Later, machines were developed after the fashion of a crossbow mounted upon a small wooden carriage which usually was a hollowed trough open on top and upon which a heavy stone was laid. The thong of the crossbow was drawn by a powerful screw operated by man power, and the crossbow arrangement when released would throw a stone weighing many pounds quite a distance over the walls of a besieged city or from such walls into the camps and ranks of the besiegers. This again was an attempt by mechanical means to develop and lengthen the stroke of the arm and the weight of the projectile. With the development of explosives, which was much earlier than many people suppose, there came a still greater range and weight of projectile thrown, although the first guns were composed of staves of wood fitted together and hooped up like a long, slender barrel, wound with wet rawhide in many folds, which, when dried, exerted a compressive force upon the staves of the barrel exactly as do the steel hoops of barrels used in ordinary commercial life to-day. This, the first gun, sufficed for a long while until the age of iron came. And then the same principle of gun construction was followed as is seen in that historical gun, the "Mons Meg," in the castle at Edinburgh. The barrel of that gun is made of square bars of iron, placed lengthwise, and similar bars of iron were wrapped hot around the staves to confine them in place and to give more resisting power than was possible with the wooden staves and the rawhide hooping. Thus, all during the age of iron, gun development went steadily forward. Every military power was always striving by the aid of its best engineers, designers, and manufacturers to get a stronger gun, either with or without a heavier projectile, but in every case striving for greater power. As a special development we find in March, 1918, the now famous long-range gun of the Germans, which was at that time trained upon Paris, where it successfully delivered a shell approximately 9 inches in diameter, punctually every 20 minutes for a good part of each day until the gun was worn out. This occurred after a comparatively small number of shots, probably not more than 75 in all. The rapid wearing out was due to the immense demands of the long range upon the material of the gun. The Germans in the shelling of Paris used three of these long-range weapons and 183 shells are known to have fallen in the city.