AT1K' "iii:i:i-. 



, .,j 



whole solar system. Dr. WollMlon argues that this finite character of 

 UM etmoapbore is more oonformabU to the atomic theory than to that 

 at UM infinite divisibility of matter, since, in the first oass, a boundary 

 is possible, and will exist at the point where the weight of a single 

 > is as great as the repulsive force of the medium, which has been 

 ' ' id to be 16,000 miles distant from the earth's equatorial 

 while in the latter eaee it is difficult to see the possibility of 



amj OB OK TW} 

 it u by no met 

 atmospheres of t 

 account of the 



It has been observed, that Bucko's comet appears, in successive 

 revolutions, to show in a slight degree the effect of some medium 

 rnekllng ita motion ; and the same thing has been said of that of 

 Bwk.lt might therefore appear that the preceding argument is 

 weakened in force by this circumstance, or rift vend, since the Urge 

 planet* might collect sensible atmospheres of the resisting fluid, what- 

 ever it be. But on this we must observe, that supposing the fact of 

 UM resisting medium to be established (and several astronomers are of 

 that opinion), it by no means follows that it is common air, or anything 

 approaching to it m the proportion of ita density to ita elastic power. 

 On the contrary, the facto observed with regard to the motion of the 

 planets (which show no signs whatever of a resisting medium), and the 

 extreme tenuity of the comets themselves (through which very faint 

 stars may be seen), justify us in supposing that the resisting medium 

 may be of a very high degree of elasticity as compared with air ; and 

 no means improbable that the planet* actually may have 

 of this same medium, not sensible to our instruments, on 

 very small increase of density which is sufficient to 

 counterbalance the action of a planet To elucidate this subject, see 

 KLASTICITT, FLUID (ELASTIC). 



The preceding argument* go to show, that even supposing the 

 temperature of the atmosphere to be uniform throughout, there is no 

 inconsistency in the supposition of a finite atmosphere. But a very 

 strong presumption in favour of such an hypothesis is derived from 

 the rapid decrease of temperature which takes place as we recede from 

 the surface of the earth. ThU decrease is conspicuous from the fact 

 that the tops of high mountains are perpetually covered with mow. 

 But the law of decrement is not known with much certainty. According 

 to the latest observations, it would appear that for every 100 yards of 

 ascent, there is an average decrement of 1 F. A slower rate of decrease 

 U evidently given by observations taken during balloon ascents, since 

 it U determined by the barometer, so that they contain the very 

 element in question. Thus, the celebrated ascent of Gay-Lussao at 

 the commencement of this century, gave a decrement of 1 for 316 feet, 

 while Mr. Green and others have made it 485 feet ; so that the general 

 average from balloon ascents would seem to be 400 feet, instead of 

 800. The causes of this decrement are, 1. That, in receding from the 

 earth's surface, we are leaving a heated body, and interposing a badly- 

 conducting medium. Indeed, Fourrier and Pouillet give 226 F. 

 as the temperature of the interplanetary spaces. 2. The heat con- 

 ducted from the earth is nearly all absorbed by the lower strata of the 

 atmosphere ; and 8, Leslie has shown that when air is dragged upwards 

 by nisssni of rising vapour, it expands, and consequently absorbs 

 caloric ; while that which descends contracts, and gives out caloric. 



If we take the general law of decrement given above as a basis of 

 calcination (and it ia probably nearly correct for heights not exceeding 

 20,000 feet), we get the following remarkable connection between x, 

 UM height in yards, and >, the barometric pressures compared at the 

 higher and lower levels 



X\S-584. 



i = 180 yards (273 + T), T being the centigrade reading at the 

 lower station, and X = a x = the drprrmon of the upper station 

 Mo* the height a, "This formula," says Sir John Herschel, in the 

 article METKOKOLOOT, in the ' Encyclopedia BriUnnica,' " moke* it \ ,-.\ 

 apparent how completely the law of equable decrease ia subversive ol 

 the received notion of a diminution of pressure in geometric progression 

 upwards from the sea-level." 



The old theories on the subject of the decrement of temperature as 

 we ascend in the atmosphere may probably be considered as mere 

 hypotheses, and unfounded. Such is that of M. Biot, which assigns a 

 decrement in geometric progression for heights in arithmetic progres- 

 sion, setting out from the assumption that the whole heat of the air 

 at any point ia due to the extinction of heat radiated or conductor 

 from the earth to that point 



The argument in favour of the finite extent of the atmosphere 

 derived from the preceding, is as follows. If we. suppose an elevation 

 of 800 yards to produce a fall of 1 of Fahrenheit's thermometer 

 (which, a* we have seen, is likely to be below the truth, that is, to 

 give the higher regions of the atmosphere a higher temperature than 

 they really have), it follows, that at a height of forty miles above the 

 level of the sea, the temperature of the air must be 850 of Fahrenheit 

 below that of the sea, or certainly more than 800 below the freering 

 point There is the strongest reason to suppose that no gas that we 

 are acquainted with would preserve ite gaseous state at this low tern 

 perature, but would become liquid : and though no gas has yet been 

 rendered liquid by reduction of temperature alone, yet several have 

 bean reduced to that state by cold and pressure united. 



