The distance to which a shell can be hurled depends to a large extent upon the angle of the gun. If the gun is tilted up to an angle of 15 degrees, the shell will go only about half as far as if it were tilted up to 43½ degrees, which is the angle that will carry a shell to its greatest distance. If the long-range German gun was fired at that angle, the shell must have risen to a height of about twenty-four miles.
BEYOND THE EARTH'S ATMOSPHERE
Most of the air that surrounds our globe lies within four miles of the surface. Few airplanes can rise to a greater height than this, because the air is so thin that it gives no support to the wings of the machine. The greatest height to which a man has ever ascended is seven miles. A balloon once carried two men to such a height. One of them lost consciousness, and the other, who was nearly paralyzed, succeeded in pulling the safety-valve rope, with his teeth. That brought the balloon down, and their instruments showed that they had gone up thirty-six thousand feet. What the ocean of air contains above that elevation, we do not know, but judging by the way the atmosphere thins out as we rise from the surface of the earth, we reckon that nine tenths of the air lies within ten miles of the surface of the earth. At twenty-four miles, or the top of the curve described by the shell of the German long-range guns, there must be an almost complete vacuum.
If only we could accompany a shell on its course, we should find a strange condition of affairs. The higher we rose, the darker would the heavens become, until the sun would shine like a fiery ball in a black sky. All around, the stars would twinkle, and below would be the glare of light reflected from the earth's surface and its atmosphere, while the cold would be far more intense than anything suffered on earth. Up at that height, there would be nothing to indicate that the shell was moving—no rush of air against the ears. We should seem detached from earth and out in the endless reaches of space.
It seems absurd to think that a shell weighing close to a quarter of a ton could be retarded appreciably by mere air. But when we realize that the shell left the gun at the rate of over half a mile a second—traveling about thirty times faster than an express-train—we know that the air-pressure mounts up to a respectable figure. The pressure is the same whether a shell is moving through the air or the air is blowing against the shell. When the wind blows at the rate of 100 to 120 miles per hour, it is strong enough to lift houses off their foundations, to wrench trees out of the ground, to pick up cattle and carry them sailing through the air. Imagine what it would do if its velocity were increased to 1,800 miles per hour. That is what the shell of a big gun has to contend with. As most of the air lies near the earth, the shell of long-range guns meet with less and less resistance the higher they rise, until they get up into such thin air that there is virtually no obstruction. The main trouble is to pierce the blanket of heavy air that lies near the earth.
WAYS OF INCREASING THE RANGE
The big 16-inch guns that protect our coasts fire a shell that weighs 2,400 pounds. Nine hundred pounds of smokeless powder is used to propel the shell, which leaves the muzzle of the gun with a speed of 2,600 feet per second. Now, the larger the diameter of the shell, the greater will be its speed at the muzzle of the gun, because there will be a greater surface for the powder gases to press against. On the other hand, the larger the shell, the more will it be retarded by the air, because there will be a larger surface for the air to press against. It has been proposed by some ordnance experts that a shell might be provided with a disk at each end, which would make it fit a gun of larger caliber. A 10-inch shell, for instance, could then be fired from a 16-inch gun. Being lighter than the 16-inch shell, it would leave the muzzle of the gun at a higher speed. The disks could be so arranged that as soon as the shell left the gun they would be thrown off, and then the 10-inch shell, although starting with a higher velocity than a 16-inch shell, would offer less resistance to the air. In that way it could be made to cover a much greater range. By the way, the shell of the German long-range gun was of but 8.2-inch caliber.
Another way of increasing the range is to lengthen the gun. Right here we must become acquainted with the word "caliber." Caliber means the diameter of the shell. A 16-inch gun, for instance, fires a shell of 16-inch caliber; but when we read that the gun is a 40-or 50-caliber gun, it means that the length of the gun is forty or fifty times the diameter of the shell. Our biggest coast-defense guns are 50-caliber 16-inch guns, which means that they are fifty times 16 inches long, or 66-2/3 feet in length. When a gun is as long as that, care has to be taken to prevent it from sagging at the muzzle of its own weight. These guns actually do sag a little, and when the shell is fired through the long barrel it straightens up the gun, making the muzzle "whip" upward, just as a drooping garden hose does when the water shoots through it.
