A T M

544

A T M

merable operations, sublimation':, separations, compositions, diges-tions, fermentations, putrefactions, Sec. The discoveries of mdern chemists have however shewn us," that atmospiieric air has

for its essential constituents,

By measure. By weight.

Nitrogen gas- 77.50 - 75.55

Oxygen gas - 21.00 23.32

Aqueous vapour ... 1.42 1.03

Carbonic acid gas .Ob - .10

100 . 100 .

It is thought by some that a small portion of hydrogen also ex-ist: in the atmosphere. The mechanical force of this fluid is otgreat importance, being employed in communicating motion toships, turning mills, and in various other ways serving the purposes^of life. The several phenomena of the atmosphere have been :1c-curatelv examined, and described by different philosophers sincethe time of lord Bacon . Were the earth and atmosphere at rest,and left entirely to the gravity of their own parts, they would na-turally assume a globular form, but ou account of the greater cen-trifugal force about the equatorial parts arising from the diurnalrotation, the atmosphere as well as the earth, will resolve itself intoa spheroidical figure whose greater axis passes through the equator.

A very great elevation of the atmosphere in the regions about thetorrid zone will be produced by the degree of heat to which it isexposed in consequence of receiving the direct rays of the sun. )The atmosphere likewise continually changes its figure from the |attractions of the sun and moon,which must affect the'air as well asthe water, and produce tides in this fluid. To -these might beadded other secondary causes of variation, such as tire direction ofthe wind, nature of the soil in particular places, See/ Mr. Kirwanobserves, that in the natural state of the atmosphere, that is, whenthe barometer would, every w here-at the level of the sea, stand at30 inches, the weight of the atmosphere at the surface of the sea jmust be equal all over the globe; and in order to produce thisequality, as the weight proceeds from its density and height, it-myst be lowest where the density is greatest, and highest wherethe density is least, that is, highest at the. equator, and lowest atthe poles, with the intermediate gradations. On this and other ac-counts, in the highest regions of the atmosphere, the denser equato-rial air not being supported by the collateral tropical columns, gra-'dually flows over and rolls down to the north and south; these supe-rior tides have been supposed to consist, in a great measure, of hydro-gen gas, and to furnish the matter of the aurora borealis and austra-lis. The weight and pressure of the atmosphere, in common with allother matter, arises from the general principle of gravity; it is foundby undeniable experiments, that the pressure-of the atmospheresustains a column of quicksilver in the tube of a barometer of 1about 30 inches in height; the same is also deduced trom thepressure of the atmosphere on the surface of water which sup-ports in pumps a column of water 34 j feet high. from these data,Mr. Cores computed the pressure of the atmosphere on the wholesurface of the earth to be equivalent to that of a globe of lead 00miles in diameter. Or. Vince and others have given the weightat 77070237973503429 tons. This weight is however variable itsometimes being much greater than at others. If the surface of aman, for instance, be equal to 14J square feet, the pressure Uponhim, when the atmosphere i< in it< lightest state, is equal to 13^tons, and when in the heaviest 14 j- Ions ; the difference is less nearthe equator and more at a greater distance from it. Such weightswould certainly destroy the human frame, if all the parts of ourbody were not endowed w ith some elastic spring, whether of air |or other fluid, sufficient to counterbalance the w eight of the atmos- Iphere, altering its force as tiie density of the atmosphere varies.— iIn countries at a considerable distance from the equator, the baro-meter varies in its height, from 23 to 31 inches, or about one-lothof the whole,; so that the pro-sure of the atmosphere will he some-time, one-10th ot its whole weight less than at other times, this dif- jference amounts to about a ton ami a half upon the body of a man.On the increase ofthis natural weight the weather is commonly flue,and we feel as we sav braced and more active; but on tiie contrary,when the weight of’the air diminishes, the weather is had, ;.udpeople feel a listnessness and inactivity about them. Hence w henthe variations of the barometer and atmosphere are sudden andgreat, we feel the alteration and effect ou our hollies and spirits

