For these reasons the ante-bellum regulations prescribed that machine-guns should work in pairs, so that one would always be ready to take up the fire, if the other should, for any reason, go out of action. This regulation is still adhered to, but only so far as circumstances permit. In cases of emergency, for instance, when an attack has to be repulsed, the simultaneous use of all the guns becomes necessary, especially against an enemy who possesses a superior number of similar guns. Each belligerent has captured many machine-guns and much ammunition from the enemy. Thus France has complete German batteries, and Germany possesses both French and English batteries. The principal structural difference between the German and French machine-guns consists in the mechanism for cooling off the guns; the Germans use for this purpose water circulation, and the French air circulation. To avoid serious burns resulting from contact with the barrels of the guns, the gunners wear gloves covered with very thick steel mail. We have seen photographs of American machine-gun batteries carried on motorcycles. The French no longer make use of this method, and although it may have proven excellent in Mexico, it is entirely impracticable on the French Front. The ground for a long distance behind the lines of defence has been so torn up and rendered impassable by prolonged bombardment that motor vehicles cannot get through. Horse-drawn vehicles can approach much nearer but at the entrance of the communication trenches even pack transport has to be abandoned, and from this point (in default of such recent devices as narrow-gauge hand-operated tracks, or pack-donkeys) all war material has to go forward to the advance lines on men’s backs. In case of an advance beyond the front, difficulties would be doubled, since the devastated ground behind the enemy’s lines would have to be traversed. Pack transport therefore is best suited for machine-gun batteries and their supplies. Where the number of horses or mules is insufficient, light vehicles, each drawn by a single animal, can be used; especially for the machine-gun sections which are to occupy more or less permanent positions. In order to counterbalance her losses, Germany has constantly increased the number of her machine-guns, using them as a defensive weapon to check the advance of the enemy, and to enable herself to cling to her positions with a small number of men. Germany does not hesitate to sacrifice machine-guns in order to gain time, and German machine-gunners were often found in their shelters chained to their guns, and so obliged to serve them until killed or released by the enemy. Machine-guns and grenades are certainly the most powerful arms against assaulting waves. Seldom will the reconnoitring airplanes detect all the numerous machine-gun shelters. Some of them always remain after a bombardment to show activity at the time of the infantry attack.