OF THE UNITED STATES ARMY. 



135 



tions have been taken by the meteorological service of the Signal 

 Corps. From these observations the altitude of the balloon is deter- 

 mined 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 32,000 



Uniform rate of ascent of pilot balloon up to 4,500 meters. 



meters, or approximately twenty miles. For 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 airplanes 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 chang- 



