In important voyages it is customary to send up pilot balloons, to discover the direction of the wind currents at the different levels, so that the level which promises the best may be selected before the balloon leaves the ground. A study of the weather conditions throughout the surrounding country is a wise precaution, and no start should be made if a storm is imminent. The extent of control possible in ballooning being so limited, all risks should be scrupulously avoided, both before and during the voyage, and nothing left to haphazard.
Chapter XVI.
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.