OF THE UNITED STATES ARMY. 143 



V 



■'W 



This formula is found to fit the observational data within the ranges 

 used in the Signal Corps work to an accuracy of somewhat less than 

 10 per cent., which is sufficient for most work at the front. 



2. Meteorology in the Aid of the Artillery.— In former times 

 when guns did not shoot to a greater distance than eight or ten 

 miles, it was usually possible to observe where the projectile hit and 

 to correct errors by " spotting." This made unnecessary the cor- 

 rection of the trajectory for the influence of the w'ind and the chang- 

 ing density of the air with increasing altitude. In the present war, 

 however, guns have been built to shoot much farther and in addition 

 camouflage has prevented the visual location of guns even at the 

 old ranges. Hostile batteries have been located in many instances 

 solely by the new art of sound-ranging which has itself demanded 

 for the high accuracy attained aerological data. The answering 

 battery has been obliged to fire wholly by the map, so that it is 

 obvious that it has become necessary to make careful allowances 

 both for the density of the air and the direction and speed of the 

 wind at various altitudes. Some of the modern projectiles remain 

 in the air as long as seventy seconds and a moderate wind blowing 

 across the path of such a projectile might easily cause it to drop 

 half a mile away from the point at which it would strike if fired in 

 still air. The wind-direction and speed at various altitudes have 

 been obtained, as already indicated by pilot balloons, while the tem- 

 perature has been determined at the proving grounds by sending 

 self-recording instruments aloft in specially constructed box-kites, 

 as well as by sending self-recording instrvmients and meteorological 

 observers aloft in airplanes. It has been with the aid of observa- 

 tions of this sort that the new range tables for the Ordnance De- 

 partment of the United States Army have been constructed. The 

 importance of this work may be understood when it is considered 

 that these range tables will be used in connection with the firing of 

 all guns, and errors in them would produce errors in the range of 

 every gun fired with their aid. 



3. TJie Development of Long Range Propaganda Balloons. — In 

 view of the fact that above an altitude of 10,000 feet 95 per cent, of 



