MECHANICS. 
722 
ing of greafe, fflould elevate the fluid in which it is im- 
merfed. But, as the intervention of an attenuated film of 
greafe deftroys capillary aftion, there is reafon to conclude, 
that it does not extend to fenfible diftances. The fame 
conclufion isdeducible from the fafl in the preceding pa¬ 
ragraph. 
From thefe fafils Laplace concludes, that the attraction 
of capillary tubes has not any influence on the elevation 
or depreflion of the fluids which they contain, except by 
determining the inclination of the firft planes of the fur- 
face of the interior fluid, which are extremely near the 
Tides of the tube. Ke fuppofes that, when the attraction 
of the tube upon the fluid exceeds the attraction of the 
fluid upon itfelf, the fluid will in that cafe attach itfelf to 
the tube, and form an interior tube, which alone will raife 
the fluid. 
“ It is interefting,” fays Laplace,” to afcertain the ra¬ 
dius of curvature of the furface of water included in ca¬ 
pillary tubes of glafs. This may be known by a curious 
experiment, which fhows at the fame time the effeCts of 
the concavity and convexity of furfaces. It confifts in 
plunging in water, to a known depth, a capillary tube of 
which the diameter is likewife know n. The lower ex¬ 
tremity of the tube is then to be clofed with the finger, 
and, the tube being taken out of the water, its external 
furface mull be gently wiped. Upon withdrawing the 
finger in this lalt fituation, the water is feen to fubfide 
in the tube, and form a drop at its lower bafe ; but the 
height of the column is always greater than the elevation 
of the wafer in the tube above the level in the common 
experiment of plunging it in water. This excels in the 
height is owing to the aCIion of the drop upon the column 
on account of its convexity ; and it is obfervable that the 
increafe in the elevation of the water is more confiderable, 
the fnialler the diameter of the drop beneath. The length 
of the fluid column which came out by fubfidence to form 
the drop, determines its mafs; and, as its furface is fphe- 
rical as well as that of the interior fluid, if we know the 
height of the fluid above the fummit of the drop, and the 
difiance of this fummit from the plane of the interior 
bore of the tube, it will be eafy to deduce the radii of 
thefe two furfaces. Some experiments lead me to con¬ 
clude that the furface of the interior fluid approaches very 
nearly to the figure of an hemifphere.” 
“ The theory which I have adopted,” obferves the fame 
philofopher, “ likewife gives the explanation and meafure 
of a Angular phenomenon prefented by experiment. Whe¬ 
ther the fluid be elevated or deprefied between two vertical 
planes, parallel to each other, and plunged in the fluid at 
their lotfer extremities, the planes tend to come together. 
Analyfis (hows us, that, if the fluid be raifed between 
them, each plane will undergo from without inwards a 
preflure equal to that of a column of the fame fluid, of 
which the height would be half the fum of the elevations 
above the level of the points of contaft of the interior and 
exterior furfaces of the fluid with the plane, and of which 
the bafe fliould be the parts of the plane comprifed be¬ 
tween the two horizontal lines drawn through thofe points. 
If the fluid be deprefled between the planes, each of them 
will in like manner undergo from without inwards, a pref- 
fure equal to that of a column of the fame fluid, of which 
tile height w ould be half the fum of the deprelfions below 
the level of the points of contact of the interior and exte¬ 
rior furfaces of the fluid with the plane, and of which the 
bafe thould be the part of the plane comprifed between the 
two horizontal lines drawn through thofe points.” 
PNEUMATICS ; or, the THEORY of ELASTIC FLUIDS. 
Of all the fenfible c.omprefiible fluids, air is the moft 
familiar; it was the firfi ftudied, and has-been the molt 
minutely examined. It has therefore been generally taken 
as the example of their mechanical properties ; while thole 
mechanical properties which are peculiar to any of them, 
and therefore charafteriftic, have ulually been treated as 
an appendix to the general fcien.ee of pneumatics. No 
objection occurs to us againfl this method, which wrll 
therefore he adopted in the prefent fedtion. 
