

 whirlwinds of importance in the air, certainly none that could be 

 called tornadoes, at the times and places where aeronauts have 

 reported holes, and therefore even half holes, in the sense of places 

 sufficiently vacuous to cause a fall, must also be discarded as unreal, 

 if not impossible. 



Along with these two impossibles, the hole and the half hole, the 

 vacuum and the half vacuum, should be consigned to oblivion that 

 other picturesque fiction, the "pocket of noxious gas." Probably 

 no other gases, certainly very few, have at ordinary temperatures 

 and pressures, the same density as atmospheric air. Therefore a 

 pocket of foreign gas in the atmosphere would almost certainly either 

 bob up like a balloon, or sink like a stone in water; it could not float 

 in mid air. It is possible, of course, as will be discussed a little later, 

 to run into columns of rising air that may contain objectionable gases 

 and odors, but these columns arc quite different from anything likely 

 to be suggested by the expression "pockets of gas." 



The above are some of the things that, fortunately alike for those 

 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 occasional disastrous falls. 



AERIAL FOU'NTAINS. 



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 

 dowai. Hence warm and therefore expanded and liglit air is buoyed 

 up whenever the surrounding air at the same level is colder; and as 



