PNEUMATICS. 
gree of the thermometer above or below 
temperature : hence the cubic foot of air, 
of water, and of quicksilver, may be 
taken as li ounce, 1000 ounces, and 13,600 
ounces. 
The gravity of the air being once known, 
it should seem that it could not be difficult 
to infer that the ascent of water, in the 
body of a pump, must be occasioned by the 
pressure of that fluid. This, however, was 
not the case : Galileo had no notion of it. 
Some Italian conduit-makers being asked 
if they would construct sucking-pumps, 
whose tubes should be more than 33 feet in 
height, remarked, with surprise, that the 
water refused to rise above that limit. They 
requested of Galileo the explication of this 
singular fact; and it is affirmed that the 
philosopher, being taken unawares, replied 
that nature did not entertain the horror of 
a vacuum beyond 33 feet. Torricelli, a 
disciple of Galileo, having meditated upon 
this phenomenon, conjectured that water is 
elevated in pumps by the pressure of the 
exterior air ; and that this pressure has only 
the degree of force necessary to coun- 
terbalance the weight of a column of water 
of 33 feet. He verified this conjecture by 
an experiment, for which natural philosophy 
owes him a double obligation, since it serves 
to render evident an important discovery, 
while it has procured us the barometer. 
Torricelli saw the mercury stand §9 or 30 
inches in a glass tube, sealed at its upper 
part, and situated vertically; and the height 
thus under consideration being to that of 
33 feet in the inverse ratio of th^ densities 
of water and of mercury ; he concluded 
that the phenomenon belonged to statics, 
and that it was really, as he had conjectur- 
ed, the pressure of the air which caused 
water or mercury to rise until an equili- 
brium was produced : this occurred in 164.3. 
The year following, the news of Torricelli’s 
experiment was disseminated in France by 
a letter written from Italy to Father Mer- 
senne. The experiment was performed 
again in 1646, by Mersenne and Pascal; 
and tlie latter devised, in 1647', a method of 
rendering it still more decisive by making 
it at different altitudes. He invited, in con- 
sequence, his friend Perrier to repeat the 
experiment upon the mountain Puy-de- 
Dome, and to observe whether the column of 
mercury would descend in the tube in pro- 
portion as it became more elevated. We 
may see from the letter of Pascal to Per- 
rier, where he seems to avoid the name of 
Torricelli, that he had not yet entirely re- 
nounced the chimera of the horror at a 
vacuum which was attributed to nature, 
and that by admitting that this horror was 
not invincible, he was not bold enough to 
assert that it never obtained. The success 
of the experiment completely removed tlie 
delusion. Yet this experiment was only a 
confirmation of that by Torricelli, and 
therefore yielded an additional ray to the 
stream of light which issued from it. The 
pressure of the atmosphere, upon a given 
surface, being nearly the same as would be 
exerted upon that .surface by a column of 
water of 33 feet high ; from this datum has 
been computed the effect of the pressure 
under consideration, with respect to a man 
of medium magnitude, and it has been 
found that it is equivalent to a weight of 
about 33,600 pounds. Considerable as this 
weight is, its pressure is exerted, unknown 
to us, because it is continually balanced by 
the re-action of the elastic fluids comprised 
in the interior cavities of our bodies ; and 
though the air is subject to continual varia- 
tions, which augment or diminish its den- 
sity, in consequence of changes of tempera- 
ture, and of the action of different natural 
causes, yet as these variations are generally 
confined within narrow limits, and succeed 
each other with comparative tardiness, they 
do not affect us commonly, except in a 
manner scarcely perceptible. But if there 
happen a sudden change, as when a man is 
raised to great heights,the rupture of the equi 
librium which ensues has a very marked in- 
fluence upon the animal economy. He then 
experiences an extreme fatigue, an absolute 
inability to continue his progress, a drowsi- 
ness under which he sinks in spits of him- 
self; the respiration becomes thick and dif- 
ficult; the pulsations take an accelerated 
motion. To explain these effects, it must 
be considered that the state of well-being, 
in all that depends upon respiration, re- 
quuss that a determinate quantity of air 
should pass through the lungs in a given 
time. If, therefore, the air that we respire 
becomes much more rare, the inspirations 
must of necessity be proportionally more 
frequent ; which will render the respiration 
more difficult, and will occasion the various 
symptoms to which we have referred. With 
regard to the inconveniences that wou'ld 
result from an air too condensed, man is 
not exposed to them by the action of natu- 
ral causes ; and it appears that, in general, 
they are less than those which are caused 
by the rarefaction of the air. 
We need only cite here, as a proof of tlie 
