The cylindrical Zeppelin balloon with approximately conical ends has already been shown (page 68). Those balloons in which the shape is maintained by internal pressure of air are usually _pisciform_, that is, fish-shaped. Studies have actually been made of the contour lines of various fishes and equivalent symmetrical forms derived, the outline of the balloon being formed by a pair of approximately parabolic curves. [Illustration: DIRIGIBLE OF DUPUY DE LOME (Man Power)] The first flight in a power driven balloon was made by Giffard in 1852. This balloon had an independent speed of about ten feet per second, but was without appliances for steering. A ballonetted balloon of 120,000 cubic feet capacity was directed by man power in 1872: eight men turned a screw thirty feet in diameter which gave a speed of about seven miles per hour. Electric motors and storage batteries were used for dirigible balloons in 1883-'84: in the latter year, Renard and Krebs built the first fish-shaped balloon. The first dirigible driven by an internal combustion motor was used by Santos-Dumont in 1901. [Illustration: TISSANDIER BROTHERS' DIRIGIBLE BALLOON (Electric Motor)] Dimensions The displacements of present dirigibles vary from 20,000 cubic feet (in the United States Signal Corps airship) up to 460,000 cubic feet (in the Zeppelin). The former balloon has a carrying capacity only about equivalent to that of a Wright biplane. While anchored or drifting balloons are usually spherical, all dirigibles are elongated, with a length of from four to eleven diameters. The Zeppelin represents an extreme elongation, the length being 450 feet and the diameter forty-two feet. At the other extreme, some of the English military dirigibles are thirty-one feet in diameter and only 112 feet long. Ballonet capacities may run up to one-fifth the gas volume. All present dirigibles have gasoline engines driving propellers from eight to twenty feet in diameter. The larger propellers are connected with the motors by gearing, and make from 250 to 700 turns per minute. The smaller propellers are direct connected and make about 1200 revolutions. Speeds are usually from fifteen to thirty miles per hour. [Illustration: THE BALDWIN Dirigible of the United States Signal Corps]