[Illustration: HENRY GIFFARD'S DIRIGIBLE
(The first with steam power)]
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)]