A T M O S 
difference between the air of the different places obferved 
by him: notwithftanding this great change, as he obferv¬ 
ed, and as he was informed by various perfons, no parti¬ 
cular change of health in the generality of people, or fa¬ 
cility of breathing, was perceived. M. Fontana, there¬ 
fore, concludes with obferving, that “Nature is not fo 
partial as we commonly believe. She has not only given 
us an air almoft equally good every where at every time, 
but has allowed us a certain latitude, or a power of living 
and being in health in qualities of air which differ to a cer¬ 
tain degree. By this 1 do not mean to deny the ex'ihence 
of certain kinds of noxious air in forne particular places; 
but only fay, that in general the air is good every where, 
and that the fmall differences are not to be feared fo much 
as fome people would make us believe. Nor do I mean 
to fpeak here of fome vapours and other bodies which are 
accidentally joined to the common air in particular places, 
but do not change its nature and intrinfteal property. This 
Hate of the air cannot be known by the ted of nitrous air; 
and thofe vapours are to be coniidered in the fame man¬ 
ner as we fhould confider fo many particles of arfenic 
fvvimming in the atmofphere. In this cafe it is the arfe¬ 
nic, and not the degenerated air, that would kill the ani¬ 
mals who ventured to breathe it.” 
With regard to the figure of the atmofphere, as it en¬ 
velopes all parts of the furface of the earth, if they both 
continued at reft, and were not endowed with a diurnal 
motion about their common axis, then the atmofphere 
would be exactly globular, according to the laws of gra¬ 
vity ; for all the parts of the furface of a fluid in 3<ftate 
of reft mud be equally removed from its centre. / But, 
as the earth and the ambient parts of the atmofpjfere re¬ 
volve uniformly together about their axis, the different 
parts «f both have a centrifugal force, the tendency of 
which is more confiderable, and that of the centripetal 
lefs, as the parts are more remote from thq. axis; and 
hence the figure of the atmofphere muft become an oblate 
fpheroid; fince the parts that correfpond to the equator 
are farther removed from the axis than the parts which 
correfpond to the poles. Befides, the figure of the atmof¬ 
phere mud, on another account, reprefenc a flattened fphe¬ 
roid, namely, becaufe the fun ftrikes more directly the air 
which encompafles the equator, and is comprehended be¬ 
tween the two tropics, than that which pertains to the po¬ 
lar regions : for, from hence it follows, that the mafs of 
air, or part of the atmofphere, adjoining to the poles, be¬ 
ing lefs heated, cannot expand fo much, nor reach fohigh. 
And yet, notwithftanding, as the fame force which contri¬ 
butes to elevate the air, diminifhes its gravity and preflure 
on the furface of the earth, higher columns of it about 
the equatorial parts, all other circumftances being the 
fame, may not be heavier than thofe about the poles. In 
the Tranladions of the Royal Irifli Academy for 1788 
Mr. Kirwin has an ingenious diflertation on the figure, 
height, weight, &c. of the atmofphere. He obferves 
that, in the natural ftate of the atmofphere, that is, when 
the barometer would every where, at the level of the fea, 
Hand at thirty inches, the weight of the atmofphere, at 
the furface of the fea, mu ft be equal all over the globe ; 
and, in order to produce this equality, as the weight pro¬ 
ceeds from its denfity and height, it mud be loweft where 
the denfity is greateft, and higheft where the denfity is 
leaft; that is, higheft at the equator and loweft at the 
poles, with feveral intermediate gradations. 
Though the equatorial air however be lefs denfe to a 
certain height than the polar, yet at fome greater heights 
it mult be more denfe : for, fince an equatorial and polar 
column are equal in total weight or mafs, the lower part 
of the equatorial column, being more expanded by heat, 
&c. than that of the polar, mud have lefs mafs, and there¬ 
fore a proportionably greater part of its mafs muft be found 
in its luperior feition; fo that the lower extremity of the 
fupericr feCfion of the equatorial column is more compref- 
fed, and confequently denfer, than the correfponding part 
of the polar column. The fame thing is to be underftood 
P H E R E. 475 
alfo of the extra-tropical columns with refpefft to each 
other, where differences of heat prevail. Hence, in the 
higheft regions of the atmofphere, the denfer equatorial 
air, not being fupported by the collateral extra-tropical 
Columns, gradually flows over, and rolls down to the north 
and foutli. Thefe fuperior tides confift chiefly of inflam¬ 
mable air, as it is much lighter than any other, and is ge¬ 
nerated in great plenty between the tropics; it furnilhes 
the matter of the aurorae borealis and auftralis, by whole 
combuftion it is deftroyed, elfe its quantity would in time 
become too great, and the weight of the atmofphere an¬ 
nually increased ; but its combuftion is the primary fource 
of the greateft perturbations of the atmofphere. 
