It is apparent that the whole of the gas is lost at the end of the journey. The cost of this is the largest expense of ballooning. For a small balloon of about 50,000 cubic feet, the coal-gas for inflating will cost about $35 to $40. If hydrogen is used, it will cost probably ten times as much. 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.