MECHANICS, 
Surface in the refervoir, is equal to the weight of a column 
ot air extending to the top of the atmofphere, and whole 
bafe is equal to the fame orifice. And fince the weight of 
this column of quickfilver, cateris paribus, is at its altitude, 
it follows that the weight of the air is proportional to the 
altitude of the mercury in the barometer. The altitude at 
which the mercury is fuftained in the barometer above the 
furface of the mercury in the refervoir is called the fandard 
altitude ; .and will be the fame in any number of tubes, 
whatever be their bore or their pofition, provided the tube 
be not fo very (lender as to expole the mercury to a fenfi- 
ble alteration from the capillary attraction. Seep. 72r. 
6. If a barometer-tube, inftead of being hermetically 
fealed at the top, be clofely covered with a piece of blad¬ 
der, the mercury will rife to the fame height as in a com¬ 
mon barometer; but, on piercing the bladder with a nee¬ 
dle fo as to admit the air, the mercury will fall ; for in 
this cafe the weight of the air preffes upon the mercury in 
the tube, and the weight of thefe two together mull ob- 
viouf.y preponderate over the contrary prelfure, and de- 
ftroy the equilibrium. 
Prop. XXXI. The preffure of the atmofphere varies at dif¬ 
ferent altitudes. —Let a gials tube, open at both ends, be 
put through a cork into a large phial containing a fmall 
quantity of coloured water; let the lower end of the tube 
be in the water; and let the cork and tube be clofely ce¬ 
mented to the neck of the bottle ; then blow through the 
tube till the quantity of air within the phial is fo increafed 
that the water will rife above the neck of the phial. Let 
this phial be placed in a veffel of fand, to keep the air 
w ithin of the fame temperature; then will the water Hand 
at different heights in the tube, according to the eleva¬ 
tion of the place where it is fet; whence it appears that 
the prefl'ureof the atmofphere varies at different altitudes. 
And the fame thing will appear more clearly in a fubfe- 
quent article. In alcending the mountain of Snowden, 
which is 3555 feet high, the barometer funk 3-8 inches. 
Hence the proportion of the fpecific gravity of air near 
the earth’s furface to that of water may be afcertained. 
Thus, if the difference in the height of the two places 
where the above experiment is made be 52. feet, and that 
difference caufe a variation of § of an inch in the height 
of the water ; it follows, that a column of water of f of an 
inch, or of a foot, is equiponderant to a column of air 
of 52 feet, having the fame bafe ; confeqnently the fpecific 
gravity of water is to that of air as 52 to or as 832 to 1. 
Prop. XXXII. Air is elajlic, or capable of compreffion and ex- 
panfion. —This is proved by various experiments: 1. By 
the great expantion of a fmall quantity of air in a bladder 
apparently nearly empty, when the air is removed from the 
external paj-ts in the receiver. 2. By the extrufion of a 
fluid from a glafs bubble, by the expanfion of a bubble 
of air contained in it. 3. By the expulfion of the white 
and yolk of an egg through a fmall hole in the little end, 
by the expanfion of the air contained in the greater end. 
4. By putting an almoft-emptied bladder into a fmall box, 
and laying a proper weight on the lid, which, on exhauft- 
ing the air, will be raifed up by the expanfion of the air 
in the bladder. 5. Alfoa bladder filled with air, and juft 
made to link with a weight, will upon exhauftion foon rife 
by the expanlion of the contained air. 6. Glafs bubbles 
and images filled with water fo as to make them juft fink 
in that fluid, will, on exhaufting the air from the furface, 
rife to the top of the veffel. 7. Beer, cyder, water, and 
porous bodies, emit great quantities of air under tile ex- 
liaulted receiver. 8. A fhriveiled apple, wdien put under 
an exhaufted receiver, will have its coat diltended by the 
internal air fo as to look fmooth. 9. If the open end of 
a tube, whole other end is clofed,be immerfed perpendi¬ 
cularly in water, the lpace occupied by the air will be di- 
minilhed, as the depth of the tube or the upward prelfure 
of the water is increafed ; or, if the (horter leg of a bent 
tube be clofed, and mercury poured into the longer, the 
air will be compreffed in the (horter leg into a fpaee con¬ 
tinually decreaflng as the quantity of prefling mercury is 
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increafed ; and, if fome of the mercury be taken from the 
longer leg, the air in the Ihorter will expand and occupy a 
proportionably larger fpaee. 10. The mercury may be 
raifed by the expanlion of a fmall quantity of confined dir 
to the fame height in an exhaufted tube above the air- 
pump, as that to which it is raifed in the mercurial gauge 
by the prelfure of the atmofphere below it. 
