Materials.

Best var

Aammable air bal

loons.

proper. An inverted cone, or truncated pyramid, with the smaller part undermost, seems then to be most proper, as it allows the heated air (which has a great tendency to expand as well as to ascend) to collect in the wide part at the top, while the useless surface, in the lower part, and which, in any other figure, would contain only the colder and heavier air, is thus thrown aside. In fact it has been found, that aerostatic machines, raised by means of rarefied air, when made of the shape of a parallelopiped, or even one deviating still more from the shape of a globe, have answered the purpose as well as they could have been supposed to do, had ever so much care been taken in forming them exactly to that shape. The very first, machine made by M. Montgolfier was in form of a parallelopiped; and though it contained only 40 cubic feet, showed a very considerable power of ascent. A very large one, 74 feet high, which M. Montgolfier had designed to exhibit before the royal family, had the middle part of it prismatic for about the height of 25 feet; its top was a pyramid of 29 feet; and its lower part was a truncated cone of near 20 feet. It weighed 1000 pounds; and, notwithstanding its shape, in a very short time manifested a power of ascent equal to 500 pounds. Another aerostatic machine of a small size, but of the figure of a parallelopiped, being suffered to ascend with 30 sheets of oiled paper fixed in a wire frame, and set on fire, rose to a great height, and in 22 minutes could not be seen. It seems therefore, that, with regard to the shape, of these machines, it is by no means necessary to adhere rigidly to that of a sphere; but that any oblong form answers very well.

For experimental purposes, both the inflammable and rarefied air balloons may be made of paper; the former being made of that kind called thin post, varnished over with linseed oil; the latter either of that or any other kind, without varnish. In order to avoid the danger of burning, however, it has been proposed to impregnate the paper of which these small rarefied air balloons are made with a solution of sal ammoniac, alum, or some other salt: but this does not seem to be necessary. Those filled with inflammable air have been made of gold-beaters skin or peeled bladders; but the cheaper material of paper is undoubtedly preferable.

For aerostatic machines of a larger size, the material mish for in- universally employed is varnished silk; and for those of the rarefied air kind, linen painted over with some size colour, or lined with paper. The best varnish for an inflammable air balloon is that made with birdlime, and recommended by M. Faujas de Saint Fond, in a treatise published on the subject. The following is his method of preparing it: "Take one pound of birdlime, put it into a new proper earthen pot that can resist the fire, and let it boil gently for about an hour, viz. till it cease to crackle; or, which is the same thing, till it is so far boiled, as that a drop of it being let fall upon the fire will burn: then pour upon it a pound of spirit of turpentine, stirring it at the same time with a wooden spatula, and keeping the pot at a good distance from the flame, lest the vapour of this essential oil should take fire. After this, let it boil for about six minutes longer; and then pour upon the whole three pounds of boiling oil of nuts, linseed, or poppy, rendered drying by means of litharge; stir it well, let VOL. I. Part I.

it boil for a quarter of an hour longer, and the varnish is made. After it has rested for 24 hours, and the sediment has gone to the bottom, decant it into another pot; and when you want to use it, warm, and apply it with a flat brush upon the silk stuff, whilst that is kept well stretched. One coat of it may be sufficient; but if two are necessary, it will be proper to give one on each side of the silk, and to let them dry in the open air while the silk remains extended."

Mr Cavallo gives the following method of preparing Mr Caval this varnish, which he prefers to that of M. de Stlo's mcFond." In order to render linseed oil drying, boil thod, it with two ounces of saccharum saturni and three ounces of litharge, for every pint of oil, till the oil has dissolved them, which will be accomplished in half an hour; then put a pound of birdlime and half a pint of the drying oil into a pot (iron or copper pots are the safest for this purpose), the capacity of which may be equal to about one gallon, and let it boil very gently over a slow charcoal fire till the birdlime ceases to crackle, which will be in about half or three quarters of an hour; then pour upon it two pints and a half more of drying oil, and let it boil for one hour longer, stirring it very frequently with an iron or wooden spatula. As the varnish, whilst boiling, and especially when it is nearly done, swells very much, care should be had to remove, in those cases, the pot from the fire, and to replace it when the varnish subsides, otherwise it will boil over. Whilst the stuff is boiling, the operator should, from time to time, examine whether the varnish has boiled enough; which is thus known :-Take some of it upon the blade of a knife, and then, after rubbing the blade of another knife upon it, separate the knives; and when, on this separation, the varnish begins to form threads between the two, you may conclude that it is done; and, without losing time, it must be removed from the fire. When it is almost, though not quite cold, add about an equal quantity of spirit of turpentine; mix it well together, and let it rest till the next day; when, having warmed it a little, strain and bottle it. If it is too thick, add some more spirit of turpentine. When this varnish is laid upon the silk, the stuff should be made perfectly dry, and stretched; so that the varnish, which ought to be used lukewarm, may fill up the pores of the stuff. The varnish should be laid once very thin upon one side of the stuff; and, about 12 hours after, two other coats of it should be laid on, one on each side; and, 24 hours after, the silk may be used, though, in cold weather, it may be left to dry some time longer."

