AEROSTATION. 
tic machines conftruJted on Montgolfier’s plan, it muft be 
an advantage to have them as large as pofiible; becaufe 
a fmaller quantity of tire will then have a greater effect in 
railing them, and the danger from that element, which in 
this kind of machines is chiefly to be dreaded, will be in 
a great meafure avoided. On this fubjeft it may be re¬ 
marked, that as the cubical contents as a globe, or any 
other figure of which balloons are made, increafe much 
more rapidly than their furfaces, there muft ultimately be 
a degree of magnitude at which the fmallett imaginable 
heat would raife any weight whatever. Thus, fuppofing 
any aeroftatic machine capable of containing 500 cubic 
feet, and the air within it to be only one degree hotter 
than the external atmofphere ; the tendency of this ma¬ 
chine to rife, even without the application of artificial 
heat, w ould be near an ounce. Let its capacity be increaf- 
ed lixteen times ; and the tendency to rife will be equiva¬ 
lent to a pound, though this may be done without making 
the machine fixteen times heavier than before. It is cer¬ 
tain, however, that all aeroftatic machines have a tendency 
to produce or preferve heat within them, which would by 
no means be imagined by thofe who have not made the ex¬ 
periment. When Meffrs. Charles and Robert made their 
longed aerial voyage of 150 miles, they had the curiofity 
to try the temperature of the air within their balloon, in 
comparifen with that of the external atmofphere; and at 
this time they found, that, w'hen the external atmofphere 
was 63°, the thermometer within the balloon flood at 104 0 . 
Such a difference of temperature mud have given a ma¬ 
chine of the magnitude which carried them a confiderable 
afcending power independent of any other caufe, as it 
amounted to forty-one grains on every cubic foot ; and 
therefore in a machine containing 50,000 fuch feet would 
have been almod 200 pounds. Hence we may eafily ac¬ 
count for what happened at Dijon, and is recorded by Mr. 
Morveau. “ A balloon, intended to be filled with inflam¬ 
mable air, being completed, w'as, by w r ay of trial, filled 
with common air, and in that date expofed to the atmof¬ 
phere. Now it was obferved, and indeed a fimilar obfer- 
vationhad been made before, that the air within the bal¬ 
loon was much hotter than the circumambient air: the 
thermometer in the former flood at 120°; whereas in the 
latter, even when the fun (hone upon it, the thermometar 
ftood at 84°. This (hewed a condderable degree of rare¬ 
faction within the balloon ; and confequennly it was fuf- 
peiled, that, by means of this rarefaction alone, efpecially 
if it were to increafe a little, the balloon might afcend. 
On the 30th of May, about noon, the wind, being rather 
ftrong, agitated the balloon fo that two men were employ¬ 
ed to take care of it; but, notwithdanding all their endea¬ 
vours, it efcaped from its confinement; and, lifting up 
about dxty-five pounds weight of cords, equatorial circle, 
&c. rofe many feet high, and, palling over feme houfes, 
went to the diftance of 250 yards, where at length it was 
properly fecured.'’ 
'1 his difference between the external and internal heat, 
being fo very condderable, muft have a great influence 
upon aeroftatic machines, and will undoubtedly influence 
thofe filled with inflammable air as well as the other kind. 
Nor is it unlikely, that the fliort time which many aerial 
voyagers have been able to continue in the atmofphere 
may have been owing to the want of a method of preferv- 
ing this internal heat. It may naturally be fuppofed, and 
indeed it has always been found, that balloons, in palling 
through the higher regions of the atmofphere, acquire a 
very condderable quantity of moiflure, not only from the 
rain or fnow. they fometimes meet with, but even from the 
dew and vapour which conderrfe upon them. On this an 
evaporation will inftantly take place ; and, as it is the pro¬ 
perty of thisoperation to produce a very violent cold, the 
internal heat of the balloon muft be foon exhaufted in fuch 
a manner as to make it become fpecifically heavier than the 
common atmofphere, and conlequentlv defcend in a much 
Ihortertime than it would have done by the mere lofs of 
air. To this in all probability we are to afcribe the de- 
Vfr 
fcent of the balloon which carried Meffrs. Blanchard and 
Jeffries ; and which feemed fo extraordinary to many peo¬ 
ple, that they were obliged to have recourfe to an imagi¬ 
nary attraction in the waters of the ocean, in order to folve 
the phenomenon. This fuppofition is rejected by Mr. 
Cavallo ; who explains the matter, by remarking, that, in 
two former voyages made with the fame machine, it could 
not long fupport two men in the atmofphere ; fo that we 
had no occafion to wonder at its weaknefs on this occafion. 
“ As for its ridng higher (fays he) juft when it got over 
the land, that may be eafily accounted for. In the firft 
place, the travellers threw out their clothes juft about.that 
time ; fecondly, in confequence of the wind’s then increas¬ 
ing, the balloon travelled at a much greater rate than it 
had done whilft over the fea ; which increafe of velocity 
leffened its tendency to defcend: beddes which, the vicif- 
fitudes of heat and cold may produce a very condderable 
efteCt; 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 feme time after ; and confequently, 
it was comparatively much lighter w'hen in the cold air 
over the land than when in the hotter air over the fea;, 
hence it floated eader in the former than it did in the lat¬ 
ter cafe.” 
It feems indeed very probable, that there was fomething 
uncommon in the cafe of M. Blanchard’s balloon while 
pafling over the fea ; for, as it rofe higher after reaching 
the land than in any former period of the voyage, and like- 
wife carried them to the diftance over land more than half 
of that which they hadpaffed over water, we can fcarcely 
avoid fuppodng, 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 ; on the con¬ 
trary, there is every reafon to believe, that the fuperior re¬ 
flective power of the land renders the-atmofphere above it 
warmer than the fea can do : but it is very natural to f up¬ 
pofe, that the air above the fea is more moift than that 
above land; and confequently, by letting fell its moiflure 
upon the balloon muff have occafioned an evaporation: 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 ffiall now proceed to the conftruflion of aeroftatic 
machines ; of which the fmaller are only for amufement, 
or fomq flight experiments, and are very eafily made. As 
in all of them, however, it is of the utmoft confequence to 
have the weight as little as pofiible, the fliape becomes an 
object of great confideration. For this purpofe a fpheri- 
cal figure has been mathematically demonftrated to be the 1 
bell; as capable of containing a greater quantity under a 
fmaller furface than any other. Thus a perfect fphere- 
contains lefs furface in proportion to its folidity than a 
fpheroid ; a fpheroid lefs than a cylinder ; the latter lefi 
than a cube ; and a cube (till lefs than a parallelopiped. 
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, whether 
their fize be great or final!; but in the rarefied-air ones, 
where the under part muft neceffarily be much colder than 
the upper, the globular fliape feems not fo proper. An 
inverted cone, or truncated pyramid, with the fmaller 
part undermoft, feems then to be moff proper, as it allows 
the heated air (which has a great tendency to expand as 
well as to afcend) to collect in the wide part at the top,, 
while the ufelefs furface in the low'er part, and w hich, in 
any other figure, would contain only the colderand heavier 
air, is thus thrown afide. I i feCt it has been found, that 
aeroftatic machines, raifed by means of rarefied air, 
w hen made of the fliape of a parallelopiped, or even one 
deviating (till more from the fhape of a globe, have anfwer- 
ed the purpofe as well as they could have been fuppofed 
to do, had ever fo much care been taken in forming 
tfeeiU: 
