yETKOROLOQY. 





immortality of the soul As soon as the body begins to decay , the koul 

 rsasi from MM animal to mother ; and when it has passed through 

 th. forms at all animal*. terrestrial, aquatic, and winged, it again enter* 

 a human form. This period of transmigration U completed in 3000 

 years. Some of the Ureeks, he add*, both in early time* and more 

 recently, hare maintained thu doctrine and claimed it u their own ; 

 and though he could mention name*, he decline* to do to. 



METEOROLOGY. in ita extended a*n>e. embrace, all physical 

 oauM* which affect the itate of the atmosphere or are affected by it. 

 Hence U is OMiaeeted with the phenomena of heat and cold, dew, rain, 

 hail and now, clouds, winds, aurora borsales and aostrales, or polar 

 lights. haloes, parhelia, *c The sense in which Aristotle (M.T.-^AO- 

 lati. L) usea the term U still more eztensiTe,eomprebending, in addition 

 to what are now called niete .r, every affection (vsitoi) common to thu 

 air and water, with the characters of the different parts of the earth, 

 and their affections, as winds and earthquakes, and everything incident 

 to s ich kind* of motion. 



Our ftnt inquiry shall be, what U the nature and what the probable 

 extent of the terrestrial atmosphere I Essential as it U both to animal' 

 and rentable life, to the distribut on of heat, an 1 to various inclina- 

 * of light, 



, 



light, the knowledge of its nature and composition is eminently 

 useful The air, though composed of several elastic fluids, obeys the 

 same Uws to which they are individually subject, namely, iu elasticity 

 and density at a given temperature are proportional to the pressure 

 which it sustains, and for every degree of the centigrade thermometer 

 under a given pressure it expands very nearly ,&, of its volume at the 

 temperature tero. Hence if its density be represented by 8 and its 

 . temperature by I (in centigrade degrees), its elastic force will be pro- 

 portional to (1 + at) S (where a represents the decimal 00375) as well 

 as the pressure sustained. Lastly, the pressure U equal to the weight 

 of a vertical column of atmosphere, having the portion pressed on as a 

 base, and extending upwards to its extreme limit. 



According to Datum's views, the various gases constituting the 

 air are not chemically combined by the law of definite prop 

 but only mechanically mixed, co-existing in the same space, and 

 producing by the sum of their incli>endent pressures the 1 1 

 of the mercury in the barometric tube. In modern chemical physics, 

 they are said to be mingled by the piinciple of Dinr.-iox, which is 

 the form in which Dalton's conception i< now held. 



We have seen that heat fncrauea the elastic power of air, ami hence 

 the equilibrium of a mass of air unequally heated U constantly dis- 

 turbed. The currents of warm and cold air charge place*, the cold 

 air moving to the warm region, and thence, when Wanned, repeating 

 the course of the previous warm air. Thus the atmosphere is a great 

 agent in lending to equalise the mean temperatures of climates in 

 various latitudes. Besides, the aerial currents are vehicles for the 

 transfer of clouds, for producing electrical discharges, for clear! 

 malaria, and ore turned by the ingenuity of man to promote his 

 industry and extend hi* knowledge of the globe which he inhabits. 



The atmosphere, considered as a transparent medium, has also great 

 effects on li.;ht by iu refractive power, and the reflection of the aqueous 

 msssns it contains. Hence arises twilight, which mitigates the transition 

 of day to night, and from the duration of which it is easy for the astro- 

 nomer to compute that altitude of the atmosphere at which it ceases to 

 act sensibly ou light, rither from its total absence or extreme tenuity. 

 This altitude is from forty to fifty miles above the level of the sea. 

 Again, by the refractive power of the atmosphere distant terrestrial 

 objects are elevated to the view when the spherical curvature of the 

 earth would otherwise have caused their concealment ; various optical 

 illusions, u the mirage, fate morgana, 4c., ore all easily explained from 

 the same refractive power under peculiar circumstances of temperature. 

 By this medium sound Is conveyed and odours are, at least in part, 

 dfaswninsliid ; the clouds which float in it soften the direct glare of 

 the solar beams, and iu aqueous particles, fluid or frozen, produce the 

 beautiful phenomena of haloes, rainbows, false suns, &c. Its greater 

 specific gravity elevates the balloon, by means of which the nature of 

 the upper strata of the air below the altitude of betwe n four and five 

 miles, may be ascertained, and the barometric elevation and tempera- 

 ture observed, which furnish data for calculating the physical limits of 

 the atmosphere. 



With regard to the extent of the atmosphere, we may consider it 

 under to points of view: first, the extreme limits to which it is 

 possible fur it to extend, considered mathematically as a moss rotating 

 round the terrestrial axis in the name time ; secondly, the much 

 narrower phyical limits founded on its nature as an elastic fluid, and 

 having regard to the great diminution of temperature at high altitn.l, 



Any particle of the revolving atmosphere is acted on by two forces, 

 namely, gravity, which is directed nearly to the centre of the earth, 

 and the centrifugal force produced by rotation, which is directed 

 according to the line by which that point is orthographically projected 

 on th earth's axis, and tends directly from that axis. The former 

 force varies inversely as the square of the distance from the earth's 

 centre; the latter, directly as ita perpendicular distance from the 

 earths axi*. At any point taken in the external siirfa.-e of the atmos- 

 phere, th resultant aruini{ from both forces must be normal to that 

 surface, in ordi-r that its furra may be permanent. At tuo terrestrial 

 e ,u ,t..r the tatio ul these two forces is known ; as we ascend in the 

 along an equatorial radius produced, gravity diminishes 



and centrifugal force increases, both in this instance being directly 

 opposite : hence it is easy to calculate the distance of a point in that 

 radius where the two forces are exactly equ d Beyond that |>oint the 

 c.-ntiifugal force predominated, and no particle there sitmu-d could 

 remain attached to our atmosphere, revolving with the earth I > 

 its diurnal and orbital motions. 



