ADVANCES IN SIGNALLING 243 



definite part in the future development of flying, in peace as 

 well as in war. 



Improvements in Ground Telegraphy. In addition to radio 

 communication without wires, the Signal Corps employed in 

 our army, and in common with our Allies, another system of 

 so-called wireless electric communication, which consists in 

 laying a short length say 75 meters of insulated wire on 

 the surface of the ground in a straight line parallel to the front 

 trenches, grounding each end of this wire, usually by steel 

 spikes driven into the soil, and inserting in this wire, say at the 

 middle of its length, a relatively powerful electric buzzer send- 

 ing apparatus, worked from a portable storage battery. Any 

 similar length of wire parallel to this, and not more than say 

 2 or 3 kilometers distant therefrom, with its ends also grounded, 

 and with a delicate telephone receiver inserted in it, enables 

 the buzzer signals from the distant sending wire to be picked 

 up and read by the listening operator, who may, perhaps, be 

 located in a dugout on the front line. This communication 

 between two short parallel grounded wires depends upon 

 ground conduction, magnetic induction, and radio action, all 

 combined in certain proportions, that vary from case to case. 

 The French who, as a nation, are always logical in thought, and 

 precise in language, have called this telegraphde par sol, abbre- 

 viated T. P. S., or as it has been repeated in our army, *' ground 

 telegraphy." 



Ground telegraphy apparatus has its advantages in being 

 portable and well adapted to rapid infantry advance over a 

 short range. It can be put down and picked up again, as 

 rapidly as the men can drive short steel spikes and pull them 

 up again. The short length of wire is likely to escape de- 

 struction from exploding shells, for a little while, during an 

 advance. Disadvantages are, on the other hand, that the coded 

 messages can be read as easily by foe as friend. As soon as 

 T. P. S. messages are picked up, the terrain from which they 

 may emanate at once invites the attention of opposing artillery. 

 The Germans, who also employed a T. P. S. system, resorted 



