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an increase of bulk, under an equal pressure in the to loftier regions, where its tenuity is so much increased, Meteor
ology. ology. permanently elastic fluid; not, however, in proportion that it rapidly eludes all observation. to the measure of the vapour communicated to it, but (75.) With regard to the various points of conden
Points of in proportion to its elasticity. The second result is, sation, it is probable, continues Mr. Daniell, that, as in
condensathat the Specific Gravity of the gas is diminished, but the atmosphere of pure steam, no cloud will be formed tion. not exactly in proportion to its expansion ; for while at any of them. The process of evaporation would be the atoms constituting the vapour are all dilated, their so gentle under these circumstances, that little above own weight is added to the mixture. But this weight, six grains of water would be raised per minute from a though increasing with the elasticity, being, in all cases, square foot of surface; so that, as the gradual precipiless than that of an equal bulk of common air, a tation of this quantity took place between the different decrease of density must follow. This diminution of stages, it would instantly be redissolved by the excess temperature becomes greater with every increment of of heat into which it would be naturally inclined to fall. temperature.
The circulation thus becomes a process of equalization, Progress of
(74.) To place all the circumstances connected with by which the temperature of the upper regions is raised: rapoar in a this interesting subject in a clear point of view, let us,
the heat which is abstracted below by evaporation is perfectly
continues Mr. Daniell, trace the progress of vapour just evolved by condensation, the pressure of the vapour spaere. beginning to form in a perfectly dry atmosphere. For becomes increased, and all the changes tend to that
this purpose, let the temperature of the sphere be 77o. peculiar distribution of heat, which we before contemThe first arrangement will be the same as that repre- plated as the natural state of an emixed atmosphere of sented under the parallel of 10° in Table I. If we now
steam. suppose water suddenly to overflow the surface, evapo- (76.) The average quantity of vapour which would Approximaration will instantly commence. No atmosphere of exist upon the hypothesis we have just assumed, while tion to the vapour exists to impede its progress, and the nascent the atmosphere maintained its proper progression of quantity of steam will merely assume such a degree of tension, as temperature may be roughly approximated as follows: which is necessary to overcome the vis inertiæ of the air which -A stratum, of the force of .616 inch, extends to the would exist obstructs its motion. What this force may be, we have height of 3600 feet; another, of the force of .401 inch, upon such not, perhaps, sufficient data to determine. For the reaches 3900 feet higher; a third, of only .200 inch, an hypo
thesis. present, we must give to it an arbitrary value, and stretches almost as far as both the former together; assume, that, at the temperature of 77°, and pressure making a total of 13,500 feet. The mean, therefore, of 30 inches, it amounts to .200 of an inch. The con
.616 .401 .200
to this point is nearly + stituent heat of vapour of this elasticity is 32°, so that
+ =.354 inch.
4 4 2 at the height of 13,500 feet it arrives at its point of For the further distance of 17,500 feet, Mr. Daniell condensation. An aqueous atmosphere of such a degree thinks we cannot greatly err in taking .064 inch, as the of force being now established, a resistance proportioned mean pressure, making the average to the height of to this amount is made to the progress of evaporation ; 31,000 feet, .209 inch. One-third of the atmosphere and the elasticity of the rising steam must in conse- beyond this being considered free from vapour, reduces quence be doubled. Its constituent temperature, there the mean to .139 inch. fore, by Table V. p. 333 of our Essay on Heat, is raised (77.) Changing now our hypothesis of a sphere of Change of to 52°, and it cannot pass the height of 7500 feet with- uniform temperature, for one whose temperature in- hypothesis out decomposition. The resistance upon the surface creases from the Poles to the Equator, let us assume to that of a now amounts to .601 inch, to overcome which, vapour that the Barometer stands every where upon its surface
sphere at 65° must be emitted. The first point of precipita- at the constant height of 30 inches. The constituent
perature intion, in ascending from the surface, would thus be temperature of the vapour, also, in the different columns creases from fixed at about 3600 feet; and it may now further be of this mixed atmosphere, at the surface of the sphere, the Poles to remarked, that the diffusion of vapour does not cease at is to approach to within eleven degrees of the tempera
the Equator. the height of 3500 feet, to which point we had first ture of the several zones. The succeeding Table has traced it, but that the mechanical obstruction is pro- been calculated by Mr. Daniell, to develope the conportionably reduced, and is carried by successive stages sequent arrangements,
Table XIII.-- Showing the Specific Gravity, Elasticity, Temperature, and Dew Point, of a mixed Atmosphere of Air and Aqueous Vapour, at different
Altitudes and different Latitudes from the Pole to the Equator.
