AEROSTATION. 
172 
them exactly to that lhape. The very firfl machine made 
by M. Montgolfier was in form of a parallelepiped ; and, 
though it contained only forty cubic feet, fhewed a very 
confiderable power of afeent. A very large one, feventy 
four feet high, which M. Montgolfier had defignedto ex¬ 
hibit before the royal family, had the middle part of it 
prifmatic for about the height of twenty-five feet; its top 
was a pyramid of twenty-nine feet; and its lower part was 
a truncated cone of near twenty feet. It weighed 1000 
pounds; and, notwithftanding its fnape, in a very fhort 
time manifefted a power of afeent equal to 500 pounds. 
Another aeroliatic machine of a final 1 lize, but of the 
figure of a parallelopiped, being fuffered to afeend with 
thirty (heets of oiled paper fixed in a wire frame, and fet 
on fire, rofe to a great height, and in twenty-two minutes 
could not be feen. It feems therefore, that, with regard 
to the fiiape of thefe machines, it is by no means necellary 
to adhere rigidly to that of a fpliere ; but that any oblong 
formanfwers very well. 
For experimental purpofes, botli the inflammable and 
rarefied air-balloons may be made of paper ; the former 
being made of that kind called thin pcjl, varnifhed over 
with linfeed-oil; the latter eitiier of that or any other 
kind, without varnifli. In order to avoid the danger of 
burning, however, it has been propofed to impregnate 
the paper of which thefe fmall rarefied-air balloons are 
made with folution of fal-ammoniac, allum, or fome other 
fait; but this does not feem to be neceffary. Thofe filled 
with inflammable air have been made of gold-beaters flein 
or peeled bladders ; but the cheaper material of paper is 
undoubtedly preferable. 
For aeroftatic machines of a larger fize, the material 
univerfally employed is varnifhed (ilk ; and, for thofe of 
the rarefied-air kind, linen painted over with fome fize 
colour, or lined with paper. The belt varnifh for an in¬ 
flammable-air balloon is that made with bird-lime, and re¬ 
commended by Mr. Faujas de Saint-Fond, in a treatife 
publifhed on the fubjedi. The following is his method of 
preparing it : “ Take one pound of bird-lime, put it into 
a new proper earthen pot that can refift the fire, and let it 
boil gently for about one hour, viz. till it ceafes to crac¬ 
kle ; or, which is the fame thing, till it is fo far boiled, 
as that a drop of it being let fall upon the fire will burn : 
then pour upon it a pound of fpirit of turpentine, ftirring 
it at the fame time with a wooden fpatula, and keeping the 
pot at a good diftance from the flame, left the vapour of 
this effential oil fhould take fire. After this, let it boil 
for about fix minutes longer; then pour upon the whole 
three pounds of boiling oil of nuts, linfeed, or poppy, 
rendered drying by means of litharge ; dir it well, let it 
boil for a quarter of an hour longer, and the varnifh is 
made. After it has refted for twenty-four hours, and the 
fediment has gone to the bottom, decant it into another 
pot; and, when you want to ufe it, warm, and apply it with 
a flat brufh upon the (ilk fluff, whilft that is kept well 
flretched. One coat of it may be fufficient; but, if two 
are necellary, it will be proper-to give one on each fide of 
the filk, and to let them dry in the open air while the filk 
remains extended.” 
Mr. Cavallo gives the following method of preparing 
this varnifh, which he prefers to that of M. de St. Fond: 
—“ In order to render linfeed-oil drying, boil it with two 
ounces of faccharum faturni and three ounces of litharge, 
for every pint of oil, till the oil hasdiffolved them, which 
will be accomplifhed in half an hotir ; then put a pound of 
bird-lime and half a pint of the drying oil into a pot (iron 
.or copper pots are the fafefl for this purpofe), the capacity 
of which may be equal to about one gallon, and let it boil 
very gently over a flow charcoal, fire till the bird-lime cea¬ 
fes to crackle, which will be in about half or three quar¬ 
ters of an hour; then pour upon it two pints and a half 
more of drying oil, and let it boil for one hour longer, 
ftirring it very frequently with an iron or wooden fpatula. 
