The seaplane must swoop sufficiently close to the water to release the torpedo and let it drop without damage. And this must be done from a sufficient distance to safeguard the seaplane from the vessel's guns. The superior speed and mobility of the seaplane gives it a great advantage over the ship attacked. Another of the weapons or instruments of warfare devised largely for use in destroying the evil submarine is the "blimp." This is nothing more nor less than a small dirigible balloon, hundreds of which the United States government started to build when it entered the war. The blimp is an aerial sea-scout. Its principal employment is for observation. It is a watcher of enemy movements on the water. But it is also serviceable for attack, and especially for assailing submarines. The British used blimps for the latter purpose, and to great advantage. The dirigible sausage-balloon, when a submarine is descried, can hover over it (as an aeroplane cannot), remaining as nearly stationary as may be desired, and waiting for an opportunity to drop a bomb with accurate aim. If the submarine be under water, and its presence betrayed by the peculiar surface-ripple that marks its wake, a bomb with a delay-action fuse can be dropped upon it, the projectile not exploding until it reaches a depth of fifty feet or so. In case the first bomb does not score a hit, there are others to follow, with better luck perhaps. THE IMPORTANCE OF THE "BLIMP." Thus, it will be seen that the blimp is an important auxiliary of the flying-machine in the pursuit of the submarines. Both together, in this exciting sport, supplement the swift power-boats called "submarine-chasers." For some time the Navy Department has trained enlisted men and officers for this work, chiefly at a Gulf port, where a school--it is no war secret--of aviation and ballooning has been maintained. Six officers and 40 men are required for each coast station. The Navy Department adopted for the blimp a standardized pattern, with definite published specifications, in accordance with which contractors turned them out in numbers. It is a sausage-shaped balloon 160 feet long, with a great diameter of 31-1/2 feet, and containing, when inflated, 77,000 cubic feet of hydrogen gas. The fabric of the "envelope"--that is to say, of the gas-bag--is coated both outside and inside with rubber. It is required that the balloon shall not lose more than 1 per cent of its gas-content in 24 hours. When inflated it must be able to carry (including its own weight) a total of 5275 pounds. If the "Zeppelin" be excepted, the blimp is the most highly-developed and scientific heavier-than-air flying machine ever devised. It has a cruising speed of 35 miles an hour, but at a pinch can travel ten miles an hour faster. At the "cruising" rate, it carries enough gasoline to keep going for sixteen hours; at 45 miles, its load of "petrol" will suffice for ten hours.