Starting with the artillery, there was first in order of size the baby two-man cannon of 37 millimeters (about an inch and a half) in the diameter of its bore--a European development new to our experience, so light that it could be handled by foot troops in the field, used for annihilating the enemy's machine-gun emplacements. Then the mobile field guns--the famous 75's, the equivalent in size of our former 3-inch gun, the 155-millimeter howitzer, the French 155 millimeter G. P. F. (Grand Puissance Filloux) gun of glorious record in the war, and its American prototypes, the 4.7-inch, 5-inch, and 6-inch guns--all of these employed to shell crossroads and harass the enemy's middle area. Beyond these were the 8-inch and 9.2-inch howitzers and the terrific 240-millimeter howitzer, for throwing great weights of destruction high in air to descend with a plunge upon the enemy's strongest defenses. Then there were the 8-inch, 10-inch, 12-inch, and 14-inch guns on railway mounts, for pounding the depots and dumps in the enemy's back areas. These weapons were so tremendous in weight when mounted as to require from 16 to 24 axles on the car to distribute the load and the recoil of firing within the limits of the strength of standard heavy railway track. All of these guns had to be produced in great numbers, if the future requirements of the American forces were to be met, produced by the thousands in the cases of the smaller ones and by the hundreds and scores in the cases of the larger. These weapons would be ineffective without adequate supplies of ammunition. In the case of the mobile held guns this meant a requirement of millions of shell or shrapnel for the incessant bombardments and the concentrated barrages which characterized the great war. The entire weight of projectiles fired in such an historic engagement as Gettysburg would supply the artillery only for a few minutes in such intensive bombardments as sowed the soil of Flanders with steel. The artillery demanded an immense amount of heavy equipment--limbers, caissons, auto ammunition trucks, and tractors to drag the heavy and middle-heavy artillery. Some of them were fitted with self-propelled caterpillar mounts which could climb a 40° grade or make as high as 12 miles an hour on level ground. These, the adaptations to warfare of peaceful farm and construction machine traction, for the first time rendered the greater guns exceedingly mobile, enabling them to go into action instantly upon arrival and to depart to safety just as soon as their mission was accomplished.