PNEUMATICS. 
small magnitude of these inconveniences, 
that which happens to divers, when they 
have been shut up witliin a bell which de- 
scended vertically in the water, and in 
which the air, pressed by the weight of the 
surlfeunding columns, contracts itself more 
and more, in proportion as the vessel is 
found at a greater depth. The accidents 
which have occurred to those who have 
continued for a certain time under the bell, 
have arisen in great part from the alteration 
produced in the air by respiration, and that 
which was most dangerous in this fluid was 
the defect of renewing it. See Diving bell, 
Barometer, &c. 
The elasticity of the. air, is verified by 
several well-known experiments. One of 
the most ordinary is that in which we em- 
ploy the machine called the ar tificial foun- 
tain. It consists of a metallic vessel of a 
rounded form, its sirmmit being pierced with 
an orifice, through which the vessel may 
be filled with water to about two-tliirds 
of its capacity. In this aperture a tube is 
then fixed, which descends into the vessel 
until it is within a little distance of the bot- 
tom, while its upper part, which projects 
from the orifice, is furnished with a cock. To 
this same part a tbrcing pump is adapted, 
and the cock being opened, a great quan- 
tity of air is injected into the vessel: 
this air, being lighter than water, rises 
above it, and its elasticity augments with 
its density, in proportion as new strokes 
are given to the piston. Then after closing 
the cock, the pump is removed, and a 
kind of little hollow cone is substituted for 
it, open at its summit, which is turned 
upwards; as soon as the cock is again 
opened, the condensed air exerts its force 
upon the surface of the water, and drives 
it through the canal that is immersed 
into that liquid, whence it is seen to shoot 
out, under the form of a jet of more than 
twenty or thirty feet in height. An ana- 
logous elFect may he obtained, solely by 
diminisliing the natural elasticity of the air, 
by placing under the receiver of an air- 
pump a little vessel, in which all is similar 
to what the artificial fountain presents at 
the moment when the cock is opened to 
give a free passage to the water, except 
that the air situated aboye this hquid is 
in its ordinary state. 
Wiiile the exhaustion is going on, the air 
included in the vessel, and whose pressure 
upon the water is no longer balanced by 
that of tlie exterior air, dilates itself, and 
gives birth to a jet which rises under the 
receiver. (See fig. 5). But the most in- 
teresting experiment relative to this object 
is that of Boyle, and of Mariotte, to show 
that the air contracts itself nearly in the 
ratio of the weights with which it is pr&sed. 
These kinds of experiments merit llie pre- 
ference, since they are not confined to 
merely proving the existence of a pheno- 
menon, but make known also how it exists, 
by determining the law to which it is sub- 
ject.! akeaglass tube a 6 (Plate Pneumatics, 
fig- i ), bent into two branches, the shortest 
of which is about twelve inches high; it 
must he equally thick throughout, and her- 
metically sealed at its extremity b. The 
other branch, which is open at a, should 
be at least five feet, but if it were eight 
feet in height, so much the better. The 
whole is fixed upon a plate which carries divi- 
sions adapted to the two tubes. First, let 
there he poured into the bent part a little 
mercury, to obtain a line of level, xz, that 
we may estimate the number of degrees 
comprised between that line and the supe- 
rior extremity of the shortest branch. In 
this state of things the air which occupies 
that branch maintains an eqnilibrinni by its 
elasticity, with the piessnre of the column 
of atmospheric air gravitating in the other 
branch, and whose pressure is transmitted 
by means of the mercury comprised in the 
inferior curvature. This pressure, as we 
have seen in the article Barometer, is 
equal to that of a mercurial column of about 
twenty-nine or thirty inches in height. 
Afterwards, let mercury be poured into the 
longest branch, and at tlie same time the 
air in the other branch will be condensed; 
by the excess of the resulting pi essnre the 
mercury will rise in the shorter branch 
until an equilibrium is again produced. 
Then measure, on one part, the length of 
that column of compressed air, and on the 
other the excess of the column of mercury 
contained in the longest branch, above tliat 
whicli occupies the shortest. We will sup- 
pose, for more simplicity, that this excess 
is equal to thirty inches ; in that case, we 
shall find that the column of compressed 
air is reduced to the half of the height 
which it occupied previously to the introduc- 
tion of the fresh mercury. But that column 
is charged with a weight double of the 
former, since a pressure of thirty inches of 
mercury is added to an equal, pressure 
exerted by the atmospheric air, and which 
is not considered as being diminished ; for 
we may neglect the small difference which 
results from this, that the thirty inches 
