In the sixteenth century Europe was busy in adopting these and other changes. Cannon were made of all sizes and calibres, but were not arranged in battle with much precision. Case shot were invented in Germany but not brought into general use. Shells were invented by the Italians and fired from mortars, but their mode of construction was preserved in great secrecy. The early breech-loaders had been discarded, as it was not known how to make the breech gas-tight, and the explosions rendered the guns more dangerous to their users than to the enemy. In the seventeenth century Holland began to make useful mortar shells and hand grenades. Maurice and Henry Frederick of Nassau, and Gustave Adolphus, made many improvements in the sizes and construction of cannon. In 1674, Coehorn, an officer in the service of the Prince of Orange, invented the celebrated mortar which bears his name, and the use of which has continued to the present time. The Dutch also invented the howitzer, a short gun in which the projectiles could be introduced by hand. About the same time Comminges of France invented mortars which threw projectiles weighing 550 pounds. In this part of that century also great improvements were made under Louis XIV. Limbers, by which the front part of the gun carriage was made separable from the cannon part and provided with the ammunition chest; the prolonge, a cord and hook by which the gun part could be moved around by hand; and the elevating screw, by which the muzzle of the gun could be raised or depressed,--were invented. In the early part of the eighteenth century it was thought by artillerists in England that the longer the gun the farther it would carry. One, called “Queen Ann’s Pocket Piece” still preserved at Dover, is twenty-five feet long and carries a ball only twenty-five pounds in weight. It was only after repeated experiments that it was learned that the shorter guns carried the projectile the greatest distance. The greatest improvements in the eighteenth century were made by Gribeauval, the celebrated French artillerist, about 1765. He had guns made of such material and of such size as to adapt them to the different services to which they were to be put, as field, siege, garrison, and sea coast. He gave greater mobility to the system by introducing six-pound howitzers, and making gun carriages lighter; he introduced the system of fixed ammunition, separate compartments in the gun carriages for the projectiles, and the charges of powder in paper or cloth bags or cylinders; improved the construction of the elevating screw, adapted the tangent scale, formed the artillery into horse batteries, and devised new equipments and a new system of tactics.