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upper, but it is they which first receive the radiations emitted by the ground Each constituent molecule of the atmosphere radiates in all directions, like the earth itself. The rays directed downward, are received by the lower strata, and by the earth, whilst those directed upward are partly lost in celestial space.

All these circumstances explain to us the influence of the seasons, of day, and of atmospheric variations, on the phenomena that we are analysing. When the weather is serene, and the sun high, the ground is much more heated than during cloudy weather, or when the sun is at a very small height above the horizon; so that radiation is much more active. But as this radiation heats the lower strata chiefly, we may imagine that in summer the temperature decreases with extreme rapidity. If we ascend to a tableland, instead of climb a mountain, the great surface of the former heating the air in contact with it, the decrease of temperature is not so rapid.

We also commonly regard the change of volume in the same mass of air as one of the most influential causes of the decrease of temperature. For if a piston is moved with friction in a glass or metal cylinder, and is suddenly thrust down, amadou placed in the cylinder is immediately kindled. An apparatus of this kind is called a pneumatic tinder-box. Reciprocally, if the air is rarefied beneath the receiver of an air-pump, there is a reduction of temperature. It follows from these facts, that the calorific capacity of the air increases in proportion as it is rarefied; a greater quantity of heat is therefore necessary in order to elevate its temperature the same number of degrees; and it borrows this heat from all neighbouring bodies, and from the thermometer, which falls rapidly.

When, therefore, the luminous and calorific rays of the sun traverse the rarefied strata of the atmosphere, the thermometer does not rise so much as it does in the neighbourhood of the ground, even when a cubic metre of this rarefied air has not absorbed more heat than a cubic metre of denser air; like as a kilogramme of water is only elevated in temperature 4° under the influence of a source of heat that would raise the temperature of a kilogramme of iron 36° (p. 11).

By combining all these elements, we can perfectly understand the decrease of temperature with the height. The influence of ascending and descending currents does not appear to me so great. Indeed, in proportion as the heated air ascends, the pressure to which it is subjected diminishes:

it expands, and consequently becomes cool: other aërial masses descend, contract, and are heated. However, these displacements can contribute to the production of the phenomenon only in an adventitious manner.

VEGETATION OF MOUNTAINS.-The fall of temperature, according to the height, has the greatest influence over the life of organised beings in mountains; and although these details enter into physical geography, I think they cannot be read without interest.

In the plains of Switzerland, at the foot of the Alps, we admire the very beautiful vegetation, the orchards, the corn-fields and the meadows destined to support, during the winter, the cattle that feed on the mountains during the summer. The plants of the high Alps are found there, springing from seeds brought thither by the torrents, and which are entirely wanting in France and Germany. Examples: Pyrethrum alpinum, Lepidium alpinum, Linaria alpina. At the foot of the mountains are beautiful forests of beech, fir, and sometimes pine.

If we ascend five or six hundred metres we find the bear's ear (Primula auricula), which covers the rocks with its bright yellow flowers; the stalkless gentian (Gentiana acaulis), the large petals of which, of an ultra-marine blue, incline toward the earth; the wolf's bane (Aconitum napellus), the ranunculus with monk's-hood leaves (Ranunculus aconitifolius), the Trollius europaus, &c. &c. At the height of 1000 metres, the soldanelle (Soldanella alpina), grows in the hollows watered by melting snow, which it frames in a violet border. The Crocus vernus is found in the same localities, and passes away as quickly as the soldanelle. The declivities are covered with rhododendrons (Rhododendron ferrugineum and R. hirsutum), shrubs covered with red flowers producing the most beautiful effect, and often ornamenting large surfaces.

At the height of 2000 metres the greater part of the vegetation of the plain has disappeared, and cannot be propagated. In North Switzerland, the vine does not rise higher than about 550 metres; on the south declivity of the Alps, and in Valois, it attains 650 metres; and in some favourable localities, such as Val-Sesia, at the foot of Monte Rosa, it is found even at the height of 1000 metres.* It is the same with the cereals; the more we ascend, the later is the harvest. In July, 1832, the harvest was over on the plains of Switzerland; but it was yet in hand in the Haut

Antei (Val Tornanche), 1042; south side of St. Bernard, 1040; Val d'Aoste, below Courmayeur, 1024.-M.

