Among the largest battleships built for the United States navy are the _Delaware_ and the _North Dakota_. Each of these battleships is five hundred and ten feet long, a little more than eighty-five feet wide, sinks to the depth of nearly twenty-seven feet in the water, and travels at the rate of twenty-one knots per hour. Each vessel weighs twenty thousand tons, and is armed with ten great guns a foot in diameter at the mouth. The _North Dakota_ required 4688 tons of steel armor at a cost of more than four hundred dollars per ton. Each of its great twelve-inch guns cost nearly $110,000, weighs fifty-two tons, and hurls a projectile weighing 850 pounds a distance of twelve miles. Three hundred and eighty-five pounds of powder are consumed at a single discharge. At a distance of more than a mile and a half the projectiles of the _North Dakota_ will penetrate steel armor to a depth of nearly twenty inches. When these projectiles leave the guns, they fly through the air at the rate of 2,800 feet in a second. When one hundred shots have been fired from one of these guns, it is worn so that it will be useless until repaired. The cost of a single discharge from one of these guns is about $350. Sub-marine navigation has always been attended by the most woeful catastrophes, but in spite of numerous accidents the development of the submarine boat has progressed uninterruptedly. Each new model presents new preventive devices. Flasks of oxylithic powder are carried for purifying the air in the water-tight compartments in which the crews live while the boat is below the surface of the water. There is also a special apparatus for signalling other vessels or the shore, in case of danger. In 1904 three vessels, designated X, Y, and Z, were completed, which could achieve submersion in the short space of two minutes. The boats were armed with six torpedoes each. France owns the largest fleet of under-water warships in the world. England stands next, and the United States government is third. CHAPTER VIII ASTRONOMICAL DISCOVERIES AND INVENTIONS "When I consider thy heavens, the work of thy fingers, the moon and the stars, which thou hast ordained, what is man, that thou art mindful of him?" The Hebrew psalmist feels the insignificance of man compared with the infinitude of the heavens. Victor Hugo expresses the opposite thought: "There is one spectacle grander than the sea--that is the sky; there is one spectacle grander than the sky--that is the interior of the soul."