ON SOARING FLIGHT. 193 



at the summit of the Washiugton Monument 7 degrees colder than 

 that at its base, 500 feet below, whereas neutral equilibrium would 

 have required but 2 degrees. The equilibrium was, therefore, unstable 

 and continued so for more than half an hour. 



EXPERIMENTS IN GENERATING ASCENDING CURRENTS. 



If the condition of unstable equilibrium is a possible one, and if the 

 normal condition of the air near the earth's surface on a warm day is 

 unstable the fact should be susceptible of verification, and I accord- 

 iugly made a large number of experiments in the open air, under the 

 conditions known to be favorable to soaring flight. On a warm day in 

 August, with light irregular winds and intervals of calms I tried the 

 effect of producing artificial ascending currents by means of a fan ; 

 reasoning that if the air was in a condition of unstable equilibrium I 

 should be able, with little difficulty, to generate a current which, once 

 started, might extend to a considerable height. These experiments 

 were made in the open park surrounding the Smithsonian Institution 

 building. In order to detect the presence of ascending currents I pro- 

 vided strands of white China silk, 1 foot in length, pulled apart so that 

 they were as light almost as spider's webs and yet visible by irradia- 

 tion in the sunlight at a considerable distance. Holding one of these 

 strands aloft in one hand, I fanned the air upward underneath it; then 

 liberating the strand I continued to toss the air beneath it until it was 

 well under way upward. When once started the silk would be borne 

 up to a height of 20, 30, 50, or 100 feet, and often to much greater 

 heights. In numerous instances the strands Avere carried upward 

 entirely out of sight, and in the bright sunlight they could be followed 

 to a height of 200 feet, as estimated by comparison with the height of 

 the adjacent towers of the Smithsonian Institution building. I repeated 

 these experiments during the extremely hot days of September, 1897, 

 choosing those localities where the ground was level, and where there 

 Avere no buildings or trees near-by which might give rise to ascending- 

 currents. I succeeded in sending the strands of silk upward almost as 

 often as I made the attempt, and by changing the locality sought to 

 eliminate, as much as possible, the probability that the strands were 

 caught up by currents already in existence. Often a single stroke 

 of the fan sufficed to produce the necessary current, the silk rising 

 steadily and often rapidly. When strands ceased to rise beyond a 

 height of some 25 or 30 feet, I could usually send them on again by 

 tossing the air upward beneath them with the fan, and frequently when 

 falling they were sent upward out of sight. On one occasion a strand had 

 risen to a height of 75 feet, and immediately afterwards a bit of thistle 

 doAvu, which chanced to float horizontally to the spot where I stood, 

 turned iij)ward without help and followed the path of the silk, continu- 

 ing upward until lost to view. The current produced thus appears to 

 have had a certain degree of ijermanence. On cold cloudy days no 

 SM 97 13 



