Now that airplanes have been invented, the disappearing-mount has lost much of its usefulness. Big guns have to be hidden from above. They are usually located behind a hill, five or six miles back of the trenches, where the enemy cannot see them from the ground, and they are carefully hidden under trees or a canopy of foliage or are disguised with paint. The huge guns recently built to defend our coasts are intended to fire a shell that will pierce the heavy armor of a modern dreadnought. The shell is arranged to explode after it has penetrated the armor, and the penetrating-power is a very important matter. About thirty years ago the British built three battle-ships, each fitted with two guns of 16-1/4-inch caliber and 30-caliber length. In order to test the penetrating-power of this gun a target was built, consisting first of twenty inches of steel armor and eight inches of wrought-iron; this was backed by twenty feet of oak, five feet of granite, eleven feet of concrete, and six feet of brick. When the shell struck this target it passed through the steel, the iron, the oak, the granite, and the concrete, and did not stop until it had penetrated three feet of the brick. We have not subjected our 16-inch gun to such a test, but we know that it would go through two such targets and still have plenty of energy left. Incidentally, it costs us $1,680 each time the big gun is fired. THE FAMOUS FORTY-TWO-CENTIMETER GUN One of the early surprises of the war was the huge gun used by the Germans to destroy the powerful Belgian forts. Properly speaking, this was not a gun, but a howitzer; and right here we must learn the difference between mortars, howitzers, and guns. What we usually mean by "gun" is a piece of long caliber which is designed to hurl its shell with a flat trajectory. But long ago it was found advantageous to throw a projectile not at but upon a fortification, and for this purpose short pieces of large bore were built. These would fire at a high angle, so that the projectile would fall almost vertically on the target.