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.

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.