AEROSTATION. 
bular, with a bent tube, but in the for,m of a 
grotesque human figure, and the blast pro- 
ceeds from the mouth. 
vEOLUS’S harp, or Eolian harp, a musi- 
cal instrument so named from its producing 
an agreeable harmony merely by the action 
of the wind. See Acoustics. 
AiRA, a fixed point of time, from which 
any number of years is begun to be reck- 
oned. See Chronology. 
AEROSTATION, in the modern appli- 
cation of the term, signifies the art of navi- 
gation through the air, both in its principles 
and practice. Hence also the machines 
which are employed for this purpose, are 
called aerostats, or aerostatic machines ; and 
on account of their round figure, air balloons. 
The fundamental principles of this art 
have been long and generally known ; al- 
though the application of them to practice 
seems to be altogether a modern discovery, 
It will be sufficient, therefore, to observe, 
in this place, that any body, which is spe- 
cifically, or bulk for bulk, lighter than the 
atmospheric air encompassing the earth, 
will be buoyed up by it, and ascend; but 
as the density of the atmosphere decreases, 
on account of the diminished pressure of 
the superincumbent air, and the elastic pro- 
perty which it possesses at different eleva- 
tions above the earth, this body can rise 
only to a height in which tire surrounding 
air will be of the same specific gravity with 
itself. In this situation it will either float, 
or be driven in the direction of the wind or 
current of air, to which it is exposed. An 
air-balloon is a body of this kind, the whole 
mass of which, including its covering and 
contents, and the several weights annexed 
to it, is of less specific gravity than that of 
the air in which it rises. Heat is well 
known to rarefy and expand, and conse- 
quently to lessen the specific gravity of the 
air to which it is applied ; and tire diminu- 
tion of its weight is proportional to the heat. 
One degree of heat, according to the scale 
of Fahrenheit’s thermometer, seems to ex- 
pand the air about one four-hundredth part ; 
and about 400, or rather 435, degrees of 
heat, will just double the bulk of a quan- 
tity of air. If, therefore, the air inclosed 
in any kind of covering be heated and 
consequently dilated to such a degree, as 
that the excess of the weight of an equal 
bulk of common air above the weight of 
the heated air, is greater than ffie weight 
of the covering and its appendages, this 
whole mass will ascend in the atmosphere, 
till, by the cooling and condensation of the 
included air, or the diminished density of 
the surrounding air, it becomes of the same 
specific gravity with the air in which it 
floats; and without renewed heat, it will 
gradually descend. If, instead of heating 
common air inclosed in any covering, and 
thus diminishing its weight, the covering be 
filled with an elastic fluid, lighter than at- 
mospheric air, so that the excess of the 
weight of an equal bulk of the latter above 
that of the inclosed elastic fluid be greater 
than the weight of the covering and its ap- 
pendages, the whole mass will, in this case, 
ascend in the atmosphere, and continue to 
rise till it attains a height at which the sur- 
rounding air is of the same specific gravity 
with itself. Inflammable air, or, as it is called, 
hydrogen gas, is a fluid of this kind. For 
the knowledge of many of its properties we 
are indebted to Mr. Henry Cavendish, who 
discovered that, if common air is eight 
hundred times lighter than water, inflam- 
mable air is seven times lighter than com- 
mon air ; but if common air is eight hun- 
dred and fifty times lighter than water, 
then inflammable air is 10.8 times lighter 
than common air. The construction of air- 
balloons depends upon the principles above 
stated ; and they are of two kinds, as one 
or the other of the preceding methods of 
preparing them is adopted. 
In the various schemes that have been 
proposed for navigating through the air, 
some have had recourse to artificial wings ; 
which, being constructed like those of birds” 
and annexed to the human body, might 
bear it up, and by their motion, produced 
either by mechanical springs, or muscular 
exertion, effect its progress in any direction 
at pleasure. This is one of the methods of 
artificial flying suggested by Bishop Wil- 
kins, in the seventh chapter of his “ De- 
dalus, or Treatise on Mechanical Motions ;” 
but the success of it is doubtful, and expe- 
riments made in this way have been few 
and unsatisfactory. Borelli having com- 
pared the power of the muscles which act 
on the wings of a bird with that of the 
muscles of the breast and arms of a man, 
finds the latter altogether insufficient to 
produce, by means of any wings, that 
motion against the air, , which is necessary 
to raise a man in the atmosphere. Soon 
after Mr. Cavendish’s discovery of the spe- 
cific gravity of inflammable air, it occurred 
to the ingenious Dr. Black, of Edinburgh, 
that if a bladder, sufficiently light and thin, 
were filled with this air, it would form 
a mass lighter than the same bulk of at- 
