Fig. 3.
As for plan No. 2, the representation of which is annexed, it is much more simple than it looks, as you will gather when I tell you that the furnace part of the apparatus need be no more nor less than a flower-pot having the necessary holes knocked in ([Fig. 3]). Mind, however, to bind your flower-pot with wire, as it is sure to crack when fuel is first lighted within it. Let it crack as much as ever it pleases, your furnace will be none the worse for present purposes, and that is all you require.
I dare say you have heard of a dear departed lady, much respected in her time, a Mistress Glass. ‘First catch your hare,’ wrote Mistress Glass, when about to give directions for making hare-soup. First catch, or learn how to catch, your gas, say I. By this time you know and can judge for yourself which to use, whether hydrogen pure and simple, or else coal-gas.
V.—THE CONSTRUCTION OF THE BALLOON.
Now about making your balloon. The first question is what you shall make it of. Large gas-balloons, such as ascend at public exhibitions with aëronaut travellers, are usually made of silk, covered with some sort of elastic varnish, though the material of very large balloons may be calico, linen, or even canvas. The general statement holds good that in proportion as the balloon is larger so does it admit of being made coarser and heavier. There is a sound mathematical reason for this which any sharp boy ought to perceive, and which, if he do not perceive, I would advise him for his own good to go to his mathematical master and accept castigation.
When the question was how to make a fire-balloon, solution was easy. Tissue-paper, plain and simple, leaves for this purpose nothing to be desired. If a few holes should happen to find themselves in your material, the consequence would not be very serious, inasmuch as the balloon carries its flame, its hot-air generator, along with it; but when you come to have one charge of gas, the gas not renewable, you are obliged to be particular, not merely as to holes, but even loosely packed fibres.
I am entitled to speak magisterially on this point, having recently had some professional talk over the matter with Mr. Brock, the pyrotechnist. ‘Gold-beater’s skin is the very best material for small balloons,’ was that gentleman’s remark to me, ‘but it is very expensive. I want to be able to use paper, and every sort of varnish I have tried has been attended with very indifferent success.’ Mr. Brock went on to say that anybody who might succeed in rendering tissue-paper effectively gas-proof might make a fortune. Now I do not believe in the fortune-making part of the business at all, but I think I shall presently be able to tell you how to make tissue-paper effectively gas-proof. I say effectively, because I know that no varnished silk, cotton, or even canvas material—all used for making balloons—ever is gas-proof. If you fill the thickest ox-bladder you can get with hydrogen or coal-gas, tie the neck of it, and varnish the bladder into the bargain, still gas will come out and atmospheric air will go in. By simply looking at the closed bladder you would not fancy this had happened, because the bladder seems plump and well distended, but were you incautiously to allow a small jet of the contained gas to escape, and to ignite the jet, the whole bladder would explode with a loud report. Now neither hydrogen nor coal-gas will explode when unmixed with oxygen; it simply burns.
Having got the necessary sheets of tissue-paper, hold each of them successively between your eye and a bright light, which will disclose any absolute holes, should such exist. Having discovered them, your first care must be to stop them, and this you do by a plaster of tissue-paper smeared with solution of india-rubber in benzoline. I shall give you no proportions for this solution, but leave you to rule-o’-thumb guidance. Having charged a wide-mouthed bottle—a pickle-bottle does quite well-about half full with benzoline, put into it piece after piece of india-rubber—not vulcanised rubber, mind, which will not dissolve in benzoline, but raw rubber, and keep adding rubber, bit by bit, stirring occasionally with a table knife until you get a paste, not too thick for spreading with a knife-blade. The solution will not be complete for some days. With this, when complete, smear a breadth of tissue-paper, and consider it your plaster.
Remember well the following injunction. In any case when you desire to stick together two surfaces with this india-rubber cement, both surfaces must be smeared, as it does not suffice to smear only one. The next injunction is a very peculiar one, and would not be likely to come self-suggested. Do not bring the surfaces together whilst the solution is wet. Let your work stand until evaporation of the solvent benzoline has left the surfaces what may be called dry. Thus you see the working with this india-rubber paste is the very reverse of working with all ordinary sticking materials.
Repeat the plastering on of solution at least twice, and plaster thick; the result is thin enough when dry. Having in this manner stopped the holes, proceed to make the paper gas-proof. For this purpose you will need a special solution, made in this wise:—To a pint of benzoline add an ounce of solid paraffin—some of a paraffin candle, for example—and a drachm of raw, not vulcanised, rubber. When the two have dissolved, which will require some days’ waiting, lay your tissue-paper on a table, and by means of a flat varnish-brush spread the solution all over it. Wait till evaporation leaves the paper dry, then repeat the operation up to six times, when the paper will have been rendered sufficiently gas-proof for making a balloon. You are to cut this out into fillets, just as you did when making a fire-balloon, and you are to stick the fillets together with the cement already described, remembering well the two injunctions already given. You are not restricted to shape as you are in fire-balloon making, but may adopt any shape, remembering, however, that to promote convenience of gas-collection, the balloon neck should be much narrower than that of a fire-balloon; tubular, so to speak, a tube of one inch diameter being quite sufficient. The reason for making a narrow tubular neck will be apparent from study of the accompanying [diagram]. It will be evident to you that on account of the narrow neck the mode of car, or weight attachment, cannot be similar to that adopted in the case of fire-balloons. The neatest and, at the same time, most obvious plan of procedure is that adopted in large balloons, by throwing a net, with strings dependent, over the whole balloon, and attaching the car or other weights to the pendent strings. Such elaborate work is, however, quite unnecessary in toy balloons. A length of cord tied round the neck and allowed to hang down affords every necessary facility for further attachment.