ON SOARING FLIGHT. 187 



quickly and inevitably come to the earth, and though he may, and 

 sometimes does, soar continuously for hours in a wind whose velocity 

 does not exceed 4 miles an hour, and may do so without flapping and 

 without apparent effort, he can accomplish this only in one way, by 

 frequently soaring in circles. Whenever he wishes either to maintain 

 his elevation or to increase it he must resort to circular or spiral flight, 

 by which maneuver he may rise to incredible heights 5 but whenever 

 he wishes to traverse the country in direct flight he is constrained to 

 do so by descending, the distance to which he may sail deijending upon 

 the height to which he may previously have risen. In high winds, there- 

 fore, the bird need never soar in spirals, and indeed it seldom does; 

 whereas in light winds there is no other way in which it can remain 

 long in the air. Our task will therefore be an easier one if we recog- 

 nize at the outset that there is a radical difference in the character 

 of flight in light and strong winds, and that nature does not demand 

 of us a single method of universal application. In presenting a new 

 hypothesis as a possible explanation of soaring flight in light winds, I 

 accordingly wish to state distinctly that it is applicable only to spiral 

 flight, and does not apply to direct flight in strong winds. 



My hypothesis is based upon the fact, as I believe it to be, that when 

 the surface of the earth is exposed to the heating influence of the sun 

 the normal condition of the lower atmosphere is one of unstable equi- 

 librium ; by which I mean that the rate of decrease in temperature in 

 the lower strata is greater than 1 degree Fahrenheit for each 183 feet of 

 ascent, this being the rate of decrease when the atmosphere is in neu- 

 tral equilibrium. This unstable equilibrium results from the viscosity 

 of the air and the diffusion of gases, in consequence of which the warm 

 air near the surface of the earth does not readily penetrate the colder 

 strata above, and there is thus a tendency at all times toward an 

 accumulation of heat in the lower strata. 



In order to fix our ideas we may consider a specific, if imaginary, 

 case, and suppose that on a clear, calm morning a large, level field, sur- 

 rounded on all sides by woodlands, is exposed to the influence of the 

 sun's rays; and further, that during the preceding night the condition 

 of the atmosphere has become one of stable equilibrium. As the air in 

 contact with the earth becomes heated it will tend to rise in slender 

 streams or bubbles from the irregularities of the surface, and being 

 lighter than the encompassing colder air, it will be carried upward, the 

 velocity of its ascent and the height to which it will rise depending in 

 part upon its temperature and in part upon the resistances to be over- 

 come, which are due partly to the displacement of the overlying masses 

 of the air and partly to friction. The frictional resistances, though 

 readily overcome where large volumes are involved, are greatly increased 

 when a mass of air is broken up into smaller masses, as seen in the 

 case of large and small bubbles of air rising through water. The 

 tendency of a mass of air to disintegration and diffusion when pene- 



