The failure of some of his solid-cast guns led Krupp, about 1865, to the adoption of the built-up principle. With few exceptions, the inner tube of a Krupp gun is forged out of a single ingot, and in every case without any weld. The ingot destined to form the tube has first to undergo a prolonged forging under the steam-hammers, by which the utmost condensation of its particles is effected. It is then rough-bored and turned, and subsequently carefully tempered in oil, whereby its elasticity and tensile strength are much increased. It is afterwards fine-bored and rifled, and its powder-chamber hollowed out. The latter has a somewhat larger diameter than the rest of the bore, this having been found an improvement. The grooves of the rifling are generally shallow, and they widen towards the breech, so that the leaden coat of the projectile is compressed gradually and with the least friction. The jacket and hoops of steel are forged and rolled, without weld, and after being turned and tempered, are heated and shrunk around the tube in their several positions, the greatest strength and thickness being of course given to the breech end, where the force of explosion exerts the utmost strain. The completed gun is mounted on its appropriate carriage, and having been thoroughly proved and tested and fitted with the proper sights, is ready for service. The testing range is at Meppen, where a level plain several miles in extent affords a suitable site for the purpose. For many years all guns of the Krupp manufacture have been on the breech-loading system, and he has devoted much time and ingenuity to perfecting the breech arrangements. The subject of recoil has also largely occupied his attention. In the larger Krupp guns the force of recoil is absorbed by two cylinders, filled with glycerine and fitted with pistons perforated at the edges. The pistons are driven by the shock of the recoil against the glycerine, which is forced through the perforations. In England a similar arrangement of cylinders, containing water as the resisting medium, has been found effective; and in America, petroleum is employed for the same purpose. The advantages of the use of glycerine are that in case of a leak it would escape too slowly to lose its effect at once, and it is also more elastic than water, and is less liable to become frozen.