25000|| 9.915 30000 7.852
(78.) By comparing the results contained in this course be increased by its motion in opposition. When Meteorsbagyon Table with those recorded in Table I., it will be seen, this is stopped, as it soon is, by any small diminution
ology. that the Specific Gravity and elasticity of the air is but of the mercurial column, the vapour will rush forward Srecibe very slightly influenced by the intermixture of the with its whole force, retarded only by the kind of fil
on aqueous vapour; so slightly, indeed, that the course tration which must take place through the quiescent te ur but and velocity of the currents, as represented in Table II., air; and the temperature of the higher latitude being sebly in- may with safety be adopted, without producing any unable to support its elasticity, precipitation must foltaxed by sensible error in the investigation. Their balance also low. From the operation of these causes, the tem- Precipi
is to be that by which the Barometer is maintained at perature of the latitude will be partially affected, the tation of an unvarying height. It will also be remarked, that density of the air be still further reduced, and the moisture, while the great aerial ocean is divided into two great aerial column become reversed. The course of the strata, flowing in opposite directions from North to vapour being thus greatly accelerated, an abundant South, and South to North, the aqueous part, which is precipitation must follow. nearly confined to the lower current, presses in a con- (81.) The progress of the precipitated moisture, Further trary direction. The adjustment of these particulars from the time when its first streaks would be found to effects of remaining as now supposed, the compensating winds shoot in a visible form across the atmosphere, to the precipitaflow on in the courses which have been described, and period when it descends in rain upon the globe, is not the balance remains undisturbed.
without its interest. In proportion to the density of the Enporation (79.) The admixture of vapour, hitherto considered, vapour, must be the magnitude of the condensed sa atiect has not yet affected the gradation of temperature, re- particles. When first formed in the higher regions, the 12. sulting from the decreasing density of the atmosphere the cloud would probably assume a light cirriform treature
in its upper parts, but the process of evaporation must appearance, but at lower elevations, their precipirealitia in time necessarily induce such an alteration. The' tation would be more dense, and the attraction of tha the
steam, as it reaches its point of condensation, must aggregation stronger. The mass would thus gently de testing give out its latent heat, and, during its precipitation, subside to a lower station, where the density of the dexty of
combining with a fresh proportion, it must again air would oppose a greater resistance to its descent. parts of the ascend, and again be evolved in the middle regions. Here, in a higher temperature, the cloud would begin ascephere. The atoms of steam may thus be considered as carry- again to be dissolved, and assume a rounded and more
ing caloric from the surface of the sphere to the higher compact form, and thus the equalization of the temstrata; and it is obvious how a considerable section of perature, and the diffusion of the vapour, would be any one column may thus have its temperature carried on from several points at once. The different equalized and fully saturated with aqueous particles. beds obey the impulse of the winds, and, as they sail The currents thus become affected both by the expan- along, enlarge the sphere of their action, till, at length, sive power of the vapour and of the liberated heat; the natural equilibrium of the atmosphere can be no -causes, the influence of which so applied, must be further checked. Hence precipitation will increase,
partial, and cannot reach the higher regions. This the strata of the clouds unite, and the air no longer be : Bus- unequal action must produce a fall in the Barometer. capable of buoying up their load. (80.) Again, as, on the one hand, this effect upon
Force of the (82.) In the next Table Mr. Daniell furnishes us
aerial curBarometer is produced by the augmentation of the with a view of the force of the aerial current, and the quantity of aqueous vapour, so, on the other, a rapid counter pressure of the vapour in a mixed atmosphere, tween the increase of the latter may be produced by a fall in the surrounding a sphere unequally heated in the manner Poles and former. The mechanical resistance of the air must of already set forth.
Equator. TABLE XIV.–Showing the Force of the different Currents in a Mixed Atmosphere of Air and Vapour, between
the Poles and Equator.
