When we consider the problem of flying, the first obstacle we encounter is the attraction which the earth has for us and for all other objects on its surface. This we call weight, and we are accustomed to measure it in pounds. Let us take, for example, a man whose body is attracted by the earth with a force, or weight, of 150 pounds. To enable him to rise into the air, means must be contrived not only to counteract his weight, but to lift him--a force a little greater than 150 pounds must be exerted. We may attach to him a bag filled with some gas (as hydrogen) for which the earth has less attraction than it has for air, and which the air will push out of the way and upward until a place above the earth is reached where the attraction of air and gas is equal. A bag of this gas large enough to be pushed upward with a force equal to the weight of the man, plus the weight of the bag, and a little more for lifting power, will carry the man up. This is the principle of the ordinary balloon. Rising in the air is not flying. It is a necessary step, but real flying is to travel from place to place through the air. To accomplish this, some mechanism, or machinery, is needed to propel the man after he has been lifted into the air. Such machinery will have weight, and the bag of gas must be enlarged to counterbalance it. When this is done, the man and the bag of gas may move through the air, and with suitable rudders he may direct his course. This combination of the lifting bag of gas and the propelling machinery constitutes the dirigible balloon, or airship. [Illustration: Degen’s apparatus to lift the man and his flying mechanism with the aid of a gas-balloon. See Chapter IV.] The airship is affected equally with the balloon by prevailing winds. A breeze blowing 10 miles an hour will carry a balloon at nearly that speed in the direction in which it is blowing. Suppose the aeronaut wishes to sail in the opposite direction? If the machinery will propel his airship only 10 miles an hour in a calm, it will virtually stand still in the 10-mile breeze. If the machinery will propel his airship 20 miles an hour in a calm, the ship will travel 10 miles an hour--as related to places on the earth’s surface--against the wind. But so far as the air is concerned, his speed through it is 20 miles an hour, and each increase of speed meets increased resistance from the air, and requires a greater expenditure of power to overcome. To reduce this resistance to the least possible amount, the globular form of the early balloon has been variously modified. Most modern airships have a “cigar-shaped” gas bag, so called because the ends look like the tip of a cigar. As far as is known, this is the balloon that offers less resistance to the air than any other.