jyo A E R O S X 
ment in knowledge, have now funk the fcience of aerofta- 
tion fo low in the opinion of mod: people, that, before giv¬ 
ing any account ot the mod proper method of condructing 
thefe machines, it may (eem neceffary to premife lbme- 
thing concerning the ufes to which they may poflibly be 
applied. Thefe, according to Mr. Cavallo, are the fol¬ 
lowing. 
“ The fmall balloons, efpecially thofe made of paper, 
and raifed by means of fpiritof wine, may ferve to explore 
the direction of the w'inds in the upper regions of the at¬ 
mofphere, particularly when it is a calm below': they may 
ferve for iignals in various circumflances, in which no other 
means can be ufed; and letters or other fmall things may 
be eafilv fent by them, as for indance from fltipsthat can¬ 
not fafely land on account of dorms, from befieged places, 
idands, or the like. The larger aerodatic machines ir.ay 
anfwer ail the above-mentioned purpofesina better man¬ 
ner; and they may, befides, be ufed as a Help to a perfon 
who wants to afcend a mountain, a precipice, or to cr.ofs a 
river ; and perhaps one of thofe machines, tied to a boat 
by a long rope, may be, in feme cafes, a better fort of fail 
than any that is ufed at prefent. The larged fort of ma¬ 
chines, which can take up one or more men, may evident¬ 
ly be fubfervient to various ceconomical and philofophical 
purpofes. Their conveying people from place to place 
with great fwiftnefs, and without trouble, may be of effen- 
tial ufe, even if the art of guiding them in a direction dif¬ 
ferent from that of the wind fhould never be difeovered. 
By means of thofe machines the diape of certain feas and 
lands may be better afeertained ; men may afcend to the 
tops of mountains they never vifited before ; they may he 
carried over marfliy and dangerous grounds ; they may by 
that means come out of a befieged place, or an idand ; and 
they may, in hot climates, afcend to a cold region of the 
atmofphere, either to refrefii themfelves, or toobfervethe 
ice, which is never feen below ; and in fliort, they may be 
thus taken to feveral places, to which human art hitherto 
knew of no conveyance. 
“ The philofophical ufes, to which thefe machines may 
be fubfervient, are numerous indeed ; and it may be fuf- 
ficient to fay, that hardly any thing which paffes in the at¬ 
mofphere is known with preciiion, and that principally for 
w'ant of a method of afeending into it. The formation of 
rain, of tlninder-dorms, of vapours, hail, inow, and me¬ 
teors in general, require to be attentively examined and af¬ 
eertained. The adtion of the barometer, the refraction 
and temperature of the air in various regions, the defeent 
of bodies, the propagation of found, &c. are fubjedts 
which all require a feries of obfervations and experiments, 
the performance of which could never have been properly 
expedted before the difeovery of aerodatic machines.” 
To thofe ufes we may add the gratification of curiofity 
and pleafure as a very drong inducement to the pradtice 
of an art, in which, with any tolerable degree of caution, 
there appears not to be the fmalled danger. Every one 
who has tried the experiment tedifies, that the beauty of 
the profpedt afforded by an afeent, or the pleafure of be¬ 
ing conveyed through the atmofphere, cannot be exceed¬ 
ed. No one has felt the lead of that giddinefs confequent 
Upon looking from the top of a very high building or of a 
precipice, nor have they any of the ficknefs arifing from 
the motion of a velfel at fea. Many have been carried by 
balloons at the rate of thirty, forty, or even fifty, miles an 
hour, without feeling the lead inconvenience, or even agi¬ 
tation of the wind ; the reafon of w hich is, that, as the ma¬ 
chine moves with nearly the velocity of the wind itfelf, 
they are always in a calm, and without uneafinefs. Some 
have apprehended danger from the electricity of the at¬ 
mofphere ; and have thought that a droke of lightning, 
or the fmalled cledtric fpark, happening near a balloon, 
might fet fire to the inflammable air, and dedroy both the 
machine and the adventurers. Mr. Cavallo has fuggeded 
feveral confiderations for diminidiingapprehenfions of this 
kind. Balloons have been already raifed in every feafon 
of the year, and even when thunder has been heard, with- 
A T I O N. 
