METEOROLOGICAL WORK 51 



3. The development of long range propaganda balloons. 



4. The charting of the upper air in the United States and 

 overseas in aid of aviation. 



i. The Extension of Our Knowledge of the Law of Motion 

 of Pilot Balloons. Prior to the development of the meteoro- 

 logical service of the army there had been made in the United 

 States perhaps one hundred pilot balloon flights in which the 

 balloons had been followed by the two-theodolite method 

 the only method which permits of real accuracy and in sev- 

 eral European countries there had been a somewhat greater 

 number, but the data were incomplete and fragmentary. 



Within the past year approximately five thousand such ob- 

 servations have been taken by the meteorological service of the 

 Signal Corps. From these observations the altitude of the bal- 

 loon is determined with great accuracy by triangulation, the 

 base line being usually a mile or more in length. The balloon 

 is kept in sight up to distances as great as sixty miles, and up 

 to heights as great as 32,00x3 meters, or approximately twenty 

 miles. P^or the practical uses of the artillery and the air 

 service, observations need not be carried higher than 10,000 

 meters (six miles), which is the extreme height to which air- 

 planes have thus far ascended, or to which projectiles usually 



go- 

 In view of the number of variables which enter into the rate 

 of ascent of pilot balloons, such as the changing density and 

 the changing temperature of the surrounding air, the changing 

 size of the balloon and consequent changing tension of the rub- 

 ber envelope, the changing temperature of its interior because 

 of the absorption of the sun's rays, the diffusion of hydrogen 

 through its walls, etc., it is one of the most striking facts to be 

 found anywhere in the annals of empirical science that these 

 balloons rise to great heights without deviating appreciably from 

 the simplest possible law of ascent, namely that of constant 

 speed. Graph No. 5 * shows a beautiful example of this con- 



1 Graphs i, 2, 3, and 4 are omitted from this volume. 



