236 THE NEW WORLD OF SCIENCE 



nals and delivered the message by telephone to the artillery of- 

 ficer. The reply was sent back from the ground to the airman 

 by flags or ground signals ; because although radio signals sent 

 from the ground station might be received in the plane ; yet the 

 loud whirring of the airplane motor made such signals very 

 difficult to detect in the telephone. 



At the entrance of America into the war, one of the first 

 projects of our Chief Signal Officer, in connection with radio 

 development, was to create a practicable airplane telephone set. 

 Events happen so rapidly in the air, that airplane telegraphy 

 with its dots and dashes may be all too slow at some critical 

 period. The telephone becomes essential to successful com- 

 munication. The difficulties in the way were very great, if 

 only on account of the great noise in airplanes aloft. It has 

 occasionally happened for example, that the pilot and the 

 observer in one and the same airplane, have desired to com- 

 municate with each other, the distance between them being 

 perhaps only a couple of meters. Shouting was of no use, 

 and even an umbrella spanning the distance between the men 

 would be unavailing for intersignalling purposes ; so that they 

 have actually given up the attempt to come to an understanding 

 aloft, and have wended their way down to the earth, in order 

 to stop the engine and talk together. The problem of tele- 

 phoning aloft from one airplane to another or to a radio station 

 on the ground was thus most ambitious and difficult. 



The problem was successfully solved, however, with the aid 

 of the experts of the large telephone companies. A vacuum- 

 tube transmitter and a two-stage vacuum tube receiver were 

 developed, which could readily be carried on an airplane. A 

 carefully designed helmet, with rubber caps enclosing a tele- 

 phone tightly over each ear, eliminated most of the engine 

 noise, and then a trailing antenna from each airplane enabled 

 signals to be exchanged. A reel, like a large fishing-rod reel, 

 mounted on the outside of an airplane fusilage pays out and 

 reels in the antenna wire with a metallic weight on the end, 

 which from its shape is called a " fish." 