Much has been said in France of their elastic gum varnish, and its composition kept a secret; but Mr Baldwin, after many expensive trials, declares to the world what he considers as the secret; and it is merely this: "Take any quantity of caoutchouc, as two ounces avoirdupois; cut it into small bits with a pair of scissars; put a strong iron ladle (like that used by plumhers) over a common pitcoal or other fire. The fire must be gentle, glowing, and without smoke. When the ladle is hot, much below a red heat, put a single bit into the ladle. If black smoke issues, it will presently flame and disappear, or it will evaporate without flame the ladle is then too hot. : When the ladle is less hot, put in a second bit, which will proF f duce

Of cutting the gores

duce a white smoke. This white smoke will continue
during the operation, and evaporate the caoutchouc
therefore no time is to be lost; but little bits are to
be put in, a few at a time, till the whole are melted.
It should be continually and gently stirred with an iron
or brass spoon.
Two pounds or one quart of the best
drying oil (or of raw linseed oil, which together with a
few drops of neats foot oil, has stood a month, or not
so long, on a lump of quicklime, to make it more or less
drying), is to be put into the melted caoutchouc, and
stirred till hot, and the whole poured into a glazed ves-
sel through a coarse gauze or fine sieve. When settled
and clear, which will be in a few minutes, it will be fit
for use either hot or cold." Mr Baldwin is not at li-
berty, he observes, to publish the art of laying on the
varnish but says, that it consists in making no intes-
tine motion in the varnish, which would create minute
bubbles; that therefore brushes are improper. M.
Blanchard's method of making elastic gum varnish for
the silk of a balloon, is the following. "Dissolve ela-
stic gum (caoutchouc) cut small, in five times its weight
of spirit of turpentine, by keeping them some days to-
gether; then boil one ounce of this solution in eight
ounces of drying linseed oil for a few minutes; lastly,
strain it. It must be used warm." The pieces of silk
for the balloon must be cut out of a proper size, accord-
ing to the dimensions, after the varnish is sufficiently
dry. They may be joined by laying about half an inch
of the edge of one piece over the edge of the other, and
fewing them by a double stitching. M. Blanchard
uses expeditiously the following method: He lays about
half an inch of the edge of one piece flat over the edge
of the other, and passes a hot iron over it; in doing
which a piece of paper ought to be laid both under and
over the silk. The joining may be rendered more se-
cure by running it with a silk thread, and sticking a
ribband over it. The ribbands laid over seams may be
stuck with common glue, provided the varnish of the
silk is properly dried. When the glue is quite dry,
the ribbands should be varnished over, to prevent their
being unglued by the rain.

The best method of cutting the pieces of silk that are to form a balloon, is to describe a pattern of wood or for a globe. stiff card paper, and then to cut the silk upon it. As the edges of such a pattern are not perfect circles, they cannot be described by a pair of compasses; but the best method of drawing them is as follows. First, Draw on a flat surface two right lines AE and BC, fig. 2. perpendicular to each other. Secondly, Find the circumference answering to the given diameter of the balloon in feet and decimals of a foot; and make AD and DE each equal to a quarter of the circumference, so that the whole length AE of the pattern may be equal to half the circumference. Thirdly, Divide AD into 18 equal parts; and to the points of division apply the lines fg, hi, kl, &c. parallel to each other, and perpendicular to AD. Fourthly, Divide the whole circumference in twice the given number of pieces, and make DC and BB each equal to the quotient of this division so that the whole, BC, is equal to the greatest breadth of one of these pieces. Fifthly, Multiply the above-mentioned quotient by the decimals annexed to fg, viz. 0.99619, and then the product expresses the length of fg; again, multiply the same length of DE by the decimals annexed to hi, and the product ex3

tern.

