A model fire balloon. A model parachute. Inflating a modern balloon wath gas. A model airship.
A MODEL ﬁre balloon can be easily made. Gurn together two sheets of tissue paper by their narrowest edges, and having prepared twelve such pieces (twenty four sheets in all) lay them aside to dry. From each of the double sheets so prepared, cut a form similar to that shown (Fig- 7).
If the edges of these gores are laid over each other and gummed together a globe will be formed, open at top
and bottom, the top aperture is to be covered in by a circular piece of tissue paper, overlying the ends of the segments which form the sides, and the lower opening must have a circle of ﬁne wire attached by turning in and gumming round it the lower edges of the paper segments. A cross wire, to which is fastened a small piece of sponge or cotton-wool, is ﬁxed across the aperture, and the paper balloon is ﬁnished. On saturating the sponge or cotton wool with methylated spirits, set it on ﬁre and the balloon will ascend. Be careful not to set the balloon on ﬁre and destroy it.
A model parachute (Fig. 8) can be made as follows:
take a square piece of tissue paper and to each of the corners attach a piece of thread twice as long as the paper is broad. Tie the ends of the strings evenly in a knot together, and aﬂix to the knot a cork. Let the apparatus be dropped from an upper window of the house, the paper will at once expand getting into shape like an umbrella, and the increased resistance experienced in the air allows the cork to come slowly to earth. As a contrast, drop a piece of cork by itself out Of the window at the same time, and observe how much more rapidly it will go to the ground.
Inflating a modern balloon witn gas
A working model of a drifting balloon is easily constructed. Purchase one of the small toy balloons (the larger the better) sold in any of the rubber and gutta-percha shops. One 3% to 4 inches in diameter will do well enough. Soften the balloon, by warming it slightly, then distend to its full capacity with air, by blowing through the hollow stem or neck. When fully distended, with a pair of large callipers or a foot rule, measure roughly the diameter and note this so that you can distend the balloon with gas to the same diameter, and stop short without any chance of bursting it. This event happening with air would not matter, but with gas it would not be at all pleasant, but, by taking the above precaution, the accident is not likely to happen. A balloon of the above size when empty will distend to about 9% or 10 inches diameter quite safely, without any fear of bursting. To ﬁll the balloon with gas it is of no use to slip it over a gas bracket, tie it tightly on, and then turn on the gas, the pressure in the mains is not sufﬁcient to distend it fully, and it would not have sufﬁcient buoyancy to ascend.
To inﬂate the balloon we require a small force pump (a vertical or a horizontal one, such as would be used to supply a model steam boiler with water) provided with a suction and a delivery valve (Fig. 9). The pipe, leading to the suction valve SV, is attached, by rubber tubing, to a gas-pipe bracket, the two ends of the tube being tied ﬁrmly on so as to be quite gas-tight. The outer end of the pipe, leading from the delivery valve DV, if of too large a diameter for the neck of the balloon to be slipped over, must have a small tube soldered into its oriﬁce, about the same outside diameter as the inside diameter of the neck of the balloon. In this case, a brass tube T55 inch outside diameter will do. Tie on the balloon ﬁrmly, with a thread, twisted round the neck two or three times, but, before doing so, see that all residual air is emptied out of
the balloon by squeezing it quite ﬂat. It is a good plan, before putting the balloon on to the delivery pipe of the pump, to join up the pump to the gas supply pipe or bracket, turn on the gas, and by hand give two or three strokes to the pump plunger P, to empty it of air, and in order to make sure it is pumping gas, light the gas, at the end of the delivery pipe, DP, with a taper. When all is ready, tie the balloon on, and then start pumping by hand. The pump can be supported by the left hand while pumping with the right, or a temporary stand may be erected. When the balloon is fully distended, or is at the same stage of distension, as shown by callipers, that it was when previously ﬁlled with air, stop pumping, and tie the neck very ﬁrmly to obliterate the opening, just above the mouth of the pipe (lint thread is best to use, as it is strong and does not readily break). Remove the balloon from the gas connections , insert a very small plug of cork into the end of the neck and from this suspend, by threads, a small piece of cork, or half a walnut shell, or a small paper pill-box without the lid (it will carry only a few grains in weight) for a car. The buoyancy of the balloon will now be great enough to allow it to ascend, which it will do, either in the house or out of doors. (This experiment is best carried on out of doors on a very calm day.) By attaching a thread to the car, with the other end on a bobbin held in the hand, we can retain the balloon captive and watch it ascend as the thread is being let out from the bobbin. This it will do going higher and higher until the weight of the suspended thread overcomes the buoyancy, when the balloon will descend till it is free of the surplus weight of thread then it will begin to ascend again and, in this way, we get a demonstration of the way in which the guide rope (which trails on the ground) acts in a real balloon. The gas will tend to leak away from the balloon in time, but it will make a good ascent for more than one hour if properly distended to start with.
