THE ATMOSPHERE. 



101 



Immortal work on the Alps, and in later years 

 the labours of Venetz, of Charpentier, and of 

 Agassiz, endowed with perseverance that set 

 danger at naught, have thrown much interest- 

 ing and new light. 



We know only the inferior, not the superior, 

 boundary of the eternal snow ; for the mount- 

 ains of the earth do not rise into the ethereal 

 or Olympic empyrean, into the thin dry strata 

 of the atmosphere, which we may presume 

 with Bouguer no longer contain any vesicular 

 vapour turned into crystals of ice, and thus 

 made visible. The lower snow-limit, howev- 

 er, is not merely a function of the geographical 

 latitude, or the mean annual temperature ; the 

 tropics, even the equator itself, is not the sit- 

 uation, as was long believed and taught, where 

 the snow-limit attains its highest elevation 

 above the level of the sea. The phenomenon 

 which we here advert to is, in fact, an ex- 

 tremely complicated one, and depends general- 

 ly on various relations of temperature, moist- 

 ure, and mountain configuration. If these re- 

 lations themselves be subjected to a more spe- 

 cial analysis, as a great number of new meas- 

 urements permit us to do(^"), we discover as 

 coefficient causes determining the snow-line : 

 Differences in temperature of the different I 

 seasons of the year ; direction of the prevail- I 

 ing winds, and their contact with the sea and ! 

 land ; the degree of dryness or moistness of 

 the upper strata of the atmosphere ; the abso- 

 lute magnitude or thickness of the deposited 

 and accumulated snow ; the relation of the 

 snowy summit to the total height of the mount- 

 ain ; the relative position of the particular 

 mountain considered in the chain ; the steep- 

 ness of the declivities ; the vicinity of other 

 mountains likewise capped with perpetual 

 snow ; the extent, lay, and height of the plain 

 or level from which the snowy mountain rises 

 isolated, or as one in a group or chain, and 

 which may be a sea-coast, or the interior 

 of a continent, covered with wood, or with a 

 thick short turf, which may be sandy, barren, 

 and strewn with naked rocks, or a wet mossy 

 bottom. I 



While the snow-line in South America reach- 1 

 es a height under the equator which equals j 

 that of the summit of Mont Blanc, and in the I 

 high lands of Mexico, near the northern tropic, j 

 in 19° North latitude, according to recent 

 measurements, descends from that by a quan- | 

 tity equal to about 960 feet, it rises, according I 

 to Pentland, in the southern tropical zone (lat. \ 

 up to 18^ south), and in the western or Chil- 

 ian Andes, not in the eastern chain, to more 

 than 2500 feet higher than it is under the equa- 

 tor, on Chimborazo, Cotopaxi, and Antisana, 

 not far from Quito. Dr. Gillies states, indeed, 

 that much farther to the south, namely, on the 

 declivity of the volcanic mountain Penguenes 

 (33'= S. lat.), he found the snow-line at an ele- 

 vation between 2270 and 2350 toises above the 

 level of the sea. The evaporation of the snow, 

 in consequence of the radiation into an atmo- 

 sphere which is excessively dry in summer, 

 into skies which are scarcely obscured by a 

 cloud, is so rapid, that the volcano of Aconca- 

 gua, to the north-east of Valparaiso (lat. 32^° 

 south), which was found by the Expedition of 

 the Beagle to be more than 1400 feet higher 



than Chimborazo, was once seen without 

 snow(372). 



In almost the same parallel of North lati- 

 tude (303'= to 31'=), the snow-limit of the south- 

 ern slopes of the Himalaya is found nearly at 

 the elevation which various combinations and 

 comparisons might lead us to expect, viz., 

 12,180 feet; on the northern slopes, however, 

 under the influence of the lofty table-land of 

 Thibet, the mean height of which appears to 

 be 10,800 feet, the snow-limit is only met with 

 at an elevation of 15,600 feet. This phenom- 

 enon, which has often been the subject of dis- 

 cussion both in Europe and in India, on the 

 cause of which I have myself made known my 

 views in several papers(3^2), possesses more 

 than a merely physical interest ; it has had an 

 important influence upon the state of numerous 

 tribes of mankind. Meteorological processes 

 fit or unfit extensive districts of a continent for 

 agriculture or pasturage. 



As with the temperature the quantity of va- 

 pour contained in the atmosphere increases, 

 this, which is so important an element for the 

 whole of the organic creation, varies with the 

 hour of the day, the season of the year, the de- 

 gree of latitude, and the height above the level 

 of the sea. The recent experience so general- 

 ly obtained through the use of August's Psy- 

 chrometer, according to the ideas of Dalton 

 and Daniell, for the determination of the rela- 

 tive moistness of the air by means of the dif- 

 ference between the dew-point and the tem- 

 perature of the air,* has considerably increased 

 the extent of our knowledge of the hygromet- 

 rical relations of the surface of the earth. Tem- 

 perature, atmospheric pressure, and quarter of 

 the wind, all stand in most intimate connec- 

 tion with the vivifying moisture of the air. 

 This vivification, however, is not so much a 

 consequence of the quantity of vapour held dis- 

 solved under different latitudes, as of the man- 

 ner and frequency of its precipitation in the 

 shape of dew, fog, rain, or snow, which moist- 

 ens the ground. From the deduction of the 

 gyratory law of winds by Dove, and the views 

 of this distinguished philosopher(^^*), it appears 

 that in our northern zone "the elasticity of va- 

 pour is greatest with south-west, least with 

 north-east winds. On the west side of the 

 wind-card it diminishes, and on the contrary it 

 rises on the east side. On the west side, viz., 

 the colder, heavier, drier current, forces back 

 the warmer, lighter, much moister air ; whilst 

 on the east side the former is overcome by the 

 latter. The south-west current is the pene- 

 trating equatorial stream ; the north-east the 

 sole prevailing polar current." 



The beauty and fresh verdure of many trees 

 which grow in countries within the tropics, 

 where for five, six, or seven months together 

 there is never a cloud to be seen on the face 

 of the heavens, where no- visible dew or rain 

 ever falls, inform us that the appendages of the 

 stem or the leaves have the power, in virtue 

 of a peculiar vital process, which perhaps is 

 not one merely producing cold by radiation, of 



[* Now very conveniently ol>tained by the different read- 

 ings of twro thermometers, as like each other as possible, 

 one of which has its balb dry, the other its bulb wet. The 

 instrument is commonly sold under the name of Mason's 

 Hygrometer in England.— Tr.] 



