Small gas balloons are made of thin sheet India rubber, or gutta percha,
or tissue-paper; larger ones are made of oiled silk. Cut gores of the
material to be used, sufficient in number when fastened together, the
sides of each gore overlapping the gore fastened to it, to form a globe
of the desired size, with pear-shaped ends. Join the gores together so
as to make them completely air-tight. When the heavier materials are
used, they should be sewn together, and then covered with glue or thin
varnish. At the lower end of the balloon insert a tube, and tie all the
narrow tips of the gores firmly round it. Cover all with a solution made
of India rubber dissolved in naphtha and turpentine, and over the
balloon place a net bag that has been previously made of the proper size
and shape.
The gas with which the balloon is to be filled is made in the following
manner: Put a pound of granulated zinc or iron filings into two quarts
of water in a stone jar, and add gradually a pint of sulphuric acid.
Have a tube of glass or metal run through the bung with which the jar is
corked, and after taking the materials out-of-doors, fill the balloon by
connecting this tube with the tube already placed at its mouth. When the
balloon is filled, tie its neck very tightly, and it will rise into the
air. Common coal gas may be used when it can be obtained. A small car
made of some light material may be attached to the netting which goes
over the balloon.
* * * * *