At first the only means of combating the machine-guns seemed to be to destroy them with shell-fire; but they were carefully concealed, and it was difficult to search them out. Only by long-continued bombardment was it possible to destroy them and tear away the barbed wire sufficiently to permit of a charge. Before an enemy position was stormed it was subjected to the fire of thousands of guns of all calibers for hours and even days. But this resulted in notifying the enemy that a charge was ere long to be attempted at a certain place, and he could assemble his reserves for a counter-attack. Furthermore, the Germans learned to conceal their machine-guns in dugouts twenty or thirty feet underground, where they were safe from the fire of the big guns, and then, when the fire let up, the weapons would be dragged up to the surface in time to mow down the approaching infantry. It was very clear that something would have to be done to combat the machine-gun. If the necessary armor was too heavy for the men to carry, it must carry itself. Armored automobiles were of no service at all, because they could not possibly travel over the shell-pitted ground of No Man's Land. The Russians tried a big steel shield mounted on wheels, which a squad of soldiers would push ahead of them, but their plan failed because the wheels would get stuck in shell-holes. A one-man shield on wheels was tried by the British. Under its shelter a man could steal up to the barbed wire and cut it and even crawl up to a machine-gun emplacement and destroy it with a hand-grenade. But this did not prove very successful, either, because the wheels did not take kindly to the rough ground of the battle-field. * * * * * And here is where we come back to Mr. Holt's mechanical elephants. Just before the great war broke out, Belgium--poor unsuspecting Belgium--was holding an agricultural exhibit. An American tractor was on exhibition. It was the one developed by Mr. Holt, and its remarkable performances gained for it a reputation that spread far and wide. Colonel E. D. Swinton of the British Army heard of the peculiar machine, and immediately realized the advantages of an armored tractor for battle over torn ground. But in the first few months of the war that ensued, this idea was forgotten, until the effectiveness of the machine-gun and the necessity for overcoming it recalled the matter to his mind. At his suggestion a caterpillar tractor was procured, and the military engineers set themselves to the task of designing an armored body to ride on the caterpillar-tractor belts. Of course the machine had to be entirely re-designed. The tractor was built for hauling loads, and not to climb out of deep shell-holes; but by running the traction-belts over the entire body of the car, and running the forward part of the tractor up at a sharp angle the engineers overcame that difficulty.