The larger the balloon, the more satisfactory will this experiment be.
lnﬂating a model balloon with hydrogen gas.
Any size of balloon can be inﬂated with coal gas or hydrogen, but the latter is preferable for very small 'toy balloons, such as one of 2}, inches diameter, as it renders them very buoyant in proportion to their size. If it be decided to use hydrogen, then we require to ﬁt up a hydrogen generating apparatus, and any boy who has studied a little chemistry will easily ﬁt up a small apparatus for a few shillings.
Procure from a dealer in chemicals and chemical apparatus , a fairly large-sized glass ﬂask, with agood ﬁtting cork, a few feet of glass tubing (some rubber tubing that will ﬁt over the glass tubing), a small stopcock, a straight glass tube with funnel, a glass trough, a glass cylinder, 1-;— inch or 2 inches in diameter, a cork to ﬁt the same, a stand with sliding arm to support it, some zinc clippings and a small bottle of strong sulphuric acid or oil of vitriol (HQSOQ. From arubber shop purchase a small gas bag with two openings and rubber tubes, one at each end. A large-sized bag of this shape is used for holding a reserve supply of gas for gas engines. A force pump is also required, of the same construction as used for inﬂation with coal gas, having a suction valve and a delivery valve. (A cycle inﬂator pump is not satisfactory, as it has got no valves.) The apparatus must now be put together. (Fig. 10.)
F is the glass ﬂask, having a bent glass tube and a straight glass tube (with funnel) each passing, gas-tight, through two separate openings in the cork (the holes through the cork had best be bored by a heated spindle to about the same thickness as the glass tubes). Sealing-wax will make the openings quite tight. The lowest extremity of the bent glass tube just enters the interior of the glass cylinder GC, which sits inside the glass trough GT. The free end of GC is supported by the arm of the stand S, which is ﬁxed by a set screw at a convenient height. A glass tube, bent at right angles, passes, gas-tight, through the cork at the top end of GC, the other end of this tube is joined by rubber tubing to one end of the stopcock SP, the other end of SP is attached by bent glass tubing to cock SP'. and by rubber tubing to the gas bag GB. The other outlet of GB is joined by rubber tubing to the suction pipe of the force pump (not shown in the drawing), the same kind of pump as shown in Fig. 9 will be suitable.
Glass tubing is easily cut by making a mark round it with a ﬁne triangular ﬁle, when it will snip across at the part on pressure being applied to it with the ﬁngers. The
tubing is easily bent by heating it over a small gas Bunsen burner and then bending by hand before it has time to cool.
