The "caterpillar" feature of the tank had its origin in the caterpillar belts or shoes which were first used on the great field guns and mortars--those tremendous weapons which shoot bombs and shells weighing tons and containing 500 or more pounds of guncotton or explosive which on contact is discharged, rending everything for yards around. These guns, as well as the smaller field guns, have had attached to them great shields of steel behind which the gunners stand, so that they are protected against the old-fashioned sharpshooters whose duty it was to pick off the gunners. The caterpillar or wheel belts on the big guns consist of flat blocks, or shoes, wider than the tires of the wheels. They are hinged and fastened together so as to form a great chain, and when placed on the wheels present broad surfaces to the ground and keep the gun carriages from sinking into the soft earth. With a set of these shoes a heavy gun can be drawn over soft and irregular ground, which would be almost impassable where the gun is mounted on wheels of ordinary width. Before these belts were devised it was necessary for every gun crew to carry a supply of beams, jackscrews and devices to be used in extricating the heavy guns when they got fast in the mud. Now every gun has these belts which can be put on or detached in a few minutes. Paradoxically, this is the day of the big gun's greatest effectiveness, and the day of its greatest limitations. The war has taught us more in two years about gunnery and the effect of various types of ordnance under varying conditions than could have been learned in twenty years of theoretical research--for actual experience proves where theoretical research merely gives ground on which to base an opinion. NATIONAL RESOURCES TO DISLODGE A MAN. One of the things that we have learned is that when man takes unto himself the humble pick and shovel and proceeds to dig a hole for himself in the ground, we can get him out of that hole only by drawing on the combined resources of a nation, by constructing one of the most complex and expensive instruments in the world, and with it hurling at man dug-in a projectile weighing a good part of a ton. The blunder, perhaps unavoidable, which stands out with equal emphasis among the preliminary preparations of all the nations engaged in the struggle was the underestimation of the artillery power required for the conduct of a successful military campaign under modern conditions of warfare. It was an underestimation so great that in the light of developments it will some day prove ridiculous.