01+.178 - .003+.387| -,0101 +.575 -.028||+.810-.049|| +1.034 –,103|| +.854) -.138||+.648 - .132|+.447 - .109+.208 -.082 5010 +.057
+.281 -.014||+.375 –,032||+.570 -.060||+.454 -.082||+.392 -.1071| +.282 -.073 +.118 -.042 10100 +.019
+.217 –.023||+.215 -.044||+.165 - .050+.159 -.038+.053 - .029 15600,-.023
-.035 +.031 - .00911+.038 -.022 +.036-,019+.008 -2013 2010 -.069 -.112 -.185
-.024 - .012 - .068 –.018||- .045 -.007 3100 -.078)
(83.) We have already stated our difficulties, with manently elastic fluid at rest, and secondly, with it in stituerits of acting regard to the amount of resistance which the pores of motion. pesten the gaseous constituents of the atmosphere offer to the (84.) With regard to the state of rest, the opposition sphere. ***pas passage of the vapour in motion. Experiments are with which vapour passes through air, is in proportion ad the wanting to elucidate this subject, but the observations to its density. Saussure concluded from his experiv- of Saussure and Dalton throw some light upon the ments, that a diminution in the density of one-third
The resistance here alluded to may be doubled the rate of evaporation. regarded as twofold ; first, in connection with the per- (85.) With respect to motion, a breeze acting in
az by the subject.
Meteor- opposition to the stream of vapour, must retard its 154. More vapour, therefore, would probably pass at
it. Much obscurity, however, envelopes this inquiry, of elasticity is only .044 inches at the former station,
approximation, however, has been carried a little further
absolute velocity with which vapour travels under any three-tenths of the elasticity, that of a high wind being travels, of the circumstances mentioned; but the relative rates . six-tenths. The retarding influence of the Polar
of different parts of the same column may be approxi- current, in its regular state, may therefore be appor-
latitude 50 = 7
Table XV.- Showing the efficient lateral force of Vapour between the Poles and Equator at the Surface of the
Sphere, and at the Altitude of one-third the Density,
011-.003 –,002-.010-.008-.028–.017| - .049 –.0251-.103.042–,138 -.0691-.132 –,0931-,109 -.088 10000 -.001 -,002-.004 –.008 -,008/-,017||-.012 –,026 -.023 –.045) –,044 –.072 -.050 -.090 ..038
-.082 -.071 ..029 -.058
This last Table, Mr. Daniell remarks, will give some (87.) The permanency of the Barometric pressure, idea of the retardation of force, in the vapour, occa- on the surface of the sphere, is dependent, as already sioned by the wind, at the surface of the sphere, and remarked, upon the equal balance of the aerial curalso of the increase of velocity occasioned by diminished rents; and its fluctuations have been traced to the pressure in the upper regions. It is easy to under destruction of this equipoise, by unequal and local ex. stand that, whenever the serial current coincides with pansions and condensations. One of the chief causes the direction of the vapour, the progress of the latter is of these latter, Mr. Daniell attributes to the increase accelerated in the same proportion.
and decrease of the aqueous vapour, counteracting the
natural progression of temperature by the caloric
cause, which must exercise a powerful influence, and
' of 25 Herculis as a settling star, which passed at 16 subject, that the source of heat has been in the sphere
emanate from its surface. This so far agrees with the
, I noticed a dusky cloud condition of the atmosphere of the earth, with which
, obliterating every star by a thick uniform coating of cloud. All the and exhaust their influence upon the surface of the time the calm remained quite undisturbed. The least supposable globe. But if the atmosphere become cloudy and Efects of rapidity of propagation in this case is 300 miles an hour, in the opaque, the rays of heat, emanating from an externa cloudy ani! direction of the Sun's motion; and the cause is obviously the exact attainment of a determinate temperature in the region of the atmo. source, are in great part absorbed before they reach the opaque stat
of the atsphere, where the cloud formed, either by radiation, or by a diminu- surface, and an increase of temperature and elastic
mosphere tion of atmospheric pressure taking place in succession along the vapour must take place in the middle regions. Here on the whole zone of sky, and as it were pursuing the point of the heavens arises another source of partial and powerful expan- temperati opposed to the Sun."
sion. To this also may be added the property which
first conceived by
dieteor the clouds possess. of preventing the radiation of heat and in the case where it is the greatest, the ratio of the Meteorology. from the surface beneath them, and the greater con- Polar and Equatorial axes is as two to three.
ology. dueting power of damp, than of dry air.
