mountain springs burst forth upon vast elevated plateaux, eight or ten thousand feet above the sea (Micuipampa, Quito, Bogota), or in narrow, isolated mountain-peaks many thousand feet higher, not only include a far greater part of the surface of the earth, but also lead to the consideration of analogous thermic conditions in the mountainous countries of the temperate zones.

In this important subject it is above all things necessary to separate the cycle of actual observations from the theoretical conclusions which are founded upon them. What we seek, expressed in the most general way, is of a triple nature: -the distribution of heat in the crust of the earth which is accessible to us, in the aqueous covering (the ocean) and in the atmosphere. In the two envelopes of the body of the earth, the liquid and gaseous, an opposite alteration of temperature (diminution and increase in the superposed strata) prevails in a vertical direction. In the solid parts of the body of the earth the temperature increases with the depth; the alteration is in the same direction, although in a very different proportion, as in the aerial ocean, the shallows and rocks of which are formed by the elevated plateaux and multiform mountain peaks. We are most exactly acquainted by direct experiments, with the distribution of heat in the atmosphere,-geographically by local determination in latitude and longitude, and in accordance with hypsometric relations in proportion to the vertical elevation above the surface of the sea,-but in both cases almost exclusively in close contact with the solid and fluid parts of the surface of our planet. Scientific and systematically arranged investigations by aerostatic voyages in the free aerial ocean, beyond the near action of the earth, are still very rare, and therefore but little adapted to furnish the numerical data of average conditions which are so necessary. Upon the decrease of heat in the depths of the ocean observations are not wanting; but currents, which bring in water of different latitudes, depths, and densities, prevent the attainment of general results, almost to a greater extent than currents in the atmosphere. We have here touched preliminarily upon the thermic conditions of the envelopes of our planet, which will be treated of in detail hereafter, in order to consider the influence of the vertical distribution of heat in the solid

crust of the earth, and the system of the geo-isothermic lines, not in too isolated a condition, but as a part of the all-penetrating motion of heat, a truly cosmical activity.

36

Instructive as are, in many respects, observations upon the unequal diminution of temperature of springs which do not vary with the seasons as the height of their point of emergence increases,-still the local law of such a diminishing temperature of springs cannot be regarded, as is often done, as a universal geothermic law. If we were certain that waters flowed unmixed in a horizontal stratum of great extent, we might certainly suppose that they have gradually acquired the temperature of the solid ground, but in the great network of fissures of elevated masses, this case can rarely occur. Colder and more elevated waters mix with the lower ones. Our mining operations, inconsiderable as may be the depth to which they attain, are very instructive in this respect; but we should only obtain a direct knowledge of the isogeothermal lines, if thermometers were buried, according to Boussingault's method, to a depth below that affected by the influences of the changes of temperature of the neighbouring atmosphere, and at very different elevations above the sea. From the forty-fifth degree of latitude to the parts of the tropical regions in the vicinity of the equator, the depth at which the stratum of invariable temperature commences, diminishes from 60 to 1 or 2 feet. Burying the geothermometer at a small depth in order to obtain a knowledge of the average temperature of the earth, is therefore readily practicable only between the tropics or in the subtropical zone. The excellent expedient of Artesian wells which have indicated an increase of heat of 1° F. for every 54 to 58 feet in absolute depths of from 745 to 2345 feet has hitherto only been afforded to the physicist in districts not much more than 1600 feet above the level of the sea 37 I have visited silver-mines in the chain of the Andes, 6°45′ south of the equator at an elevation of nearly 13,200 feet and found the temperature of the water penetrating through the fissures of the limestone to be 52°.3 F.38 The waters which were heated in the baths of the Inca

36 See Cosmos, vol. i, p. 218, and vol. v, p. 40, Bohn's edition. 37 See above, p. 37.

38 Mina de Guadalupe, one of the Minas de Chota, l. c. sup. p. 41.

39

Tupac Yupanqui, upon the ridge of the Andes (Paso del Assuay), probably come from springs of the Ladera de Cadlud, where I have traced their course, near which the old Peruvian causeway also ran, barometrically to an elevation of 15,526 feet (almost that of Mont Blanc). These are the highest points at which I could observe spring water in South America. In Europe the brothers Schlagint weit have found gallery-water in the gold mine in the Eastern Alps at a height of 9442 feet, and found that the temperature of small springs near the opening of the gallery of only 33°.4 F.,40 at a distance from any snow or glacier ice. The highest limits of springs are very different according to geographical latitude, the elevation of the snow line and the relation of the highest peaks to the mountain ridges and plateaux. If the radius of our planet were to be increased by the height of the Himalaya at the Kintschindjunga, and therefore uniformly over the whole surface by 28,175 feet (4.34 English miles), with this small increase of only th of the radius, the heat in the surface cooled by radiation, would be (according to Fourier's analytical theory), almost the same as it now is in the upper crust of the earth. But if individual parts of the surface raise themselves in mountain chains and narrow peaks, like rocks upon the bottom of the aerial ocean, a diminution of heat takes place in the interior of the elevated strata, and this is modified by contact with strata of air of different temperature, by the capacity for heat and conductive power of heterogeneous kinds of rocks, by the sun's action on the forest-clad summits and declivities, by the greater and less radiation of the mountains in accordance with their form (relief), their massiveness) or their conical and pyramidal narrowuess. The special elevations of the region of clouds, the snow and ice-coverings at various elevations of the snow line, and the frequency of the cool currents of air coming down the steep declivities, at particular times of the day, alter the effect of the terrestrial radiation. In proportion as the towering cones of the summits become cooled, a weak current