The limits of the condenfation and rarefaction of air by 
human powers are not afcertained. Dr. Hales found, that 
when dry wood was put into a ftrong veffel, which it al- 
moff filled, and the remainder was filled with water, the 
fwelling of the wood, occafioned by its imbibition of wa¬ 
ter, condenfed the air of his gauge into the thoufandth of 
its original bulk. He found that pc-afe treated in the fame 
way generated elaflic air, which preffing on the air in the 
gauge condenfed it into the fifteen hundredth part of its 
bulk. This is the greateft condenfation that has been af¬ 
certained with precilion, although in other experiments it 
has certainly been carried much further; but the precife 
degree could not be afcertained. The only life to be made 
of this obfervation at prefent is, that, fince we have been 
able to exhibit air in a denfity a thoufand times greater 
than the ordinary denfity of the air we breathe, it cannot, 
as fome imagine, be only a different form of water; for in 
this ltate it isasdenfe or denfer than water, and yet retains 
its great expanfibility. 
Another important obfervation is, that, in every ftate of 
denfity in which we find it, it retains its perfeit fluidity, 
tranfmitting all preffures which are applied to it with un- 
diminilhed force, as appears by the equality conftantly ob- 
ferved between the oppofing columns of water or other 
fluid by which it is compreffed, and by the facility with 
which all motions are performed in it in the molt com¬ 
preffed ftates in which we can make obfervations of this 
kind. This fad: is totally incompatible with the opinion 
of tliofe who aferibe the elafticity of air to the fpringy 
ramified ftrudrure of its particles, touching each other 
like fo many pieces of fponge or foot-balls. A colledion 
of fuch particles might indeed be pervaded by folid bodies 
wdth confiderable eafe, if they were merely touching each 
other, and not fubjeded to any external prelfure. But, 
the moment fuch prelfure is exerted, and the affemblage 
fqueezed into a fmaller fpaee, each prelfes on its adjoining 
particles; they are individually comprefled, flattened in 
their touching furfaces, and before the denfity is doubled they 
are fqueezed into the form of perfect cubes, and compofe 
a mats, which may indeed propagate prelfure from one 
place to another in an impeded: manner, and with great 
diminution of its intenfity, but will no more be fluid than 
a mafs of foft clay. 
Prop. XXXIII. The elafic force of the air is equal to the 
force of compreffion. —For, if the air be exhaulted from an 
open tube whole lower part is immerfed in a veffel con¬ 
taining mercury, the air within the velfel being.prevented 
from efcaping, then will this air by its elafticity force the 
mercury up the tube very nearly to the fame height as it 
would be raifed by the prelfure of the atmofphere. 
This Propofition is lometinies proved, independent of 
experiment, In the following manner: If the force with 
which the air endeavours to expand itfelf when it is coin- 
prelfed were lefs than the comprefting force, it would yield 
(till further to that force ; if it were greater, it could not 
have yielded fo far. Confequently, when any force has 
fo compreffed the air that it remains at relt, the force of 
the air arifing from its elafticity can neither be greater 
nor iefs than the comprefling iorce ; that is, it mult be 
equal to it. But it mull be eonfeffed that, in this cafe, 
the experimental proof is the molt latisfadory. See the 
article Aerology, vol. i. p. 142. 
Prop. XXXIV. Heat increafes the claflicity of the air, and 
cold diminifhes it ; or, heat expands, and cold condenfes, the air. 
—This property is likewife proved by experience. 1, 
Tie a bladder very clofe with fome air in it, and lay it be¬ 
fore the fire; then, as it warms it will more and more 
dillend the bladder, and at laft burit it, if the heat be con- 
3 t-imied* 
4 
