Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 1, A-AME

224
be near an ounce. Let its capacity be increafed 16
times; and the tendency to arife -will be equivalent to
a pound, though this may be done without making
the machine 16 times heavier than before. It is cer¬
tain, however, that all aeroftatic machines have a ten¬
dency to produce or preferve heat within them, which
would by no means be imagined by thofe who have
not made the experiment. When Meffrs Charles and
Roberts made their longeft aerial voyage of 150 miles,
they had the curiofity to try the temperature of the air
within their balloon, in comparifon with that of the
external atmofphere ; and at this time they found,
that when the external atmofphere was 63°, the ther¬
mometer within the balloon flood at 104°. Such a
difference of temperature muff have given a machine
of the magnitude which carried them a confiderable
afcending power independent of any other caufe, as it
amounted to 41 grains on every cubic foot; and there¬
fore in a machine containing 50,000 fuch feet would
have been almofl 200 pounds. Hence we may ealily
account for what happened at Dijon, and is recorded
A balloon by Mr Morveau. “ A balloon, intended to be filled
at. with inflammable air, being completed, was, by way of
into theSat-w‘t^1 con3mon a‘r> anc* in that ftate expofed
mofphere. to the atmofphere. Now it was obferved, and indeed
a fimilar obfervation had been made before, that the
air within the balloon was much hotter than the cir¬
cumambient air ; the thermometer in the former flood
at 120° } whereas in the latter, even when the fun
fhone upon it, the thermometer flood at 84°. This
fhowed a confiderable degree of rarefaftion within the
balloon ; and confequently it was fufpefled, that, by
means of this rarefaftion alone, efpecially if it rvere to
increafe a little, the balloon might afcend. On the
30th of May, about noon, the wind being rather
flrong, agitated the balloon fo that two men were em¬
ployed to take care of it; but, notwith flan ding all
their endeavours, it efcaped from its confinement j and,
lifting up about 65 pounds weight of cords, equatorial
circle, &c. rofe many feet high, and palling over fome
houfes, went to the diftance of 250 yards, where at
length it was properly facured.”
Internal This difference between the external and internal
heat of the heat being fo very confiderable, mull have a great in¬
balloons fluence upon aeroflatic machines, and will undoubtedly
influence influence thofe filled with inflammable air as well as
on aerial the other kind. Nor is it unlikely, that the fhort
voyages. time which many aerial voyagers have been able to
continue in the atmofphere may have been owing to
the want of a method of preferving this internal heat.
It may naturally be fuppofed, and indeed it has always
been found, that balloons, in paffing through the high¬
er regions of the atmofphere, acquire a very confider¬
able quantity of moiflure, not only from the rain or
fnoW they fometimes meet with, but even from the dewT
and vapour which condenfes upon them. On this an
evaporation will inflantly take place ; and, as it is the
property of this operation to produce a very violent
cold, the internal heat of the balloon mull be foon ex-
haufted in fuch a manner as to make it become fpeci-
fically heavier than the common atmofphere, and con¬
fequently defcend in a much fhorter time than it would
have done by the mere lofs of air. To this, in all pro¬
bability, we are to afcribe the defcent of the balloon
which carried Meffrs Blanchard and Jeffries j and
Practit
which feemed fo extraordinary to many people, that Great td
they were obliged to have recourfe to an imaginary
attraction in the waters of the ocean, in order to L>lve(,lmrcl,,,''|
the phenomenon. This fuppofition is rejeCted by Mr loon to c|
Cavallo ; who explains the matter, by remarking, thatfcer.d arJ!
in two former voyages made with the fame machine,
it could not long fupport two men in the atmofphere j
fo that we had no occafion to wmnder at its weaknefs
on this occafion. “ As for its rifing higher (fays he),
jull when it got over the land, that may be eafily ac¬
counted for. In the firft place, the two travellers
threw out their clothes juft about that time ; fecondly,
in confequence of the wind’s then increafing, the bal¬
loon travelled at a much greater rate than it had done
whilft over the fea \ which increafe of velocity leffen-
ed its tendency to defcend : befides which, the vicifli-
tudes of heat and cold may produce a very confider¬
able effeft ; for if we fuppofe, that the air over the
land was colder than that over the fea, the balloon
coming into the latter from the former, continued to
be hotter than the circumambient air for fome time
after j and confequently, it was comparatively much
lighter when in the cold air over the land, than
when in the hotter air over the fea j hence it floated
eafier in the former than in the latter cafe.”
It feems indeed very probable, that there was fome-
thing uncommon in the cafe of Mr Blanchard’s bal¬
loon while pafling over the fea ; for, as it rofe higher
after reaching the land than in any former period of
the voyage, and likewife carried them to a diftance
over land more than half of that which they had paf-
fed over water, we can fcarce avoid fuppofing, that it
had a tendency to defcend when over the water more
than when over land, independent of any lofs of air.
Now, it does not appear that the air over the fea is at
all warmer than that above land j on the contrary, there
is every reafon to believe, that the fuperior refledlive
power of the land renders the atmofphere above it warm¬
er than the fea can do : but it is very natural to fup¬
pofe, that the air above the fea is more moift than that
above land •, and confequently, by letting fall its moi-
fture upon the balloon, muft have occafioned an evapo¬
ration that would deprive the machine of its internal
heat, which it would partly recover after it entered
the warmer and drier atmofphere over land.
We fliall now proceed to the conftrudlion of aero-^on^™c'
ftatic machines; of which the fmaller are only for^aticn
amufement, or fome flight experiments, and are very chines,
eafily made. As in all of them, however, it is of the
utmoft confequence to have the weight as little as pof-
fible, the fhape becomes an objedl of great confidera-
tion. For this purpofe a fpherical figure has been ma-|?^t*ieir
thematically demonftrated to be the bell; as capable of a^e*
containing a greater quantity under a fmaller furface
than any other. Thus a perfedl fphere contains lefs
furface in proportion to its folidity than a fpheroid ; a
fpheroid lefs than a cylinder ; the latter lefs than a
cube ; and a cube ftill lefs than a parallelepiped. In
all cafes, therefore, where we can fill the whole capa¬
city of the balloon with air equally light, the fpherical
figure is undoubtedly to be preferred : and this holds
good with regard to all inflammable air balloons, whe¬
ther their fize be great or fmall; but in the rarefied
air ones, where the under part muft neceffarily be much
colder than the upper, the globular ftiape feems not fo
proper.
aerostation.