There were two sizes of tanks. The larger ones aimed to destroy the machine-gun emplacements, and they were fitted up with guns for firing a shell. The smaller tanks, armed with machine-guns, devoted themselves to fighting the infantry. British soldiers following in the wake of the bullet-proof tank were protected from the shots of the enemy and were ready to attack him with bayonets when the time was ripe. But the tanks also furnished an indirect protection for the troops. It was not necessary for the men to conceal themselves behind the big tractors. Naturally, every Hun who stood his ground and fought, directed all his fire upon the tanks, leaving the British infantry free to charge virtually unmolested. The success of the tank was most pronounced. In the meantime the French had been informed of the plans of their allies, and they set to work on a different design of tractor. It was not until six months later that their machines saw service. The French design differed from the British mainly in having the tractor belt confined to the wheels instead of running over the entire body of the tank. It was more blunt than the British and was provided at the forward end with a steel cutting-edge, which adapted it to break its way through wire entanglements. At each end there are two upward-turning skids which helped the tank to lift itself out of a hole. The larger machines carried a regular 75-millimeter (3-inch) field-gun, which is a very formidable weapon. They carried a crew of one officer and seven men. Life in a tank is far from pleasant. The heat and the noise of machinery and guns are terrific. Naturally, ventilation is poor and the fumes and gases that accumulate are most annoying, to say the least. Sometimes the men were overcome by them. But war is war, and such discomforts had to be endured. But the tank possessed one serious defect which the Germans were not slow to discover. Its armor was proof against machine-gun fire, but it could not ward off the shells of field-guns, and it was such a slow traveler that the enemy did not find it a very difficult task to hit it with a rapid-fire gun if the gunner could see his target. And so the Germans ordered up their guns to the front lines, where they could score direct hits. Only light guns were used for this purpose, especially those whose rifling was worn down by long service, because long range was not necessary for tank fighting. [Illustration: (C) Underwood & Underwood The Speedy British "Whippet" Tank that can travel at a speed of twelve miles per hour] [Illustration: (C) Underwood & Underwood The French High-Speed "Baby" Tank]