In fact, artillery is, of all services, the one most dependent on co-operation. It is always a joint job, but the joining must be done among many factors. Its effectiveness depends first upon the precision of the mathematical calculation which goes before the pull of the lanyard. This calculation is complicated by the variety of types of guns and shells, and, in the case of howitzers, by the variable behavior of charges of different size and power. But these are things that can be learned with patience, and require knowledge rather than inspiration. It is when the air service enters that inspiration enters with it. Observation must be accurate, in spite of weather, visibility, enemy camouflage, and everything else. More than that, the observer in the plane must keep himself safe--often a matter of sheer genius. The map-maker must do his part, so that targets not so elusive as field-guns and motor-emplacements can be found without much help from the air. Finally, the artillery depends, even more than any other branch of the service, on the rapidity with which its wants can be filled from the rear. The mobility of the big pieces, and their constant connections with ammunition-stores, are matters depending directly on the training of the artillerymen. These, then, were the things in which the Americans were either tested or trained. Their mathematics were A1, as has been noted, and their familiarity with existing models of big guns sufficient to enable them to pick up the new types without long effort. They had a few weeks of heavy going with pad and pencil, then they were led to the giant stores of French ammunition--more than any of them had ever seen before--and told to open fire. One dramatic touch exacted by the French instructors was that the guns should be pointed toward Germany, no matter how impotent their distance made them. Long lanes, up to 12,000 metres, were told off for the ranges. The training was intensive, because at that time there was a half-plan to put the artillery first into the battle-line. In any case it is easier to make time on secondary problems than on primary. Throughout September, while the artillerymen grew in numbers as well as proficiency, the mastering of gun types was perfected, and the theory of aim was worked out on paper. Late in the month the French added more guns, chief among them being a monster mounted on railway-trucks whose projectile weighed 1,800 pounds. The artillerymen named her "Mosquito," "because she had a sting," although she had served for 300 charges at Verdun. It was not long before every type of gun in the French Army, and many from the British, were lined up in the artillery camp, being expertly pulled apart and reassembled.