ADVANCES IN SIGNALLING 235 



square frame sets are able to maintain communication between 

 them, over a distance of 5 km. or more, even when each is 

 operated in a deep dugout. 



This war differed from preceding wars also in the fact that 

 the artillery was fired at distant targets it very rarely saw, and 

 was directed by targets, on which it did not aim. In such 

 artillery practice, it becomes of vital importance for the artillery 

 commander to know just where his own infantry forward lines 

 are located, so as to keep his fire ahead of their advance. Port- 

 able radio loop sets are invaluable for this service, as they 

 enable communication to be maintained at all times with the 

 advancing lines. 



Airplane Radio Communication During the War. In the 

 early days of the war, the airplane became the eyes of the army. 

 It was by reports from airplanes, that the British were able 

 to make good their retreat from Mons to the Marne, during 

 the fateful closing days of August, 1914. It was reports from 

 airplanes that gave Foch the inspiration for his daring and 

 epoch-making thrust through the German lines at the Marne 

 on September 9th, 1914. It was airplanes that enabled a 

 trench stalemate to be maintained for nearly three years there- 

 after, by eliminating the possibility of complete surprise on 

 either side in large-scale strategy. Above all, it was airplanes 

 that enabled long-range artillery fire to be controlled, the air- 

 plane observer signalling to the artillery outpost the effect of , 

 each shell round. 



At first, the signalling between the airplane and the observer 

 on the ground was entirely visual. The airplane showed a flag 

 or released a visible signal. The ground observer replied to 

 the airman by means of large flags, or other visible signals, dis- 

 played on the ground. These could only be read at a compara- 

 tively short range. Later in the war one-way radio communi- 

 cation was introduced into airplane fire control. The observer 

 in the airplane let out a wire or trailing antenna and sent his 

 reports in radio dots and dashes on this wire. A receiving 

 operator in a radio station on the ground received these sig- 



