In the _Zeppelin_, the rigid aluminum frame is braced every forty-five
feet by transverse diametral rods which make the cross-sections
resemble a bicycle wheel (page 68). This cross-section is not circular,
but sixteen-sided. The pressure is resisted by the framework itself,
the envelope being required to be impervious only. The seventeen
compartments are separated by partitions of sheet aluminum. There is a
system of complete longitudinal bracing between these partitions. Under
the main framework, the cars and machinery are carried by a truss about
six feet deep which runs the entire length. The cars are boat-shaped,
twenty feet long and six feet wide, three and one-half feet high,
enclosed in aluminum sheathing. These cars, placed about one hundred
feet from the ends, are for the operating force and machinery. The
third car, carrying passengers, is built into the keel.
[Illustration: WRECK OF THE "ZEPPELIN"]
In non-rigid balloons like the _Patrie_, the connecting frame must be
carefully attached to the envelope. In this particular machine, cloth
flaps were sewed to the bag, and nickel steel tubes then laced in
the flaps. With these tubes as a base, a light framework of tubes
and wires, covered with a laced-on waterproof cloth, was built up
for supporting the load. Braces ran between the various stabilizing
and controlling surfaces and the gas bag; these were for the most
part very fine wire cables. The weight of the car was concentrated
on about seventy feet of the total length of 200 feet. This accounts
for the deformation of the envelope shown in the illustration (page
77). The frame and car of this balloon were readily dismantled for
transportation.
In some of the English dirigibles the cars were suspended by network
passing over the top of the balloon.