192 ON SOARING FLIGHT. 



air rises with difficulty from a surface. Let a quantity of tobacco 

 smoke be gently exhaled upon the top of a table; instead of rising, as 

 it would do if free of the table, it will remain a surprisingly long time 

 as a dense layer upon the surface, like a fog upon a river, and perhaps 

 for the same reason. Still, the crucial test must be applied in the 

 open air, and I shall presently show that when such a test is applied 

 experimenb seems to favor the theory here set forth. Instead of unsta- 

 ble equilibrium, unstable motion would perhaps be the more accurate 

 term, but the former will be used as sufficiently exact for our purpose. 



My hypothesis is that when the air is in this condition of unstable 

 equilibrium the soaring bird, by the mere act of moving in circles, so 

 disturbs the equilibrium as to produce within the circle of its flight a 

 feeble ascending current of warm air, which grows in magnitude, slowly 

 at first but with increasing rapidity, through the pressure of outlying 

 masses of heavier air above, until it attains a sufficient force to bear 

 the bird up with it. Thus a natural chimney, which gradually enlarges, 

 is produced, through which the warm air over an extended area finds 

 the means of escape, the column thus formed often rising to a great 

 height. The heat of the sun is stored up in vast quantities in the 

 lower strata of the air on every warm day, and by tapping these nat- 

 ural reservoirs of energy the bird obtains all the power necessary for 

 flight in light winds. The process of j)enetration and permeation in 

 slender streams must at all times be a slow one, and if the lower strata 

 receive heat from the sun and the earth more rapidly than it can in 

 this way be carried off, an accumulation of heat must result and a 

 correspondingly increased tension throughout the masses involved. 

 Sooner or later the lower air must burst through the overlying masses, 

 the violence of the resulting disturbances depending upon the degree 

 of instability existing at the time of the upheaval. Under such con- 

 ditions the bird by moving in circles either with or without flapping 

 easily destroys the equilibrium and produces an ascending current 

 which it utilizes in ascending along a spiral course, by which means it 

 keeps witLiu the ascending column. 



I carefully examined the records of temperature taken by aeronauts 

 in balloon ascents, as well as those obtained by means of kites, and 

 these records all go to show that the normal condition of the atmos- 

 phere taken as a whole up to a height of several thousand feet is one 

 of stable equilibrium. But it is to be noted that these temperatures 

 have usually been recorded at great altitudes and that a stable condi- 

 tion of the upper air is in nowise incompatible with a condition of 

 UMstable equilibrium in the lower strata, in which the flight of the 

 birds usually begins.* On several occasions I have found the outer air 



* Since the above was written Mr. Eddy and Jiis assistants have made some experi- 

 ments in the lower atmos]^)here which showed that at the time the experiments were 

 made the equilibrium was unstable up to an altitude of 1,500 feet and much more so 

 up to a height of 400 feet. Thus the average rate of decrease for 1,500 feet was found 

 to be 1° for 163 feet of ascent, while for 400 feet the average was 1° for 67 feet of 

 ascent. 