presses the length of hi; and, in short, the product arising from the multiplication of the length of DC by the decimals annexed to each of the parallel lines, gives the length of that line. Lastly, Having found the lengths of all these lines, draw by hand a curve line passing through all the extremities of the said lines, and that is the edge of one quarter of the patThe other quarters may be easily described, by applying to them a piece of paper cut according to that already found. Suppose, for example, that the diameter of the balloon to be constructed is 20 feet, and that it is required to make it of 12 pieces; then, in order to draw the pattern for those pieces, find the circumference of the balloon, which is 62.85 feet, and dividing it by four, the quotient is 15.7 feet; make therefore AD equal to 15.7 feet, and DE likewise of the same length. Divide the circumference 62.83 by 24, which is double the number of pieces that are to form the balloon, and the quotient, 2.618 feet, is the length of DC and likewise of BD; so that BC is equal to 5.236 feet. Then having divided the line AD into 18 equal parts, and having drawn the parallel lines from those points of division, find the length of each of those lines by multiplying 2.618 by the decimals annexed to that line. Thus, 2.618 multiplied by 0.99619, gives 2.608 feet for the length of fg; and again, multiplying 2.618 by 0.98481, gives 2.578 feet for the length of hi; and so of the rest. In cutting the pieces after such a pattern, care should be taken to leave them about three quarters of an inch all round larger than the pattern, which will be taken up by the seams.

To the upper part of the balloon there should be adapted, and well fitted in, a valve, opening inwards; to which should be fastened a string passing through a hole made in a small piece of round wood fixed in the lowest part of the balloon opposite to the valve, and the end of this string fastened in the car below, so that the aeronaut may open the valve when occasion requires. The action of this valve may be understood from fig. 3. A round brass plate AB has a round hole CD, about two or three inches diameter, covered on both sides with strong smooth leather. On the inside there is a shutter E, also of brass, covered with leather, which is to close the hole CD; being about two inches larger in diameter than the hole. It is fastened to the leather of the plate AB; and by a spring, which need not be very strong, it is kept against the hole. The elasticity of the gas itself will help to keep it shut. To this shutter the string is fastened, by which it is occasionally opened for the escape of gas. A small string or other security should be fixed to the shutter and the plate, so as not to admit the shutter to be opened beyond a certain safe distance. To the lower part of the balloon two pipes should be fixed, made of the same stuff as the envelope; 6 inches diameter for a balloon of 30 feet, and proportionably larger for balloons of a greater capacity. They must be long enough for the

car.

For balloons of 18 feet and less diameter, one neck or pipe will be sufficient. These pipes are the apertures through which the inflammable gas is introduced into the balloon.

The car or boat is best made of wicker work, covered with leather, and well painted or vanished over; and the proper method of suspending it, is by ropes proceeding

ceeding from the net which goes over the balloon. This net should be formed to the shape of the balloon, and fall down to the middle of it, with various cords proceeding from it to the circumference of a circle about two feet below the balloon; and from that circle other ropes should go to the edge of the boat. This circle may be made of wood, or of several pieces of slender cane bound together. The meshes of the net may be small at top, against which part of the balloon the inflammable air exerts the greatest force; and increase in size as they recede from the top. A hoop has sometimes been applied round the middle of the balloon to fasten the net. This, though not absolutely necessary, is best made of pieces of cane bound together, and covered with leather.

With regard to the rarefied air machines, Mr Cavallo recommends first to soak the cloth in a solution of sal-ammoniac and common size, using one pound of each to every gallon of water; and when the cloth is quite dry, to paint it over in the inside with some earthy colour, and strong size or glue. When this paint has dried perfectly, it will then be proper to varnish it with oily varnish, which might dry before it could penetrate quite through the cloth. Simply drying linseed oil will answer the purpose as well as any, provided it be not very fluid.