In the seventh chapter of the third Book of the Efects of (88.) Amongst the numberless modifications of con- Mécanique Céleste, Laplace has entered on the consi- Limit to the friber is dition, to which an atmosphere of the nature we have deration of the figure of the atmospheres of the celes
been considering is liable, there are yet two or three to tial bodies, with his usual generalization and skill. face of the which it will be necessary shortly to refer. The surface
Kepler. quere. of the sphere has hitherto been chiefly considered as
On the Limits of the Atmosphere. perfectly plain, and either thoroughly dry or everywhere covered with water. Let us now contemplate it (92.) The existence of a definite limit to the as covered with water to the extent of three-fourths of altitude of the atmosphere was first conceived by its surface, and the remaining fourth of dry earth, un- Kepler; and he sought to determine its elevation by even and inters ected by eminences; conditions which means of the duration of the twilight. By an ingenious assimilate it still more closely to the actual terrestrial and interesting process, which most writers on Astroatmosphere. Such an intermixture of land and water nomy explain, it is known to extend to between 40 or must at once introduce inequalities of temperature of a 50 miles above the Earth's surface, and even at that different character from those which have been hitherto altitude it still continues to possess a density sufficient considered. These will arise chiefly from the greater for refracting and reflecting the rays of light. rapidity both of heating and cooling, in the dry surface, (93.) The authors* who have written, says Mr. dependent upon the peculiar constitution of the watery Ivory,t on the height and figure of the atmosphere, element. As the processes by which their impressions have likewise assigned a boundary beyond which it are communicated to the incumbent air are slow and cannot reach. But in this they have rather fixed a gradual, they mostly affect the different columns in an limit to the domain peculiarly belonging to the Earth, equable manner; so that their influence upon the atmo- than reasoned upon any distinguishing properties of spheric currents resolves itself into the cases which the atmosphere itself. If we conceive a body that cirhave been already proposed, of total and regular ex- culates round the Earth by the force of gravitation, in pansion. With respect to the vapour, however, the the time of a diurnal revolution, the path which it decase is different. The parts of the atmosphere above scribes will mark the limit whereat the centrifugal force the dry spaces cannot remain free from its admixture, arising from the rotatory motion of the Earth will just because the elasticity of the surrounding medium will balance the opposite centripetal force. Therefore any soon supply the vacuum. The law of this equalization body that participates in the rotatory motion common will depend upon the mechanical obstruction of the air, to all, if placed beyond the boundary we have menwhich is influenced also by the conditions of the wind. tioned, would continually recede from the Earth, and When once diffused over the land, it will be more sub- be eventually lost in the immensity of space; or, if ject to condensation; and the amount of precipitation placed within the same boundary, would fall to the must be restored from the great expanse of waters. common centre. The radius of the orbit described by
(89.) Finally, unevenness of surface will tend, says the revolving body is about 25,000 miles, or something Mr. Daniell, to modify the atmosphere in some minor more than three diameters of the terrestrial globe. degree. Any elevation will obviously partake of the Now the air surrounding the Earth cannot reach so far, temperature due to the stratum of air into which it may for if it did it would be continually dissipated ; a suprise; but the action must be reciprocal; and as the position which is extremely improbable, since we are beating surface is raised to higher regions, those re- acquainted with no source from which a constant waste gions must be proportionally and unequally affected. of so necessary a fluid could be supplied.
We shall now pass to the consideration of some of The first writert who attempted, from Physical con- Dr. Wollasthe phenomena connected with the actual atmosphere siderations, to fix a definite limit to the atmosphere, was ton's atof the earth.
Dr. Wollaston, in the Philosophical Transactions for tempt to fix
1822, who remarks in his interesting Paper, that, in a definite Figure of the Atmosphere. attempting to estimate the probable height to which the
atmosphere. Earth's atmosphere extends, no phenomenon caused by (90.) The problem of the figure of the atmosphere is its refractive power in directions at which we can view re Earth's connected with some very refined and delicate points it, or by reflection from vapours that are suspended in
Had the Earth been truly of a spherical it, will enable us to determine it. From the law of its
• D'Alembert, Opus, tom. vi. Laplace, Mécanique Céleste, liv. iii.
| Mr. Cavendish communicated to Mr. Davies Gilbert, President
of the Royal Society, in the year 1808, the substance of a very in. atmosphere is flattened towards the Poles, and dis
teresting investigation relative to the definite limits of the atmotended at its Equator; but this ellipticity has limits, sphere; but this appears never to have been published.
limit to the
Emisphere. of analysis.