by buzzer and there is already a wire laid passing both of us but in use already for ordinary telegraphy. We only need to add a "condenser" to our apparatus and we can manage all right. As a matter of fact, the service instruments generally have condensers partly for this very purpose. Each of us then connects his instrument to the wire and to earth, after which we can signal to each other while the telegraphist is unaware of the fact. The reason that is possible is the reverse of what we saw just now. There we had a coil which obstructed buzzer or telephone currents but passed ordinary telegraph currents. Here we use condensers which will pass our buzzer currents but not the ordinary telegraph currents.
Thus the soldier telegraphist is up to many dodges whereby he can save time or save material, both of which may be precious. As in bridge building and other branches, he needs to be quick to adapt himself to circumstances, to utilize to the full any opportunities which may present themselves. But his principles are quite simple and do not differ in any way from those of peace. It is only in applying them that the differences arise.
CHAPTER XXIII
HOW WAR INVENTIONS GROW
The inventor of one of the devices described later on in this book modestly claims that he did not invent it but it invented itself. What he means is that he worked step by step, from simple beginnings, each step when complete suggesting the next. To put it another way, many inventions grow in the inventor's mind, sometimes from unpromising beginnings, the most unlikely start often resulting in the most successful ending.
Who has not heard of the "tanks" which made such a name for themselves when they suddenly appeared in Northern France? The British Commander-in-Chief simply mentioned that a new type of armoured car had come into use with good results, but the newspaper men set the whole non-Teutonic world laughing with droll stories of huge monsters suggestive of prehistoric animals which suddenly began to crawl through the slime and mud of the battle-field, pouring death and destruction upon the astounded Germans.
How they came to be called tanks no one seems to know clearly but that is how they will be known for all time. It has been suggested that they were so
named because tank is one of the things which they certainly are not, the intention being thereby to add to the mystification of the enemy. That is by the way, however, for we are more concerned with the things than with their name.
Their precise origin is wrapped in mystery but we have it on excellent authority that they grew out of the peaceful "tractor," originally intended to drag a plough to and fro across a field in the service of the farmer. An illustration of one of these interesting machines will be seen in this book which will well repay a little study.