on every square inch of the wall of the room, while in the other case its pressure would be almost infinitesimal. But 80 pounds of air in a room of a thousand cubic feet would exert the same pressure as the atmosphere, or 15 pounds on every square inch. And when we say that a thousand cubic feet of hydrogen weighs only a little over 5 pounds, we are talking about hydrogen at the same pressure as the atmosphere. Since the hydrogen is sixteen times lighter than air, naturally it will float in the air, just as a piece of wood will float in water because it is lighter than the same volume of water. If we surrounded the thousand cubic feet of hydrogen with a bag so that the gas will not diffuse into the air and mix with it, we shall have a balloon which would float in air provided the bag and the hydrogen it contains do not weigh more than eighty pounds. As we rise from the surface of the earth, the air becomes less and less dense, or, in other words, it becomes lighter, and the balloon will keep on rising through the atmosphere until it reaches a point at which its weight, gas-bag and all, is exactly the same as that of an equal volume of air. But there are many conditions that affect the height to which the balloon will ascend. The higher we rise, the colder it is apt to become, and cold has a tendency to compress the hydrogen, collapsing the balloon and making it relatively heavier. When the sun beats upon a balloon, it heats the hydrogen, expanding it and making it relatively lighter, and if there is no room for this expansion to take place in the bag, the bag will burst. For this reason, a big safety-valve must be provided and the ordinary round balloon is open at the bottom so that the hydrogen can escape when it expands too much and the balloonist carries ballast in the form of sand which he can throw over to lighten the balloon when the gas is contracted by a sudden draft of cold air. Although a round balloon carries no engine and no propeller, it can be guided through the air to some degree. When an aëronaut wishes to go in any particular direction, he sends up his balloon by throwing out ballast or lowers it by letting out a certain amount of gas, until he reaches a level at which he finds a breeze blowing in the desired direction. Such was the airship of Civil War times, but for military purposes it was not advisable to use free balloons, because of the difficulty of controlling them. They were too liable to fall into the hands of the enemy. All that was needed was a high observation post from which the enemy could be watched, and from which observations could be reported by telegraph. The balloon was not looked upon as a fighting-machine. ZEPPELIN'S FAILURES AND SUCCESSES