As the airship is designed to force its way through the air, instead of floating placidly in it, it is evident that it must have a certain tenseness of outline in order to retain its shape, and resist being doubled up by the resistance it encounters. It is estimated that the average velocity of the wind at the elevation at which the airship sails is 18 miles per hour. If the speed of the ship is to be 20 miles per hour, as related to stations on the ground, and if it is obliged to sail against the wind, it is plain that the wind pressure which it is compelled to meet is 38 miles per hour--a gale of no mean proportions. When the large expanse of the great gas-bags is taken into consideration, it is evident that ordinary balloon construction is not sufficient. Attempts have been made to meet the outside pressure from the wind and air-resistance by producing mechanically a counter-pressure from the inside. Air-bags are placed inside the cavity of the gas-bag, usually one near each end of the airship, and these are inflated by pumping air into them under pressure. In this way an outward pressure of as much as 7 lbs. to the square foot may be produced, equivalent to the resistance of air at a speed (either of the wind, or of the airship, or of both combined) of 48 miles per hour. It is evident, however, that the pressure upon the front end of an airship making headway against a strong wind will be much greater than the pressure at the rear end, or even than that amidships. It was this uneven pressure upon the outside of the gas-bag that doubled up the first two airships of Santos-Dumont, and led him to increase the proportional girth at the amidship section in his later dirigibles. The great difficulty of adjusting these varying pressures warrants the adherence of Count von Zeppelin to his design with the rigid structure and metallic sheathing. The loss of the second Zeppelin airship so discouraged its designer that he decided to withdraw from further aeronautical work. But the German Government prevailed on him to continue, and by October, 1906, he had the Zeppelin III in the air. This airship was larger than Zeppelin II in both length and diameter, and held 135,000 cubic feet more of gas. The motive power was supplied by two gasoline motors, each of 85 horse-power. The gas envelope had 16 sides, instead of 24, as in the earlier ship. At its trial the Zeppelin III proved highly successful. It made a trip of 69 miles, with 11 passengers, in 2¼ hours--a speed of about 30 miles an hour. [Illustration: The Zeppelin III backing out of the floating shed at Friedrichshafen. The illustration shows the added fin at the top, the rudders, dipping planes, and balancing planes.]