ATMOSPHERE. 
eouid we ourselves exist without it. Nature 
indeed, and the constitutions and principles 
of matter, would be totally changed if this 
fluid were wanting. 
The mechanical force of the atmosphere 
is of great importance in the affairs of men, 
who employ it in the motion of their ships, 
in turning their mills, and in a thousand 
other ways connected with the arts of life. 
It was not till the time of Lord Bacon, who 
taught his countrymen how to investigate 
natural phenomena, that the atmosphere be- 
gan to be investigated with any degree of 
precision. Galileo introduced the study by 
pointing out its weight ; a subject that was 
soon after investigated more completely by 
Torricelli and others. Its density and elas- 
ticity were ascertained by Mr. Boyle and 
the academicians at Florence. Mariotte 
measured its ddatibilily ; Hooke, Newton, 
Boyle, and Derham, shewed its relation to 
light, to sound, and to electricity. Sir I. 
Newton explained the effect produced upon 
it by moisture, from which Halley attempted 
to explain the changes iu its weight indi- 
cated by the barometer. 
Tire atmosphere, we have said, envelops 
the whole surface of the earth, and if they 
were both at rest, then the figure of the at- 
mosphere would be globular, because all the 
parts of the surface of a fluid in a state of 
rest must be equally removed from its cen- 
tre. But as the earth and the surrounding 
parts of the atmosphere revolve uniformly 
together about their axis, the different parts 
of both have a centrifugal force, the ten- 
dency of which is more considerable, and 
that of the centripetal less, as the parts are 
more remote from the axis, and hence the 
figure of the atmosphere must become an 
oblate spheroid, since the parts that corres- 
pond to the equator are farther removed 
from the axis than the parts which corres- 
pond to the poles. The figure of the atmos- 
phere must also, on another account, repre- 
sent a flattened spheroid, namely, because 
the sun strikes more directly the air which 
encompasses the equator, and is compre- 
hended between the two tropics, than that 
which pertains to the polar regions : hence 
it follows, that the mass of air, or part of the 
atmosphere adjoining to the poles, being less 
heated, cannot expand so much nor reach so 
high. Nevertheless, as the same force which 
contributes to elevate the air diminishes its 
gravity and pressure on the surface of the 
earth, higher columns of it about the equa- 
torial parts, other circumstances being the 
same, may not be heavier than those about 
the poles. Mr, Kit-wan observes, that in 
the natural state of the atmosphere, that is, 
when the barometer would, every where at 
the [level of the sea, stand at 30 inches, the 
weight of the atmosphere at the surface of 
the sea must be equal all over the globe ; 
and in order to produce this equality, as the 
weight proceeds from its density and height, 
it must be lowest where the density is great- 
est, and highest where the density is least, 
that is, highest at the equator and lowest at 
the poles, witli the intermediate gradations. 
On this and other accounts, in the highest 
regions of the atmosphere, the denser equa- 
torial air not being supported by the collar 
teral tropical columns, gradually flows over 
and rolls down to the north and south; these 
superior tides have been supposed to consist 
of hydrogen gas, inasmuch as it is much 
lighter than any other, and is generated in 
great plenty between the tropics; it is also 
supposed to furnish the matter of the auro- 
ras borealis and australis. 
With regard to the weight and pressure of 
the atmosphere, it is evident that the whole 
mass, in common with all other matter, must 
be endowed with weight and pressure : and 
it is found by undeniable experiments, that 
the pressure of the atmosphere sustains a 
column of quicksilver in the tube of a baro- 
meter of about 30 inches in height; it ac- 
cordingly follows, that the whole pressure of 
the atmosphere is equal to the weight of a 
column of quicksilver of an equal base, and 
30 inches in height, or the weight of the at- 
mosphere on every square inch of surface is 
equal to 15 pounds. It has moreover been 
found, that the pressure of the atmosphere 
balances, in the case of pumps, &c. a column 
of water 34§ feet high; and the cubical foot 
of water weighing just 1000 ounces, or ex- 
pounds, 34i multiply ed by 62J, or 2158 lb. 
will be the weight of a column of water, or 
of the atmosphere on the base of a square 
foot; and consequently the 144th part of 
this, or 15 lb. is the weight of the atmosphere 
on a square inch. From these data, Mr. 
Cotes computed the pressure of the atmos- 
phere on the whole surface of the earth to 
he equivalent to that of a glqbo of lead 60 
miles in diameter. Dr. Vince and others 
have given the weight at 7767029? 9735634.29. 
tons. This weight is however variable; it 
sometimes being much greater than at 
others. If the surface of a man, for instance, 
be equal, to 14j square feet, the pressure up- 
on him, when the atmosphere is in its light- 
est state, is equal to 13j tons, and when in the 
heaviest, it is about 14 tons and one-third"; 
the difference of which is about 2464 lb. It 
is surprizing that such weights should be able 