Having arranged the apparatus, remove the cork from F and put some zinc * clippings in the ﬂask, then prepare the acid, making it the proper strength, viz. 1 part of acid to 8 parts of water. Take 1} ﬂuid ounces or 10 ﬂuid drachms of strong vitriol, add this to half a pint or 10 ﬂuid ounces of water. (Measures are given in apotheca1ies’ weight.) It is best to put the water into a bowl or other receptacle, and add the acid in small quantities at a time, for great heat is evolved when the acid and water mix, and there is danger that splashing of the acid may burn the operator, should the water be poured on the acid. The safest plan is to get the acid diluted and made the proper strength by the chemist. Replace the cork and pour the weak acid through the funnel over the zinc in F; a rapid effervescence commences, and the evolved gas is collected over water in GC, placed in GT half full of water. Care must be taken that all the air is expelled from the ﬂask and bag before any gas is allowed to enter the latter. GB should be exhausted of air by the pump before opening the cock SP.
On opening these cocks, hydrogen will enter into the bag, and a stroke or two of the pump, with the delivery pipe mouth left open, will expel any residual air, and soon nothing but free hydrogen will issue from it. Next, ﬁx the balloon (being completely emptied of air) on to the delivery pipe of the pump, use the same method as before, operating the pump plunger by hand till the balloon is completely distended , but be careful not to pump too long and cause it to burst. When fully inﬂated, grasp the balloon at the neck with the ﬁnger and thumb, press the neck together, and, at the same time, cut the thread fastening it to the pump, slip a very small plug of wood or cork into the mouth and tie it ﬁrmly in with a thread to prevent the gas escaping. A piece of cork or wood can be suspended from the plug by silk threads to represent the car. When the balloon is set free, it will rise into the air to a greater height than if ﬁlled with coal gas, as hydrogen makes the balloon more buoyant in proportion to its size than coal gas does. This would make a good experiment for the lecture-room.
Two stopcocks are shown in Fig. 10, SP, SP', inserted in the tube leading from GC to GB; the use of these is to make it possible (should the supply of hydrogen run short before the balloon or airship is fully inﬂated) to detach the apparatus from the bag without letting air into the bag when we require to recharge F with fresh chemicals. Close both cocks, uncouple the tube T, reﬁll the ﬂask with fresh chemicals, open cock SP, and when the gas issues freely, close the cock for a few minutes till T is attached to the cock SP' again, then open both cocks and start pumping to ﬁll the balloon.
The dirigible balloon or airship can be purchased of the orthodox cigar shape made to any size. We inspected
one (Fig. 11) made of goldbeater’s skin to the sizes given by Messrs. Richardson & C0., Shefﬁeld. To the envelope were fastened three loops of thin twine, and to these a piece of cane, say, 1 foot II inches long by 1%, inch square (cane can be had from the model makers, machined true, at 3d. per yard) is attached horizontally under the airship. The square rudder and ﬁns are made of paper. There are two vertical and two horizontal ﬁns of the same shape. The two vertical ones are cut together from one piece of paper, the horizontal ones are cut separately. The vertical ﬁns and rudder are ﬁxed by slitting up the cane at the rear end for a short distance, inserting the two ﬁns and then the rudder in the slit, and gluing the whole together. After the glue has set, the two horizontal ﬁns can be glued on outside.
Suspended at the centre of the airship, below the cane, by threads, is a spring motor and propeller of sheet tin. A long running toy torpedo-boat motor (if one can be got sufﬁciently light) will give good results. If the motor appears too heavy on trial then a lighter one must be obtained. The airship is inﬂated through the neck N by a force pump (see p. 21), and a little bit of cork, prepared beforehand, is inserted into the opening and tied in to prevent the gas from escaping. When the envelope is fully inﬂated, then on winding up the spring and setting the airship free, on a calm day, in a ﬁeld where there are no obstacles to impede its progress, it will sail away and continue to forge ahead until the energy of the spring is exhausted or the elastic runs down, if an elastic motor be used instead of a spring one.
To prevent losing the airship, it is a good plan to fasten to it a lint thread from a bobbin, held in the hand, and let the thread out as the balloon sails away, even following it over the ﬁeld.
The rudder is put on for appearance sake merely, and does not act in any way.