The former is illustrated in our 3” and 75-mm field guns, the latter by the French 75-mm gun and 155-mm howitzer. Their purpose, of course, is to return the gun “into battery” after it has recoiled on the carriage. The recoil mechanism is a study in itself, of which there are two schools—the advocates of the spring and of the air recuperators. Great Britain, Germany and the United States have been the advocates of spring recuperation and France of air recuperation. Great Britain and the United States were of the spring school, undoubtedly, because of the lack of a satisfactory air recuperating system; which is rather strikingly proven by the fact that both countries have adopted air recuperation since they have procured or developed satisfactory types. Both schools have grounds for their position, however. The spring school has in its favor simplicity of design and manufacture and ease of replacement, which can be done in the field. On the other hand, spring recoils have many breakages and greater weight combined with a high replacement of weakened springs, the life varying from 3000 to 8000 rounds. The air school has in its favor a high order of efficiency—smoother action, general all around efficiency and less weight. But the air recuperator is difficult to manufacture, costly, and when damaged must go to the rear to be repaired; which, however, it seldom needs. In mounting the spring recuperator, the most recent practice has separated the recoil mechanism from the springs in order to distribute the piston rod pull, thus preventing whip and allowing easy access to the various parts for replacement, refilling and repairs. In order to lower the center of gravity, the gun is slung under the recoil cylinder with the two spring recuperators below and on either side. Air recuperators are invariably located below the gun for protection and because of their large size and shape, which adapt them for attaching the elevating mechanism. [Illustration: BATTERY OR FIRING POSITION IN-BATTERY OR FIRING POSITION HYDRO-SPRING RECOIL SYSTEM HYDRO-PNEUMATIC RECOIL SYSTEM WITH FLUID IN DIRECT CONTACT WITH THE AIR] In either system, the recoil is taken up by means of oil or glycerine and water passing through an orifice created by a slotted piston passing over ribs of varying height, or through a valve on the pressure side of the piston, or by a solid or perforated piston passing through a perforated intermediate cylinder. The latter type is particularly adapted to variable recoil guns as the intermediate cylinder can be rotated, thus throwing varying orifices into position for the flow of oil. Counter recoil is accomplished by the springs or by the air pressure in the hydro-pneumatic system, in which the air pressure is sufficient to hold the gun in battery at all elevations and is built up at recoil.