When you have pasted all the gores together, and they are quite dry, blow the balloon out with a pair of bellows through the neck. You will find at the top, where all the points of the gores meet, a small hole, which will let out the air with which you have filled your balloon, and à fortiori the gas with which it will have to be filled presently, the gas being lighter than the air, and so more anxious to make its escape from its imprisonment. At all events this is very probable, even in the most carefully and scientifically constructed balloons. To obviate this fatal mistake, therefore, you must cut a round piece of paper, which is called the cap, answering to the valve in a real balloon, and carefully paste it over the meeting-places of the various gores. For you must bear in mind that a balloon is not a balloon at all unless it be perfectly air-tight.
Now let out the air by gentle pressure, and fold up the balloon, gore over gore, and commence the varnishing, which is laid on as thinly as possible with a small piece of flannel. The varnish used is simply boiled oil, which can be obtained from any oil and colour shop. After you have carefully varnished the whole of the gores, blow the balloon out again, and hang it up by the neck until it is dry, a process which will take about twelve hours.
The material used in the construction of balloons for carrying passengers is Scotch cambric—not silk, as is erroneously supposed. Silk has not been largely used in the manufacture of balloons for the last forty years, and I need hardly say that it is not pasted, as in a paper balloon, but sewn with double rows of stitches, and varnished exactly in the same manner as I have already described.
In big balloons, the most important part of all is
THE NETTING.
And I shall now describe the way in which, if you desire to make your model perfect, you must set about this portion of your manufacture, which, however, you can dispense with if you please in a paper gas-balloon. As I told you before, there is scarcely any strain whatever on the balloon; in fact you could make a large paper balloon to contain 20,000 cubic feet of gas, and if it were covered with a properly fitting net it would, for one ascent, answer the same purpose as a cambric balloon, the reason for making it of a material at once light and strong being to enable it to stand the wear and tear of laying out, folding, packing, &c. The first thing to be observed in making the net of a balloon is to take the same pattern gore as the balloon itself was cut from. Now draw a longitudinal line through the centre A ([Fig. 6]), the gore being reduced to two halves. Divide one half by the same rule as that on which you originally cut the pattern gore, the object of this division being to get the meshes of the intended netting reduced to such a small scale that four meshes may cover each gore. Measure on the centre line A, the distance B C, and half as long again. Draw a line parallel to B C from D to E.
Fig. 6
Fig. 7