very much ; but when a change takes place by slow degree:, weare scarcely sensible of it, by reason of the power which we haveof accommodating ourselves to this and many other changes in thecircumstances of life. Thus when we ascend gradually from thebottom to the top of a mountain, we fed no inconvenience, al-though tiie pressure be' considerably diminished, but such as haveascended rapidly to great heights in balloons, or descended to thebottom of the sea in diving-bells, have been affected most sen-sibly by the change of pressure upon their bodies. It is difficultto assign the true reason for the variations that happen in the gra-vity of the atmosphere at the same place. One cause seems to bethe heat of the sun, for where this is uniform, the changes are smalland regular, 'rims between the tropics the barometer sinks abouthalf an inch every day, and rises to its former height in the night-time, but in the temperate zones, the barometer ranges between28 and 31 inches, shewing by its various altitudes, the clnngesabout to take place in the state of the weather, therefore we mayconclude that the causes which influence the weather, also affectthe gravity of the atmosphere. If the air had no elasticity, hutwere every where of the same density, from the surface of the'earth to the top of the atmosphere, it would be an easy matter todetermine its height, from knowing jls density, and the column 6fmercury it can sustain in the barometer; for since the weight of acolumn of the atmosphere is equivalent to a column of nn’rcurvupon the same base, and 2-| feet high, and since mercury is 11040times heavier than air, it follows that the. height of the atmospherewould be 11040 times 21 feet, or a little more than 5J miles!—But because, the .air is elastic, and the spaces which it occupies,when compressed by different weights, are reciprocally propor-tional to the weights themselves, the more the air is pressed, theless space it takes up, hence the air in the superior regions of theatmosphere must decrease in density, and according to that law itmust extend to an indefinite height. Now, if we suppose theheight of the whole divided into innumerable equal parts, thequantity of each part will he as its density : and tiie weight of thewhole incumbent atmosphere being also as its density : it follows,I that the weight of the incumbent air is every where as tire quan-tity contained in the subjacent part; which causes a difference be-tween the weights of each two contiguous parts of air. But bv atheorem in arithmetic, when a magnitude is continually diminishedby the like part of itself, and the remainders tiie same, these will hea series of continued quantities decreasing in geometrical progres-sion ; therefore if, according to the supposition, the altitude of theair, by the addition of new parts into which it is divided, continu-ally increases in arithmetical progression, its density will he dimi-nished, or, which is the same thing, its gravity decreased, in conti-nued geometrical proportion. And hence, again, it appears that,according to The hypothesis of the density being always propor-tional to the compressing force, the height of the atmosphere mustnecessarily be extended indefinitely. And farther, as an arithme-'tical series adapted to a geometrical one, is analogous to tlui lo-garithms of the geometrical one ; it follows therefore that the alti-tudes afe proportional to lire logarithms of the densities, or weights,of air ; and that any height taken from the earth’s'surface, wnichis the difference of two altitudes to tile top of the atmosphere, isproportional to the difference of the logarithms of the two densi-ties there, or to the logarithm of the ratio of those densities, ortheir corresponding compressing forces, as measured by the twoheights of tiie barometer there. 'I bis law was first observed anddemonstrated by 14r. Halley, from the nature of the hyperbola;and afterwards by Or. Gregory, by means of the logarithmic curve.S6e I’liilos. Trans. N° 181, or Abridg. vol. 2, p. 13, and Greg.Astron. lib. v. prop. 3. Tims if O denote the density of the airat one place, and d the density at the other ; both measured bythe column of mercury in the barometrical tube ; then the dif-ference of altitude between the two places will be proportional to

the log. of D —the log. of d, or to the log. ofii. But as thisfor-

d

inula expresses only the relation between different altitudes, andnot th, absolute quantity of them, assume some indeterminatebut constant' quantity h, which multiplying the expression log

J , may be equal to the real difference of altitude a ; that is, a = b

X log- of ~

d

Then,