Looking back over the development of the Zeppelins (Plate 15), one fails to find such remarkable and quick advance in any other medium of transportation. The history of engineering does not record in any other science progress comparable to that of the relatively new science of lighter-than-air as represented by the Zeppelins during the four years of war. Seventy Percent Speed Increase Their speed had increased from 46.6 to 87.5 miles per hour (75 to 130 kilometers per hour) approximately 70%. Their horsepower averaged 2,000. To carry useful loads of 44 tons their hydrogen capacity had been raised from 706,200 to 2,189,220 cubic feet (20,000 to 62,000 cubic meters). Other commercial ships were built embodying the improvements developed during the war. A description of them will be found in Chapter III. Refinement in Design This progress was made possible only by continuous experiments. Ideas and suggestions were adopted regardless of expense or chance of failure. In this way the Zeppelins had the advantage of every conceivable refinement in design. Their hulls, motor gondolas, in fact, all braces and wires were streamlined so as to offer the least air resistance. [PLATE 24: Zeppelin-Werke Staaken “Giant” All Metal Monoplane. Which carried eighteen passengers in a luxurious cabin at a speed of 145 miles per hour. Power plant consists of 4-260 horsepower Maybach Motors totaling more than 1000 H. P. Zeppelin-Dornier “Dragon Fly” All Metal Flying Boat, 1921. Carries pilot and two passengers with 60 horsepower motor at a speed of 80 miles per hour and a gasoline consumption of only four gallons per hour.] The rubberized cloth gas cells, or bags, used in 1914 had been discarded for others of light yet strong cotton cloth (and often silk), lined with goldbeater’s skin to make them hydrogen proof. Many of the experiments were as costly as they were painstaking but the Zeppelin engineers had learned early in their work that airships can not be built satisfactorily without long and arduous experiments to support each innovation. By continually striving to increase efficiency they secured simplified control systems and ships that handled more easily, hulls that were far more rigid yet lighter than their predecessors. Even the framework was lightened as by degrees it was made stronger. Many structural parts were standardized, facilitating production and repairs. One has an idea of the innumerable parts necessary in the skeleton of a Zeppelin when he learns that more than 250,000 small crossties are required in making the triangular shaped girders in the frame work of a 1,977,300 cubic foot (56,000 cubic meters) ship which crosstie is a masterpiece of construction, because of its ingenious shape and finish. Eighty-Eight Zeppelins During the War Few persons know that during the war alone Luftschiffbau-Zeppelin designed and built 88 airships at their four great construction plants, as follows: