The cables, even the most deeply buried, might be and fre quently were cut at critical moments. At once the alternative visual and wireless methods held in reserve, inadequate though they were, became of vital importance. Visual signalling could no longer, under such a fire, be carried out by the traditional soldierly figure standing square on his feet with a flag. It generally took the form instead of an electric signalling lamp working through a deep embrasure, or of a small inconspicuous shutter showing just above the parapet; and any form of visual signalling working from the rear forwards was more than likely to be seen by the enemy and to draw his fire. Wireless signalling at that time laboured under the drawback that an exposed aerial was necessary, no less liable to be shot away than the cables were to be cut. The need for something less vulnerable led to the revival of an old principle, that of earth induction, whereby an alternating current in a length of wire connected to earth at each end induces corresponding im pulses in a similar wire up to a distance of a few thousand yards.
The wires could be buried and the instruments protected in dug outs so that the whole became comparatively shell-proof. As, however, each transmitter affected every receiver within its range they could be installed only at wide intervals along a front. Finally, each side evolved a code of firework signals, such as coloured lights, to be fired by the forward infantry if they should be attacked, each signal conveying its own message to the support ing artillery.
Other means were sought, less costly in life and less vulnerable to fire. These expedients all took the form of message carrying, and their agents were pigeons, messenger dogs and rockets. Of the three, pigeons were by far the most reliable; they might be de layed by darkness, chased by hawks, lost in the smoke and uproar or killed in crossing a barrage, but they made their lofts on the whole with remarkable regularity and sometimes showed a pa thetic courage. More than one reached its loft wounded and died as it alighted, and one such in the French army was officially dec orated for valour. Messenger dogs were less successful; their in telligence made them temperamental. Success demanded special treatment and a rigid discipline, which must be enforced not only by their kennelmen but by all with whom the dogs came in con tact. Under these circumstances it was natural that the Germans, with their instinct for regulation, should achieve better results than their enemies. Nothing could prevent the British soldier from making a pet of any dog in his vicinity, and a dog which had been made much of in a forward position would not face the shell swept open to regain his kennel. None the less in proper hands
they could prove their worth. Rockets could carry a message some 2,000 yards across a region where neither runner nor signaller could live and where no wire could remain uncut ; but to be suc cessful they were required to drop a message with some certainty within reasonable reach of their destination, and this was a stage of reliability to which they never attained.
Air Co-operation.—While signalling was adapting itself to meet such changes in its old tasks, it found itself faced with a new duty, that of communicating with aircraft. A commander might have his eyes in the air above the enemy lines, but without the nerve connecting eye to brain he was little the better for it. At the outset aircraft of both sides were employed solely on recon naissance and the pilot made his report after landing. It was not long, however, before there was added to this the duty of observ ing for artillery, and this was made possible only by the adaptation of wireless telegraphy for signalling the fall of shell to the guns. At the same time the need arose for bodies of infantry, isolated after an attack and cut off by fire from their friends, to report their position and their predicament. This need was met to a limited extent in the different armies by codes of ground signals, displayed by the isolated troops to their own aircraft. The signals consisted generally of canvas strips laid out on the ground in set forms or groups of letters, each with its special meaning.
Tapping an Enemy's Communications.—The conditions of warfare on the Western front gave rise to a further new activity of signalling. A commander at all times has sought to read his enemy's mind ; to divine "what is happening on the other side of the hill." In the war each commander found himself faced by the impenetrable barrier of the enemy's front line, behind which stretched the cables carrying intelligence of inestimable value to him, could he but hear it. So the rival intelligence bureaus asked their signallers if there were no means of tapping the enemy's cables. The experts worked for a time in closely guarded secrecy, and soon on either side unaccountable miscarriages began to oc cur. An attack would be planned in scrupulous detail ; troops and staff would play their parts faultlessly, but at the critical moment an enemy bombardment, perfectly timed and directed, would turn the packed assembly trenches into a bloody chaos. Like incidents were repeated, for which the only possible explanation was that the enemy was possessed in detail of the plans for the opera tion.