The material for the envelope and ballonets should be light, strong,
unaffected by moisture or the atmosphere, non-cracking, non-stretching,
and not acted upon by variations in temperature. The same
specifications apply to the material for the wings of an aeroplane. In
addition, for use in dirigible balloons, fabrics must be impermeable,
resistent to chemical action of the gas, and not subject to spontaneous
combustion. The materials used are vulcanized silk, gold beater's
skin, Japanese silk and rubber, and cotton and rubber compositions.
In many French balloons, a middle layer of rubber has layers of
cotton on each side, the whole thickness being the two hundred and
fiftieth part of an inch. In the _Patrie_, this was supplemented by
an outside non-heat-absorbent layer of lead chromate and an inside
coating of rubber, all rubber being vulcanized. The inner rubber layer
was intended to protect the fabric against the destructive action of
impurities in the gas.
[Illustration: THE "PATRIE." DESTROYED BY A STORM]
Fabrics are obtainable in various colors, painted, varnished, or wholly
uncoated. The rubber and cotton mixtures are regularly woven in France
and Germany for aeroplanes and balloons. The cars and machinery are
frequently shielded by a fabricated wall. Weights of envelope materials
range from one twenty-third to one-fourteenth pound per square foot,
and breaking stresses from twenty-eight to one hundred and thirty
pounds. Pressures (net) in the main envelope are from three-fifths to
one and a quarter _ounces_ per square inch, those in the ballonets
being somewhat less. The _Patrie_ of 1907 had an envelope guaranteed
not to allow the leakage of more than half a cubic inch of hydrogen per
square foot of surface per twenty-four hours.
[Illustration: MANUFACTURING THE ENVELOPE OF A BALLOON]
[Illustration: INSPECTING THE ENVELOPE OF ANDRÉE'S BALLOON "L'OERNEN"]
The best method of cutting the fabric is to arrange for building up the
envelope by a series of strips about the circumference, the seams being
at the bottom. The two warps of the cloth should cross at an angle so
as to localize a rip or tear. Bands of cloth are usually pasted over
the seams, inside and out, with a rubber solution; this is to prevent
leakage at the stitches.