Prop. XXX. Air is a ponderous fluid which furrounds the 
earth. —For, that it is a fluid is obvious, becaufe its parts 
are eafily moved, and yield to the fmallefi inequality of 
preflure; and that it is ponderous will appear from the 
following conliderations: 
1. It always accompanies this globe in its orbit round 
the fun, furrounding it to a certain diftance, under the 
name of the atmojphere , which indicates the being con- 
nedled with the earth by its general force of gravity. It 
is chiefly in confequence of this, that it is continually 
moving round the earth from eaft to weft, forming what is 
called the trade-wind. 
2. It is in like manner owing to the gravity of the air 
that it fupportsthe clouds and vapours which we fee con- 
ftantly floating in it. We have feen bodies of no incon- 
fiderable weight float, and even rife, in the air. Soap- 
bubbles and balloons filled with inflammable gas rife and 
float in the fame manner as a cork rifes in water. This 
phenomenon proves the weight of the air in the fame man¬ 
ner that the fwimming of a piece of wood indicates the 
weight of the water which fupports it. 
3. But we are not left to thefe refined obfervations for 
the proof of the air’s gravity. We may obl'erve familiar 
phenomena, which would be immediate confequences of 
the fuppolition that air is a heavy fluid, and, like other 
heavy fluids, prefles on the outlides of all bodies immerfed 
in, or 1 'urrounded by, it. Thus, for inftance, if we fhut 
the nozzle and valve-hole of a pair of bellows, after having 
fqueezed the air out of them, we fhall find that a very 
great force, even fome hundred pounds, is neceflary for 
feparating the boards. They are kept together by the 
preflure of the heavy air which furrounds them, in the 
lame manner as if they were immerfed in water. In like 
manner, if we flop the end of a fyringe after its pifton has 
been prefled down to the bottom, and then attempt to 
draw up the pifton, we fhall find a confiderable force ne- 
ceflary, viz. about 15 or 16 pounds for every fquare inch 
of the feftion of the fyringe. Exerting this force, we can 
draw up the pifton to the top, and we can hold it there; 
but, the moment we ceafe acting, the pifton ruffles down 
and (trikes the bottom. This is called J'udion, as we feel 
fomething as it were draw ing in the pifton ; but it is really 
the weight of the incumbent air preffing it in. And this 
obtains in every pofition of the fyringe; becaufe the air 
is a fluid, and prefles in every dirfeftion. Nay, it prefles 
on the fyringe as well as on the pifton ; and, if the pifton 
be hung by its ring on a nail, the fyringe requires force 
to draw’ it down (juft as much as to draw the pifton up) ; 
and if it be let go, it will fpring up, unlefs loaded with 
at leaft 15 pounds for every fquare inch of its tranlverfe 
fedtion. 
4. Let the air be exhaufted from a glafs veflel, and by 
means of a cock let the veflel be kept exhaulted : if the 
veflel be weighed while it is exhaufted, and then again 
when the air is re-admitted, there will be a manifeft dif¬ 
ference, exhibiting the weight of as much air as the veflel 
contained. 
5. If a glafs tube more than 31 inches in length, one 
end of which is clofed up, be filled with mercury, and be 
held vertically, the other extremity being immerfed in a 
veflel of the fame fluid, then the mercury in the tube will 
defeend from the upper extremity, and will remain fuf- 
pended at fome altitude between 28 and 31 inches from 
the furface of the external mercury : the fufpenfion of the 
mercury is occafioned by the preflure of the external air 
upon the furface of the mercury in the veflel ; when this 
preflure is removed by placing the tube and veflel under 
a receiver and exhaufting the air, the mercury, will fink, 
in the tube, and on re-admitting the air, wit! rife. This 
is called the Torricellian experiment. The inftrument here 
ufed is called a barometer ; becaufe the weight of a column 
of mercury whole bafe is the orifice of the tube, and al¬ 
titude equal to that of the mercury in the tube above the 
furface 
