BALLOONS: HOW TO MAKE.
The fabrics used--Preliminary varnishing--Varnishes--Rubberized
fabrics--Pegamoid--Weight of varnish--Latitudes of the
balloon--Calculating gores--Laying out patterns and
cutting--Sewing--Varnishing--Drying--Oiling--The neck--The
valve--The net--The basket.
The making of a balloon is almost always placed in the hands of a
professional balloon-maker. But as the use of balloons increases, and
their owners multiply, it is becoming a matter of importance that the
most improved methods of making them should be known to the intending
purchaser, as well as to the amateur who wishes to construct his own
balloon.
The fabric of which the gas envelope is made may be either silk,
cotton (percale), or linen. It should be of a tight, diagonal weave,
of uniform and strong threads in both warp and woof, unbleached, and
without dressing, or finish. If it is colored, care should be exercised
that the dye is one that will not affect injuriously the strength or
texture of the fabric. Lightness in weight, and great strength (as
tested by tearing), are the essentials.
The finest German percale weighs about 2½ ounces per square yard;
Russian percale, 3⅓ ounces, and French percale, 3¾ounces, per square
yard. The white silk used in Russian military balloons weighs about
the same as German percale, but is very much stronger. Pongee silk is
a trifle heavier. The silk used for sounding balloons is much lighter,
weighing only a little over one ounce to the square yard.
Goldbeater’s skin and rubber have been used to some extent, but the
great cost of the former places it in reach only of governmental
departments, and the latter is of use only in small balloons for
scientific work--up to about 175 cubic feet capacity.
The fabric is first to be varnished, to fill up the pores and render
it gas-tight. For this purpose a thin linseed-oil varnish has been
commonly used. To 100 parts of pure linseed-oil are added 4 parts of
litharge and 1 part of umber, and the mixture is heated to about 350°
Fahr., for six or seven hours, and stirred constantly. After standing a
few days the clear part is drawn off for use. For the thicker varnish
used on later coats, the heat should be raised to 450° and kept at
about that temperature until it becomes thick. This operation is
attended with much danger of the oil taking fire, and should be done
only by an experienced varnish-maker.
The only advantages of the linseed-oil varnish are its ease of
application, and its cheapness. Its drawbacks are stickiness--requiring
continual examination of the balloon envelope, especially when the
deflated bag is stored away--its liability to spontaneous combustion,
particularly when the varnish is new, and its very slow drying
qualities, requiring a long wait between the coats.
Another varnish made by dissolving rubber in benzine, has been
largely used. It requires vulcanizing after application. It is
never sticky, and is always soft and pliable. However, the rubber
is liable to decomposition from the action of the violet ray of
light, and a balloon so varnished requires the protection of an outer
yellow covering--either of paint, or an additional yellow fabric.
Unfortunately, a single sheet of rubberized material is not gas-tight,
and it is necessary to make the envelope of two, or even three, layers
of the fabric, thus adding much to the weight.
The great gas-bags of the Zeppelin airships are varnished with
“Pegamoid,” a patent preparation the constituents of which are not
known. Its use by Count Zeppelin is the highest recommendation possible.
The weight of the varnish adds largely to the weight of the envelope.
French pongee silk after receiving its five coats of linseed-oil
varnish, weighs 8 ounces per square yard. A double bag of percale with
a layer of vulcanized rubber between, and a coating of rubber on the
inside, and painted yellow on the outside, will weigh 11 ounces per
square yard. Pegamoid material, which comes ready prepared, weighs but
about 4 ounces per square yard, but is much more costly.
In cutting out the gores of the envelope it is possible to waste fully
⅓ of the material unless the work is skilfully planned. Taking the
width of the chosen material as a basis, we must first deduct from ¾
of an inch to 1½ inches, in proportion to the size of the proposed
balloon, for a broad seam and the overlapping necessary. Dividing the
circumference at the largest diameter--the “equator” of the balloon--by
the remaining width of the fabric gives the number of gores required.
To obtain the breadth of each gore at the different “latitudes”
(supposing the globe of the balloon to be divided by parallels
similar to those of the earth) the following table is to be used; 0°
representing the equator, and 90° the apex of the balloon. The breadth
of the gore in inches at any latitude is the product of the decimal
opposite that latitude in the table by the original width of the fabric
in inches, thus allowing for seams.
[Illustration:
Finsterwalder’s method of cutting material for a spherical
balloon, by which over one-fourth of the material, usually
wasted in the common method, may be saved. It has the further
advantage of saving more than half of the usual sewing. The
balloon is considered as a spherical hexahedron (a six-surfaced
figure similar to a cube, but with curved sides and edges). The
circumference of the sphere divided by the width of the material
gives the unit of measurement. The dimensions of the imagined
hexahedron may then be determined from the calculated surface
and the cutting proceed according to the illustration above,
which shows five breadths to each of the six curved sides. The
illustration shows the seams of the balloon made after the
Finsterwalder method, when looking down upon it from above.]
TABLE FOR CALCULATING SHAPE OF GORES FOR SPHERICAL BALLOONS
0° 1.000
3° 0.998
6° 0.994
9° 0.988
12° 0.978
15° 0.966
18° 0.951
21° 0.934
24° 0.913
27° 0.891
30° 0.866
33° 0.839
36° 0.809
39° 0.777
42° 0.743
45° 0.707
48° 0.669
51° 0.629
54° 0.588
57° 0.544
60° 0.500
63° 0.454
66° 0.407
69° 0.358
72° 0.309
75° 0.259
78° 0.208
81° 0.156
84° 0.104
87° 0.052⅓
In practice, the shape of the gore is calculated by the above table,
and plotted out on a heavy pasteboard, generally in two sections for
convenience in handling. The board is cut to the plotted shape and used
as the pattern for every gore. In large establishments all the gores
are cut at once by a machine.