Valois, in the neighbourhood of Munster and Obergestlen. In elevated villages they are often obliged to suspend the sheaves to poles, in order to ripen the grain artificially. A particular means is also employed to make the snow disappear: they cover it with black earth, which, as it absorbs the heat, hastens its melting. De Saussure has seen this means employed in the valley of Chamouni. In the north of Switzerland the cereals may rise as high as 1100 metres, but they do not calculate on a sure harvest beyond about 900 metres. Maize ripens even at 870m.* The localities have in this case a great influence; thus, in the valley of Lugnetz (canton of the Grisons), cereals are found near Vin, at 1510 metres. On the north side of Monte Rosa, barley ceases at the height of 1300 metres; on the south

More frequently cultivation is pushed by the inhabitants as high as possible on the slopes of the mountains. The peasant of France, Switzerland, Piedmont, or Savoy, does not fear to carry the hoe, wherever he has a reasonable hope of a harvest. Cultivated fields, therefore, cease where they can no longer recompense the cultivator for his labours; but the estimation of the value of products, compared with the labour that they demand, varies with different people and different individuals. Thus, in the limit of cultivated fields, is a function of political and moral elements; and not the simple result of the change of climate, like the limits of a wild plant that is independent of man. The following is a proof:-It is an almost general law that all vegetables rise higher on the south than on the north declivity of mountains; and yet if, under this point of view, we compare the limits of cultivated fields in the Pennine Alps, which I obtained by exact barometric measurements, it is rarely so. On examining the relative heights of permanent habitations on the two sides, we obtain a result which confirms the preceding.

COMPARATIVE TABLE OF THE HEIGHT OF THE MOST ELEVATED VILLAGES, AND THE ALTITUDINAL LIMIT OF CULTIVATED FIELDS ON THE TWO SIDES OF THE PENNINE ALPS.

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The influence of exposure is explained on the contrary in the most evident manner, by the differences of level that are observed between the limit of a wild plant on the south and on the north side of an isolated mountain. Mount Ventoux, in Provence, which I have studied with this object in view,

side, it ascends in certain points, to 1950. It is the same with fruit-trees. In North Switzerland, none exist above 880 metres; only in certain favourable localities, near Disentis, for example, are they found at 1070. Cherry-trees ascend higher; the latter, which are found as standards on the Riga, are at Unter-Daechli (953m). It is with much diffi

culty that the Capuchins of the convent of Marie-à-la-Neige (Maria zum Schnee) can occasionally ripen them in espaliers at 1310 above the level of the sea. Walnut-trees (Juglans regia), which in the plains are magnificent trees, disappear at about 800m;* the chestnut (Castanea vesca) does not exist beyond 780.+ 877 metres may therefore be regarded as the mean limit of cultivation.

It is useless to insist on the local circumstances by which this limit may be modified; and on its fall, in proportion as we approach the north. In Lapland it is 100 metres above the level of the sea. In South America, maize rises to 2270 metres, but it is only abundant between 1000 and 2000; from 2000 to 3000, the cereals of Europe are found; wheat, in the lower zones, rye and barley in the higher regions; from 3000 to 4000, nothing but the potatoe is cultivated.

In Switzerland, man has carried the hoe as high as possible, and he has profited by every portion of cultivatable earth. However, at a certain elevation, the woods become predominant, and finish by occupying all the surface of the soil; but even the physiognomy of the trees changes with the height. The pointed fir (Abies excelsa) is transformed

is a remarkable example. The following are the highest limits of several plants on the two sides:

LIMITS OF DIFFERENT TREES ON THE TWO SIDES OF MOUNT VENTOUX.

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(Vide Botanic Topography of Mount Ventoux, Annales des Sciences Naturelles, t. x. pp. 129, 228. 1838.)-M.

* In the Pennine Alps the walnut-tree rises to the mean height of 1005 metres; that is, 1060 in the south, and 950 in the north.

† On the south side of the Pennine Alps its mean limit is 875 metres.

into a pyramid; its lower branches, of which the base is formed, rest on the ground: the structure of the wood also varies; the annual layers are thinner, and the wood harder. On a branch of pine twenty-seven millimetres in diameter, I counted sixty annual layers.* The trees finally disappear altogether. In the north of Switzerland, the beech does not rise above 1300 metres; the fir stops at 1800. On the south side of Monte Rosa, trees ascend as high as 2270; these are the larch (Larix europea), the cembro (Pinus cembra), the alder (Alnus viridis), and the birch (Betula alba). In the north, green trees do not pass beyond 2000 metres. The latter also vary much; on the north side of the Alps the firs attain the highest elevation ;† on the south side it is the larch; on Ararat, the birch ceases at 2530 metres; in the Caucasus, at 2360. On the south side of the Pyrenees, the firs (Abies pectinata) cease at 2570; in the north, the pines (Pines sylvestris) cease at 2420. In Lap

* In our researches on the increase of wood-fir in the north of Europe, inserted in Vol. 15 of the Mémoires de l'Académie de Bruzelles, M. BRAVAIS and myself have shewn that the mean thickness of the annual ligneous layers of the wood pine increases as we go from north to south. The following table clearly shews the powerful influence of climate :

MEAN THICKNESS OF THE ANNUAL LAYERS OF THE WOOD-PINE AT DIFFERENT LATITUDES.

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† On the Grimsel, in Switzerland, is an exception to this rule; and the succession of large vegetation on its north side, recals to mind the succession of the same vegetation along the coasts of Scandinavia. The following is the table:

LIMITS OF TREES ON THE NORTH SIDE OF THE GRIMSEL.

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(For further details, vide Annales des Sciences Naturelles, Oct. 1842.)-M.

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