If, then, we suppose the atmosphere to be of finite extent, ite form (as 



mentioned above) must be nearly that of an oblate spheroid, the lesser 

 axis passing through the poles of the earth ; at the same time the 

 action of the sun and moon must produce certain small atmtBjJteric 

 itUt; and the tidus of the sea, which are constantly disturbing the 

 jose on which the atmosphere rests, must produce periodical alteration* 

 >f form in the Utter also. If any such exist, sensibly, they may be 

 letoctod by the barometer ; for, cateru paribut, any increase in the 

 icight of the superincumbent column of air must be accompanied by a 

 small increase in the height of the counterbalancing column of m< 

 *aplace was the first who examined this curious branch of the subject 

 ie showed by analysis that the attraction of the sun and moon 

 iroduce no permanent effect upon the currents of the atmosphei 

 nstance, such as the trade winds. He also showed that the diurnal 

 ciUationa caused by the above-mentioned attractions would only 

 iroduce a very small effect upon the barometer in fact, less than one 

 Millimetre, or l-25th of an inch. The reduction of a large num 

 observations gave, at first, OSS of a millimetre for the quantity in 

 question ; those of another set gave '018 ; from which Laplace con- 

 cluded, taking into account the smallness of the quantities, and tin- 

 degree of probability which could be attached to results so differ, -nt, 

 that the tetuibte existence of the atmospheric tide was doubtful. In 

 he meanwhile, however, the diurnal variation of the barometer has 

 wen completely established by observations made in several d:: 

 ilaces. But the law and quantity of this oscillation appears t 

 10 much with latitude, climate, and other circumstances, that no 

 xjsitive conclusion can yet be drawn, either to the exclusion of 

 itmogpheric tldtt, properly * called, or the adoption of any other cause 

 ic conjunction with it. 



The average pressure of the atmosphere is found to be the same, or 

 very nearly BO, at any one place from year to year, notwithstanding the 

 various temporary alterations arising from meteorological causes. But 

 it is not yet accurately determine* 1 in a sufficient number of places to 

 settle the question, whether it is the same at the level of the sea 

 throughout the globe or not. Indeed, it is obvious that it must always 

 jo difficult to decide whether an observed difference in the mean height 

 of the barometer at two places on land arises from difference of I. \ d, 

 or from the atmosphere itself. The mean height of the barometer in 

 London is stated at 29-88 inches ; at Paris, where it has been deter- 

 mined with great accuracy, it is 0780 metres = 'M~i>'2'2 inches. 

 "BAROMETER.] 



As we advance higher in the atmosphere, the barometer falls ; and 

 the quantity of the fall b used to ascertain the height ascended. Tin- 

 method of doing this will bo explained in the article HIK.HTS tM'-\- 

 SUREMENT OP) ; we notice it here in order to mention a circumstance 

 which shows that our knowledge of the general condition- of tin- 

 atmosphere has not been overstated. In order to construct the 

 formula, it is necessary to take into account the diminution of the 

 weight of the air, not only from its rarefaction, but also from its 

 increasing distance from the earth, the variation of elastic force, OH 

 well from rarefaction as from change of temperature, the alteration 

 of density in the mercury itself, arising from the alteration of tempera- 

 ture, and to use the formula in ditlerent latitudes, the variation of 

 the force of gravity on the earth's surface. In our ignorance of the 

 variation of the temperature/ it in usual to allow to the whole column 

 of air contained between the points of observation, the average tem- 

 perature of its upper and lower extremities. This is the most doubtful 

 port of the process ; and as a verification, recourse is had to the com- 

 parison of heights measured by the barometer, and also by the processes 

 of trigonometry. It is thus found that a co-efficient which, when 

 deduced from theory alone, is 18837'46, appears from a number of 

 heights measured trigonometrieally to be 18386, differing from (In- 

 former only by about its 18,000th part. This shows the effect of 

 iein|.crature to be sufficiently well taken into account, for such heights 

 as we can measure, by the preceding supposition. 



In the article AIR some, reasons were sliqwn for supposing that its 

 component parts were not united chemically; but only mixed. Now a 

 Uw is found to prevail in the mixture of gases and vapours, ax uni- 

 versal as the one described in the article AIR, relative to the expansion 

 arising from temperature namely, that two gases in a state of mixture 

 exercise no influence one upon' the other, except communication of 

 temperature, but that each is disposed in exactly' the same manner as 

 it would be if the other were not present Thus it is found that no 

 pressure of dry air upon water exerts the least influence in preventing 



the formation of steam, which goes on exactly as* if 

 were a vacuum, and continues until further evaporation is stopped by 

 the pressure of the iteam already formed. It is found that no pressure 

 of one gas can confine another in water ; but that supposing a bottle 

 partly full of water, the gas confined in the water will escape to the 

 surface and distribute itself in precisely the same way as if the other 

 gas were not present. By this it ia not meant that the action com- 

 monly called mechanical cannot take place, or that a stream of hydrogen 

 would not trouble the air ; hut only that the permanent settlement of 

 one gas is not affected in any way by the pro* . l<m K 



as no chemical action is excited. From thin ].rin. Mr 1 1 ill. .11 



(' PhiL Trans.' 1828), taking into COT the presumptions whioh 



exist against the dirmintl union of the ingredients of the atmosphere, 

 infer* that the atmosphere does not consist altogether of the compound 

 called air, but that the nitrogen atmmphcre is higher than the 



