Even under battle conditions the purity of the water was the first consideration, for impure water can work far more havoc with an army than enemy shell. In order to provide against this contingency, mobile laboratories for water-testing purposes moved in the van of the armies, and during the drives the Water-Supply troops were provided with poison-testing kits, for, warned by the experiences of the British in German Southwest Africa, where wells were systematically poisoned by the enemy, we took no chances. Sources of supply were, wherever possible, protected, it being considered almost as serious an offense for a soldier to contaminate a water-supply as for him to sleep on post. Where water was found to be polluted, the troops, no matter how thirsty, were under no circumstances permitted to use it until it had been filtered and sterilized. It is a curious fact that the chlorine used in gas-shell to kill Germans was used by the Water-Supply Service in minute quantities to kill an equally dangerous and far more insidious enemy—the microbic disease-carriers in the water. Special motor-trucks, equipped with pumping, filtering, sterilizing, and testing apparatus, time after time demonstrated that they were able to get into action and deliver pure water from a polluted supply _within thirty minutes_ after their arrival. In many cases the position of the troops and the nature of the terrain made it possible to deliver water only by hauling. This was done by means of trains of motorized water-tanks and by special tank-cars operating over the narrow-gauge railway systems, the tank trucks and cars being emptied into reservoirs built in strategic positions near the front. A common and quickly built reservoir consisted of a hole in the ground, a waterproof canvas lining, and a camouflaged cover. The lives of the tank-train truck-drivers were hard and exciting, for though the roads over which they had to pass in approaching the front were nearly always subjected to heavy shell-fire, there could be no let-up in supplying water for the troops on the firing-line. Most of the activities of the Water-Supply troops were between the locations of the light artillery and the heavy artillery, the men consequently working almost continuously within the areas under enemy bombardment. On one occasion, during the open warfare incident to the St. Mihiel offensive, the driver of a water-truck ventured so close to a German machine-gun nest that when he came back his tank was found to be better adapted for road-sprinkling than for water-transportation purposes.