out injury. In cafe of danger, the aeronauts may either 
defeend to the earth, or afcend above the region of the 
clouds and thunder-dorms.. Befides, as balloons are form¬ 
ed of materials that are not conductors of electricity, they 
are not likely to receive drokes, efpecially as by being en- 
compaded with air they dand infulated. Moreover, in¬ 
flammable air by itfelf, or unmixed with a certain quanti¬ 
ty of common air, will not bum; fo that, if an eledtric 
fpark fhould happen to pafs through the balloon, it would 
not fet fire to the inflammable air unlefs a hole were made 
in the covering. 
The general principles of aerodation are fo little differ¬ 
ent from thofe of hydrodatics, that it rbay feeni fuperdu- 
ous to infid much upon them. It is a fadt univerfally 
known, that when a body is immerfed in any fluid, if its 
weight be lefs than an equal bulk of that fluid, it will rife' 
to the furface ; but, if heavier, it will fink ; and, if equal,, 
it will remain in the place where it is left. For this rea¬ 
fon fmoke afeends into the atmofphere, and heated air ire 
that which is colder. The afeent of the latter is fhewn in¬ 
ti very eafy and fatisfaCtory manner by bringing a red-hot 
iron under one of the feales of a balance, by which the 
latter is indantly made to afcend ; for, as foon as the red- 
hot iron is brought under the fcale, the hot air being light¬ 
er than that which is colder, afeends, and (trikes the bot¬ 
tom, which is thus impelled upwards, and the oppodte 
fcale delcends, as if a weight had been put into it. 
Upon this Ample principle depends the whole theorv of 
aerodation ; for it is the fame thing whether we render 
the air lighter by introducing a quantity of heat into it, or 
inclofinga quantity of gas fpecificaliy lighter than the com¬ 
mon atmofphere in a certain fpace ; both will afcend, and 
for the fame reafon. A cubic foot of air, by the mod ac¬ 
curate experiments, has been found to weigh about 554- 
grains, and to be expanded by every degree of heat, mark¬ 
ed on Fahrenheit’s thermometer, about part of the 
whole. By heating a quantity of air, therefore, to 500 
degrees of Fahrenheit, we diall juft double its bulk when- 
the thermometer dands at fifty-four in the open air, and 
in the fame proportion we dial! diminifh its weight; and if 
fuch a quantity of this hot air be inclofedin abag, that the - 
excefs of the weight of an equal bulk of common air 
weighs more than the bag with the air contained, in it, 
both the bag and air will rife into the atmofphere, and 
continue to do fo until they arrive at a place where the 
external air is-naturally fo much rarefied that the weight 
becomes equal ; and here the whole will float. 
The power of hot air in railing weights, or rather that 
by which it is itfelf impelled upwards, may be Ihewn ire 
the following manner : Roll up a fheet of paper into a 
conical form, and, by thrufring a pin into it, prevent it 
from unrolling. Faden it by its apex under one of the 
feales of a balance by means of a thread, and, having pro¬ 
perly counterpoifed it by weights, put it into the oppofite 
fcale;- apply the flame of a candle underneath, you will 
indantly perceive the cone to arife, and it will not be 
brought into equilibrium with the other but by a much 
greater weight than thofe who have never feen the expe¬ 
riment would believe. If we try this experiment with 
more accuracy, by getting proper receptacles made which 
contain determinate quantities of air, we diall find (hat the 
power of the heat depends much more on the capacity of 
the bag which contains it than could well be fuppofed. 
Thus, let a cubical receptacle be made of a fmall wooden 
frame covered with paper capable of containing one foot 
of air, and let the power of a candle be tried with this as 
above directed for the paper cone. It will then be found 
that a certain weight may be raifed ; but a much greater- 
one will be raifed by having a receptacle of the fame kind 
which contains two cubic feet;. a (till greater by one of three- 
feet ; a yet greater by one of four feet, &c. and this even 
though the very lame candle be made ufe of; nor is it 
known to what extent even the power of this fmall flame 
might be carried. 
From thefe experiments it appears, that in the aeroda¬ 
tic 