Of filling It now only remains to give some account of the aerostatic method by which aerostatic machines may be filled machines. with their proper gas, in order to give them their power of asscending into the atmosphere; and here we are enabled to determine with much greater precision concerning the inflammable air balloons than the others. Methods of With regard to them, a primary consideration is, the procuring most proper method of procuring the inflammable air. inflamma- It may be obtained in various ways, as will be shown

ble air.

under the article CHEMISTRY. But the most advantageous methods are, by applying acids to certain metals; by exposing animal, vegetable, and some mineral substances, in a close vessel to a strong fire; or by transmitting the vapour of certain fluids through red-hot tubes.

1. In the first of these methods, iron, zinc, and sulphuric acid are the materials most generally used. The sulphuric acid must be diluted with five or six parts of water. Iron may be expected to yield in the common way 1700 times its own bulk of gas; or one cubic foot of inflammable air to be produced by 4 ounces of iron, the like weight of sulphuric acid, and 22 ounces of water. Six ounces of zinc, an equal weight of sulphuric acid, and 30 ounces of water, are necessary for producing the same quantity of gas. It is more proper to use the turnings or chippings of great pieces of iron, as of cannon, &c. than the filings of that metal, because the heat attending the effervescence will be diminished; and the diluted acid will pass more readily through the interstices of the turnings when they are heaped together, than through the filings, which stick closer to one another. The weight of the inflammable air thus obtained by means of sulphuric acid, is, in the common way of procuring it, generally one-seventh part of the weight of common air; but with the necessary precautions for philosophical experiments, less than one-tenth of the weight of common air. Two other sorts of elastic fluids are sometimes generated with the inflammable air. These

may be separated from it by passing the inflammable air through water in which quicklime has been dissolved. The water will absorb these fluids, cool the inflammable air, and prevent its over-heating the balloon when introduced into it.

Fig. 4. of Plate II. represents an apparatus described by Mr Cavallo as proper for filling balloons of the size of two or three feet in diameter with inflammable air, after passing it through water.-A is the bottle with the ingredients: BCD a tube fastened in the neck at B, and passing through C, the cork of the other bottle, in which there is another hole made to receive the tube on which the balloon is tied. Thus it is plain, that the inflammable air coming out of the tube D will pass first through the water of the bottle E and then into the balloon. Two small casks may be used instead of the bottles A and E.

2. Inflammable air may be obtained at a much cheaper rate by the action of fire on various substances; but the gas which these yield is not so light as that produced by the effervescence of acids and metals. The substances proper to be used in this way are, pitcoal, asphaltum, amber, rock-oil, and other minerals; wood, and especially oak, camphor-oil, spirits of wine, æther, and animal substances, which yield air in different degrees, and of various specific gravities: but pitcoal is the preferable substance. A pound of this exposed to a red heat, yields about three cubic feet of inflammable air, which, whether it be passed through water or not, weighs about one-fourth of the weight of common air. Dr Priestley found, as we have elsewhere noticed, that animal or vegetable substances will yield six or seven times more inflammable air when the fire is suddenly increased than when it is gently raised, though it be afterwards made very strong. Mr Cavallo observes, that the various substances above enumerated generally yield all their inflammable air in about one hour's time. The general method is, to enclose the substances in iron or earthen vessels, and thus expose them to a strong fire sufficient to make the vessels red-hot; the inflammable air proceeding from the aperture of the vessel is received into a tube or refrigeratory, and, passing through the tube or worm, is at last collected in a balloon or other vessel. A gun-barrel has often been used for essays of this kind. The substance is put into it so as to fill six or eight inches of its lowest part, the remainder filled with dry sand: a tube, adapted to the mouth of the barrel, is brought into a bason of water under an inverted receiver; and the part of the barrel containing the substance being put into the fire and made red-hot, the inflammable air is collected in the inverted receiver. As the gun-barrel cannot serve for producing a large quantity of inflammable air, Mr Cavallo recommends, as the most advantageous shape, the following contrivance: Let the vessel be made of clay, or rather of iron, in the shape of a Florence flask, somewhat larger, and whose neck is longer. and larger (See ABC, fig. 5.). Put the substance to be used into this vessel, so as to fill about four-fifths or less of its cavity AB. If the substance is of such a nature as to swell much by the action of the fire, lute a tube of brass, or first a brass and then a leaden tube, to the neck C of the vessel; and let the end D of the tube be shaped as in the figure, so that going into the water of a tube HI, it may terminate under a sort of inverted vesFf2

sel