As to the weight and prejj'ure of the air, it is evident, 
that the mafs of the atmofphere, in common with all other 
matter, muft be endowed with weight and preflure; and 
this principle was alferted by almoft all philofophers, both 
ancient and modern. But it was only by means of the ex¬ 
periments made with pumps and the barometrical tube, by 
Galileo and Torricelli, that we came to the proof, not on¬ 
ly that the atmolphere is endued with a preflure,- but alfo 
what the meafitre and quantity of that preflure is. Thus 
it is found, that the preflure of the atmofphere fuftains a 
column of quickfilver, in the tube of the barometer, of 
about thirty inches in height; it therefore follows, that 
the whole preflure of the atmofphere is equal to the weight 
of a column of quickfilver, of an equal bafe, and thirty- 
inches height : and, becaufe a cubical inch of quickfilver 
is found to weigh nearly half a poundfttvoirdupoife, there¬ 
fore the whole thirty inches, or the weight of the atmof¬ 
phere on every fquare inch Of furface, is equal to i $lb. 
Again, it has been found that the preflure of the atmof¬ 
phere balances, in the cafe of pumps, &c. a column of wa¬ 
ter of about 34§ feet high ; and, the cubical foot of wa¬ 
ter weighing juft 1000 ounces, or 6 i±\b. 34J times 62A, or 
21581b. will be the weight of the column of water, or of 
the atmofphere, on a bafe of a fquare foot ; and confe¬ 
quently the 144th part of this, or 151b. is the weight of 
the atmofphere on a fquare inch; the fame as before. 
Hence Mr. Cotes computed, that the preflure of this am¬ 
bient fluid on the whole furface of the earth, is equivalent 
to that of a globe of lead of fixty miles in diameter. And 
hence alfo it appears, that the preflure upon the human 
body muft be very confiderable ; for, as every fquare inch 
of furface fuftains a preflure of 15U). every fquare foot 
will fuftain 144 times as much, or 2i6olb. then, if the 
whole furface of a man’s body be fuppofed to contain fif¬ 
teen fquare feet, which is pretty near the truth, he muft 
fuftain 15 times 2160, or 324001b. that is, near 14J tons 
weight, for his ordinary load. By this enormous preflure 
we fhould undoubtedly be cruflied in a moment, if all parts 
of our bodies were not filled either with air or forne other 
elaftic fluid, the fpring of which is juft fufficient to coun¬ 
terbalance the weight of the atmofphere. But, whatever 
this fluid may be, it is certain, that it is juft able to coun¬ 
teract the weight of the atmofphere, and no more: for, it 
any confiderable preflure be fuperadded to that of the air, 
as by going into deep water, or the like, it is always fe- 
verely felt let it be ever fo equable, at leaft when the change 
is made fuddenly; and if, on the other hand, the preflure 
of the atmofphere be taken off from any part of the hu¬ 
man body, as the hand forinftance, when put over an open 
receiver, front whence the air is afterwards extracted, the 
weight of the external atmofphere then prevails, and we 
imagine the hand ftrongly fucked down into the glafs. 
The difference in the weight of the air u ltich our bodies 
fuftain at one time more than another, is alfo very confi¬ 
derable, from the natural changes in the ftate of the at¬ 
mofphere. This change takes place chiefly in countries 
at fome diftance from the equator; and, as the barometer 
varies at times from twenty-eight to thirty-one inches, or 
about one-tenth of the whole quantity, it follows, tha£ 
this difference amounts to about a ton and a half on the 
whole body of a man, which he therefore fuftains at one 
time more than at another. On the increafe of this natural 
weight. 
