From the Battle of Crécy onward, the use of gunpowder spread rapidly throughout Europe, the Russians being the last to adopt it. Saltpetre, at first used in its natural state, began to be produced artificially, and then the manufacture of powder extended among the nations. During the French Revolution, according to Carlyle, the revolutionists were driven to such extremities for want of powder that they scraped old cellars seeking material for its manufacture. Many recent improvements have been made in the production of gunpowder, the most important resulting in the smokeless powder. Before the introduction of cannon using gunpowder as a propelling force, various machines were used in warfare for hurling missiles. Large stones and heavy darts or arrows were thrown by means of tightly twisted ropes, like the action of a bow, or through the aid of a lever and sling. Various names were applied to these weapons, the chief of which were the ballista and the catapult. The ballista hurled stones by means of a twisted cord or a lever; the catapult by darts or arrows could throw a projectile half a mile. Both machines were used by the Romans with great effect, in both defensive and offensive warfare. In destroying the wall of a besieged town, the Romans used a battering-ram. It consisted of a beam of wood with a mass of bronze or iron on the end resembling a ram's head. In its earliest form, the battering-ram was beaten against the wall by the soldiers; later it was suspended in a frame and made to swing with ropes. Another kind moved on rollers, the swinging movement being given to it also by means of ropes. The beam of the ram was from sixty to one hundred and twenty feet long, the head sometimes weighed more than a ton, and as many as a hundred men were necessary to swing it. For the protection of the soldiers using it, a wooden roof covered it, and the whole was mounted on wheels. Scarcely any wall could resist the continued blows of the battering-ram. The Romans were the most effective in the use of this engine, though they borrowed it from the Greeks. The first cannon were clumsy and comparatively inefficient. They were made of wooden bars held together with iron hoops, and they shot balls of stone. Cannon of bronze were next made, and in the latter part of the fifteenth century iron cannon came into use. The next improvement was the production of cannon of steel, and for some years past the best artillery has been made of this material. After stone balls ceased to be used, round balls of iron were utilized. These in time gave way to cylindrical projectiles of steel. Originally cannon were loaded at the muzzle, but in recent years breech-loading devices have been developed, so that now all of the best modern guns are loaded from the rear.