ON SOARING FLIGHT. 191 



atmosphere, from which, with any very light disturbance, arises a 

 bursting up of the air of the lower strata of the atmosphere through 

 those above." 



"The vertical circulation is the initial stage in the formation of a 

 tornado, and so the tornado can not originate without the condition of 

 unstable equilibrium which gives rise to a vertical circulation." 



"In very hot, dry climates, where there is a sandy soil, sand spouts 

 and whirlwinds are of frequent occurrence. The dry air of such cli- 

 mates, especially over a sandy soil, is often in a state of unstable 

 equilibrium from the accumulation of heat on the earth's surface." 



"Small waterspouts observed on seas and lakes in clear, calm, and 

 hot weather usually arise from a state of unstable equilibrium in the 

 lower strata of the atmosphere." 



Mirage is defined by the Century Dictionary as "An optical illusion, 

 due to the excessive bending of the light rays in traversing adjacent 

 layers of air of widely different densities. * * * The heated earth 

 rarifies the air in the lower strata faster than it can escape, so produc- 

 ing the mirage." 



Professor Tait speaks of the mirage of the desert as formed by the 

 refraction in the hot layer of air near the sand. 



Deschanel says: "Mirage is explained by the heating and consequent 

 rarefaction of the air in contact with the hot soil. The density within 

 a certain distance of the ground increases upward, and rays traversing 

 this portion are bent upward in accordance with the general rule that 

 the concavity must be turned toward the denser side." 



Guyot says: "The mirage is most frequent in arid plains where the 

 soil, exposed to the burning rays of the sun, becomes intensely heated, 

 and in consequence the strata near the ground are less dense than those 

 above." 



There thus seems to be abundant authority for the supposition that 

 the air not infrequently exists in the condition of unstable equilibrium. 

 And if the superheating of the lower strata of the air and the conse- 

 quent unstable equilibrium which ensues is at times sufficient to pro- 

 duce thunderstorms, tornadoes, and mirages, it seems not unreasonable 

 to suppose fliat a similar condition of unstable equilibrium, less 

 intensified, may be of more frequent occurrence when the results are 

 not so manifest. We are thus confronted with a question of fact about 

 which recorded experiments give us but little information. We have 

 been told so often that heated air will rise that it seems somewhat 

 incredible to suppose that there are circumstances in which it will not 

 do so. Laboratory experiments can have but little value in this con- 

 nection, as it is next to impossible to reproduce the conditions as they 

 exist in nature, nor have we the instruments for measuring with preci- 

 sion the small differences in temperature which suffice to produce 

 unstable equilibrium within the walls of a laboratory. The following 

 simple experiment, however, appears to indicate that a layer of heated 