The raw edges are hemmed, and folded into one another to give a flat
seam, and are then sewn together “through and through,” in twos and
threes: afterward these sections are sewn together. Puckering must be
scrupulously avoided. In the case of rubberized material, the thread
holes should be smeared with rubber solution, and narrow strips of the
fabric cemented over the seams with the same substance.
Varnishing is the next process, the gores being treated in turn.
Half of the envelope is varnished first, and allowed to dry in a
well-ventilated place out of reach of the sun’s rays. The other half is
varnished when the first is dry. A framework which holds half of the
balloon in the shape of a bell is usually employed. In case of haste,
the balloon may be blown up with air, but this must be constantly
renewed to be of any service.
The first step in varnishing is to get one side (the outer, or the
inner) coated with a varnish thin enough to penetrate the material:
then turn the envelope the other side out and give that a coat of the
thin varnish. Next, after all is thoroughly dry, give the outer side
a coat of thick varnish closing all pores. When this is dry give the
inner side a similar coat. Finally, after drying thoroughly, give
both sides a coat of olive oil to prevent stickiness. The amount of
varnish required is, for the first coat 1½ times the weight of the
envelope, and for the second coat ½ the weight--of the thin varnish.
For the thick coat on the outer side ⅓ of the weight of the envelope,
and on the inner side about half as much. For the olive-oil coat, about
⅛ of the weight of the envelope will be needed. These figures are
approximate, some material requiring more, some less; and a wasteful
workman will cause a greater difference.
The neck of the balloon (also called the tail) is in form a cylindrical
tube of the fabric, sewn to an opening in the bottom of the balloon,
which has been strengthened by an extra ring of fabric to support it.
The lower end of the tube, called the mouth, is sewn to a wooden ring,
which stiffens it. The size of the neck is dependent upon the size of
the balloon. Its diameter is determined by finding the cube of one-half
the diameter of the balloon, and dividing it by 1,000. In length, the
neck should be at least four times its diameter.
The apex of the balloon envelope is fitted with a large valve to permit
the escape of gas when it is desired that the balloon shall descend.
The door of the valve is made to open inward into the envelope, and
is pulled open by the valve-cord which passes through the neck of the
balloon into the basket, or car. This valve is called the manœuvring
valve, and there are many different designs equally efficient. As they
may be had ready made, it is best for the amateur, unless he is a
machinist, to purchase one. The main point to see to is that the seat
of the valve is of soft pliable rubber, and that the door of the valve
presses a comparatively sharp edge of metal or wood so firmly upon the
seat as to indent it; and the springs of the valve should be strong
enough to hold it evenly to its place.
The making of the net of the balloon is another part of the work which
must be delegated to professionals. The material point is that the net
distributes the weight evenly over the surface of the upper hemisphere
of the envelope. The strength of the cordage is an item which must be
carefully tested. Different samples of the same material show such wide
variations in strength that nothing but an actual test will determine.
In general, however, it may be said that China-grass cordage is four
times as strong as hemp cordage, and silk cordage is ten times as
strong as hemp--for the same size cords.
The meshes of the net should be small, allowing the use of a small
cord. Large cords mean large knots, and these wear seriously upon the
balloon envelope, and are very likely to cause leaks. In large meshes,
also, the envelope puffs out between the cords and becomes somewhat
stretched, opening pores through which much gas is lost by diffusion.
The “star,” or centre of the net at the apex of the balloon, must be
fastened immovably to the rim of the valve. The suspension cords begin
at from 30° to 45° below the equator of the envelope, and are looped
through rings in what are called “goose-necks.” These allow a measure
of sliding motion to the cordage as the basket sways in the wind.
For protecting the net against rotting from frequent wetting, it is
recommended to saturate it thoroughly with a solution of acetate of
soda, drying immediately. Paraffin is sometimes used with more or less
success, but tarring should be avoided, as it materially weakens the
cordage. Oil or grease are even worse.
At the bottom of the net proper the few large cords into which the many
small cords have been merged are attached to the ring of the balloon.
This is either of steel or of several layers of wood well bound
together. The ropes supporting the basket are also fastened to this
ring, and from it hang the trail-rope and the holding ropes.
[Illustration:
Sketch showing the diamond mesh of balloon cordage and the method
of distributing the rings for the goose-necks; also the merging
of netting cords into the suspension cords which support the car.
The principal knots used in tying balloon nets are shown on the
right.]
The basket is also to be made by a professional, as upon its
workmanship may depend the lives of its occupants, though every other
feature of the balloon be faultless. It must be light, and still very
strong to carry its load and withstand severe bumping. It should be
from 3 to 4 feet deep, with a floor space of 4 feet by 5 feet. It is
usually made of willow and rattan woven substantially together. The
ropes supporting the car are passed through the bottom and woven in
with it. Buffers are woven on to the outside, and the inside is padded.
The seats are small baskets in which is stored the equipment. With
the completion of these the balloon is ready for its furnishings and
equipment, which come under the direction of the pilot, or captain, as
detailed in the preceding chapter.