[Illustration: Engineers at work erecting a pontoon bridge over a river. _Photo, Newspaper Illustrations, Ltd._] An army must be fed, or it cannot fight. This is what Napoleon meant when he said that an army marches on its stomach. The work of bringing food to an army or part of an army is entrusted to a very important branch of the service known as the Army Service Corps. Then there must be a Medical Corps, to look after the sick and wounded; a Flying Corps, for scouting purposes; and a Signalling Corps, to transmit messages from one part of the field to another. Signalling is done by "flag-wagging," by flashes of light sent from mirrors (heliographs[183]) or lamps, or by means of telegraphs, both wire and wireless, and by telephones. Our army is famous all over the world for its expertness in signalling. By means of relays of flag-waggers messages can be conveyed for fifty miles with great speed and certainty. [Illustration: Signalling by means of two flags. Most signalling is now done by means of one flag.] * * * * * On page 208 you will see a little picture which compares a man with an army. An army in the field is very like a man, as you will plainly see if you study the drawing. First, let us look at the man's brain. By means of it he thinks, makes his plans, and orders all the movements of his body. What is called the _staff_ of an army is the brain of the army. It plans how to outwit the enemy, thinks out ways and means, and controls the movements of all the troops. Now consider the man's eyes and ears. With these he obtains information as to what is going on around him. Without them he is at the mercy of those who are better provided than he. The eyes and ears of the army are the _Flying Corps_, the _motor cyclists_, and the _cavalry_. They discover the enemy's movements, and keep the staff well informed of his doings. When a man is boxing, he usually leads off with a blow at the head from his left arm. We may call his left arm the _artillery_, for with its artillery an army strikes hard and far. [Illustration: A comparison between a man and an army.] A man's feet enable his body to advance. We may call the _Engineers_, the _Army Service Corps_, and the _Royal Medical Corps_ the feet of the army. There now only remains the man's body, in which lies all his power. The body of an army is the mass of _Infantry_ which comprises its chief force. [Footnote 180: Born 1761, died 1842. He invented shrapnel in 1787, and it was first employed by the British in 1804. Some of our victories in the Peninsular War were largely won by means of it, and it played an important part in the battle of Waterloo. The Prussians first used it in 1864.] [Footnote 181: Millimetre. A millimetre is 1/1000 of a metre (3-1/3 ft.). Seventy-five millimetres is about three inches. This is the bore or _calibre_ of the gun.] [Footnote 182: Apparatus for signalling by flashing the sun's rays.] CHAPTER XX.