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. It consists of a steel frame or platform upon which is mounted a four-cylinder petrol engine with a reservoir above to carry the supply of fuel and with a radiator in front to cool the water which keeps the engine from becoming too hot. Towards the back of the vehicle is what is called by engineers a worm-gear, the function of which is to reduce the one thousand revolutions per minute of the engine to somewhere near the slow speed required of the wheels of the tractor.