The above jioint therefore defines the extreme limit to which it in 

 pauMt for our atmosphere to extend, and which U at a distance of 

 about 30,000 miles from the centre, or 26,000 from the earth's 

 equatorial surface, though it by no means follows that it must extend 

 so far. Other data would be necessary to give the actual extent : for 

 instance, the height of the barometer at the surface of the sea, and t he 

 law of the diminution of temperature in the upper strata of the air. The 

 figure of the extreme surface can however be determined from these 

 considerations, which i- tint of an oblate spin -mid Battened at the 

 polex, and in which the polar axis is to the equatorial in the ratio of 

 2 to 8. 



As the phenomenon of twilight indicates an extreme degree of 

 rarefaction in the atmosphere at an inconsiderable altitude above the 

 earth's surface, we shall now consider some of the physical causes 

 which demonstrate that the actual limits of that fluid are much more 

 contracted than t! possible limits given above. 



Representing by;*, S, and t respectively, tliu pressure, density, and 

 temperature of the air at the surface of the earth, and by ;>', 6", and (, 

 like quantities for a portion of air at a certain elevation, it iollou 



the general laws of gaseous bodies that . = ' ' , where ft repre- 



p (1 + /5 S 



cents the fraction :,*-;. Now the pressure measures the elastic force of 

 d particles : this elasticity cannot become negative, and ceases 

 to exist when p' = l>, which may happen either because (1 +fl/) = 0, or 

 $' = ('. Thin but supposition would bring us to the consideration of 

 the mathematical limits above treated on ; the former, to the physical 

 limits depending on the decrease of temj>erature at high altitudes, 

 hence the air ceases to be an elastic fluid, according to thu reasoning, 



1 800 

 when f= 3= ~j~= - 266}' centigrade. Now the law of the 



ment of heat in the atmosphere proceeds in a progression quicker than 

 an arithmetical, and :w the pi. me oi' prrpciuid snow is at a compara- 

 tively small alt.it licit; in climatesofmeanteiiiper.itnre.it is easy to see that 

 from 100 to 200 miles altitude would besiilhciciit to diminish the tempe- 

 rature to the above number of 206 below Hero. Kven if the simple 

 law y> = (l + /3 () 5 was not strictly rigorous at such Id \v temperatures 

 as 2<Ji; le, still n diminution of temperature for a 



density would produce a diminution of elasticity, and scarcely ii 

 with the general conclusion nn -i\> di t law. 



But even limits thus obtained would, in all probability, bo still too 

 extensive, for it is not necessary that the elasticity should be totally 

 destroyed : it js sufficient that the repulsive power of two con- 

 tiguous particles of air at that altitude shall be less than the force of 

 gravity, and if these two forces are equal, it will be the o.v 

 at which the air can remain attached to the globe, leaving out of con- 

 sideration the centrifugal force, which at that altii u. i 

 and whi.-h would itself t n.i 



limits become still more contracted by tl i .ition, and it will 



nut be necessary that the tampsntOM should W a* low as 26. 



Some have imagined that planetary atmospheres are due to the 

 attraction of the masses of the planets on a rare elastic fluid di 

 natal through space, but this supposition will not bear investi; . 

 for putting aside the consideration of the extreme cold of the pl;n. 

 spaces, this hypothesis is not corroborated by the dimensions of the 

 atmospheres of the sun and planets, which would then depend on their 

 masses, and the present total disappearance of t ' , of Jupiter 



behind their primary would be converted into an annular appi 

 round his Uxly at the time of the eclipse of a satell: 



We have seen that the unequal distribution of heat in the atmos- 

 phere is amain source of tl. .,ns of win! 

 consequently of the distribution of clim.ite ; but OB hand, 

 the earth itself, having its own distribution and i 

 reacts on the atmo-pln-ie and produces dew, 1 \o. In like 

 m. inner the sea and the 1'ol.ir fields of ice materially allcv' 

 distribution of !: 



The general causes of the temperature of the globe are the radiation 

 of the sun, and the proper heat of the . ; their 



effects however are greatly modified by various loc-d oin 

 : such as the vicinity of seas or mountains, and the ditl'crence in 

 radiating power of the soil. ; .vei-g ,,f 



; the strata subjacent to the particular places, with other causes less 

 perman nt. 



With respect to the proper heat of the earth, we observe in any 

 latitude that at a depth which is very small complied with 

 the radius of the globe, the temperature in permanent throughout the 



..f solar radiation being confined to a sup.; 



stratum of inconsiderable thiikne-s 'Ihii fact has be.cn fully 



established by long continued observation* in the cellars of the Ob-ur- 



at Paris, and !. >ns in the mines of Cornwall, in 



.untried. If this stratum lie supposed to be 



1 off, the internal n icred as nearly a 