39 Humboldt, Views of Nature, p. 393.

40 Mine on the Great Fleuss in the Moll Valley of the Tauern, see Hermann and Adolph Schlagintweit, Untersuchungen über die physikaLische Geographie der Alzen, 1850, s. 242-273.

of heat tending towards, but never reaching an equilibrium, sets in from below upwards. The recognition of so many factors acting upon the vertical distribution of heat, leads to well-founded presumptions regarding the connexion of complicated local phenomena, but not to direct numerical determinations. In the mountain springs (and the higher ones, being important to the chamois-hunter, are carefully sought) there so often remains the doubt that they are mixed with waters, which by sinking down introduce the colder temperature of higher strata, or by ascending introduce the warmer temperature of lower strata. From 19 springs, observed by Wahlenberg, Kämtz draws the conclusion that in the Alps we must rise from 960 to 1023 feet in order to see the temperature of the springs sink 1° C. (1°.8 F.). A greater number of observations selected with more care by Hermann and Adolph Schlagintweit in the eastern Carinthian Alps and in the western Swiss Alps on the Monte Rosa, give only 767 feet. According to the great work of these excellent observers, "the decrease of the temperature of springs is certainly somewhat more gradual than that of the average annual temperature of the air, which in the Alps amounts to about 320 feet for 1° F. The springs there are in general warmer than the average temperature of the air at the same level; and the difference between the temperature of the air and springs increases with the elevation. The temperature of the soil is not the same at equal elevations in the entire range of the Alps, as the isothermal surfaces which unite the points of the same average temperature of springs, rise higher above the level of the sea, independently of the influence of latitude, in proportion to the average convexity of the surrounding soil; perfectly in accordance with the laws of the distribution of heat in a solid body of varying thickness, with which the relief (the mass-elevation) of the Alps may be compared."

41

In the chain of the Andes, and indeed in those volcanic parts of it which present the greatest elevations, the burying of thermometers may in particular cases lead to deceptive results by the influence of local circumstances. From the opinion formerly held by me, that black rocky ridges, visible at a great distance, which penetrate the snowy 41 Monte Rosa, 1853, chap. vi, s. 212-225.

region, are not always indebted for their entire freedom from snow to the steepness of their sides, but to other causes, I buried the bulb of a thermometer only three inches deep in the sand which filled the fissure in a ridge on the Chimborazo at an elevation of 18,290 feet, and therefore 3570 feet above the summit of Mont Blanc. The thermometer permanently showed 10°.5 F. above the freezing point, whilst the air was only 4°.5 F. above that point. The result of this observation is of some importance; for even 2558 feet lower, at the lower limit of perpetual snow of the volcano of Quito, according to numerous observations collected by Boussingault and myself, the average temperature of the atmosphere is not higher than 34°.9 F. The ground temperature of 42°.5 must therefore be ascribed to the subterranean heat of the doleritic mountain: I do not say of the entire mass, but to the currents of air ascending in it from the depths. At the foot of Chimborazo, at an elevation of 9486 feet towards the hamlet of Calpi, there is, moreover, a small crater of eruption, Yana-Urcu, which, as indeed is shown by its black, slag-like rock (augitic-porphyry), appears to have been active in the middle of the fifteenth century.42

The aridity of the plain from which Chimborazo rises, and the subterranean brook, which is heard rushing under the volcanic hill (Yana-Urcu) just mentioned, have led Boussingault and myself at very different times to the idea that the water which the enormous masses of snow produce daily by melting at their lower limit, sinks into the depths through the fissures and chambers of the elevated volcano. These waters perpetually produce a refrigeration in the strata through which they run down. Without them the whole of the doleritic and trachytic mountains would acquire, even at times when no near eruption is foretold, a still higher temperature in their interior, from the volcanic source, perpetually in action, although perhaps not lying at the same depth in all latitudes. Thus, in the varying struggle of the causes of heat and cold, we have to assume a constant tide of heat upwards and downwards in those places where conical solid parts ascend into the atmosphere. 42 Humboldt, Kleinere Schriften, Bd. i, pp. 139 and 147. 43 Humboldt, Op. cit., s. 140 and 203.

VOL. V.