As the varnifh, whilft boiling, and efpecially when it is 
.nearly done, fwells very much, care fhould be had to re¬ 
move, in thofe cafes, the pot from the fire, and to'replace 
it when the varnifh fubfides, otherwife it will boil over. 
Whilft the fluff is boiling, the operator fhould, from time 
to time, examine whether che varnifh has boiled enough; 
which is thus known :—Take fome of it upon the blade of 
a knife, and then, after rubbing the blade of another knife 
upon it, feparatethe knives; and' when, on this feparation, 
the 'varnifli begins,to form threads between the two, you 
may conclude that it is done ; and, without lofing time, it 
mu ft be removed from the fire. When it is almofl, though 
not quite, cold, add about an equal quantity of fpirit of 
turpentine: mix it well together, and let it reft till the 
next day; when, having warmed it a little, drain and bot¬ 
tle it. If-it is too thick, add fome more fpirit of turpen¬ 
tine. When this varnifh is laid upon the filk, the fluff 
fhould be made perfectly dry, and flretched ; fo that the 
varnifh, which ought to be ufied lukewarm, may fill up the 
pores of the fluff. The varnifh fhould be laid once Very 
thin upon one fide of the fluff; and, about twelve hours 
after, two other coats of it fhould be laid on, one on each 
fide ; and, twenty-four hours after, the filk may be ufed, 
though, in cold weather, it may be left to dry fome time 
longer.” ■ 
Much has been faid in France of their elaftic gum- 
varnifh, and its compofition kept a fecret ; but Mr. Bald¬ 
win, alter many expenfive trials, declares to the world 
what he confiders as the fecret; and it is merely this: 
“Take any quantity of caoutchouc, as two ounces averdu- 
pois ; cut it into fmall bits with a pair of feiffars; put a 
flrong iron ladle (like that ufed by plumbers) over a com¬ 
mon pitcoal or other fire. The fire mud be gentle, glow¬ 
ing, and without fmoke. When the ladle is hot, much 
below a recTheat, put a (ingle bit into the ladle. If black 
fmoke iflues, it will prefently flame and difappear, or it 
will evaporate without flame: the ladle is then too hot. 
When the ladle is lefs hot, put in a fecond bit, which will 
produce a white fmoke. This white fmoke will continue 
during the operation, and evaporate the caoutchouc : 
therefore no time is to be loft; but little bits are to be put 
in, a few at a time, till the whole are melted. It fhould be 
continually and gently ftirred with an iron or brafs fpoon. 
Two pounds or one quart of the belt drying oil (or of raw 
linfeed-oil, which, together with a few drops of neat’s- 
footoil, has flood a month, or not fo long, on a lump of 
quick-lime, to make it more or lefs drying), is to be 
put into the melted caoutchouc, and ftirred till hot, and 
the whole poured into a glazed velfel, through a coarfe 
gauze or fine fieve. When fettled and clear, which will 
be in a few minutes, it will be fit for ufe either hot or 
cold.” Mr. Baldwin is not at liberty, he obferves, to 
publifh the art of laying on the varnifli; but fays, that it 
conlifts in making no inteftine motion in the varnifh, which 
would create minute bubbles ; that therefore brufhes are 
improper. Mr. Blanchard’s method of makingelafticgum- 
varnilh for the filk of a balloon, is the following : “ Dif- 
folve elaftic gum (caoutchouc) cut fmall in five times its 
weight of fpirit of turpentine, by keeping them fome days 
together; then boil one ounce of this folution in eight- 
ounces of drying linfeed-oil for a few minutes; laftly, 
(train it. It muft be ufed warm.” The pieces of filk for 
the balloon.mult be cut out of a proper fize, according to 
the dimenfions, after the varnifh is diffidently 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 ditching. Mr. Blanchard ufes expeditioufly the 
following method. He lays about half an inch of the edge 
of one piece flat over the edge of the.other, and paffes a 
Hot iron over it; in doing which a piece of paper ought to 
be laid both under and over the filk. The joining may 
be rendered more fecure by running it with a filk thread, 
and flicking a ribband over it. The ribbands laid over 
feams may be duck with common glue, provided the var¬ 
nifh of the filk is properly dried. When the glue is quite 
dry, the ribbands fhould be varnilhed over, to prevent their 
being unglued by the rain. 
The 
