[footnote] *On the theory of isogeothermal (chthonisothermal) lines, consult the ingenious labors of Kupffer, in Pogg, 'Annalen', bd xv., s. 184, and bd xxxii., s. 270, in the 'Voyage dans l'Oural', p. 382-298, and in the 'Edinburgh Journal of Science', New Series, vol. iv., p. 355. See, also, Kamtz, 'Lehrb. der Meteor.', bd. ii., s. 217; and, on the ascent of the chthonisothermal lines in mountainous districts, Bischof, s. 174-198.
Such at any rate, is the result I have arrived at from my own observations and those of my fellow-travelers in Northern Asia. The temperature of springs, which has become the subject of such continuous physical investigation during the last half century, depends, like the elevation of the line of perpetual snow, on very many simultaneous and deeply-involved causes. It is a function of the temperature of the stratum in which they take their rise, of the specific heat of the soil, and of the quantity and temperature of the meteoric water,* which is itself different from the temperature of the lower strata of the atmosphere, according to the different modes of its origin in rain, snow, or hail.**
[footnote] *Leop. v. Buch, in Pogg., 'Annalen', bd. xii., s. 405.
[footnote] ** On the temperature of the drops, of rain in Cumana, which fell to 72 degrees, when the temperature of the air shortly before had been 86 degrees and 88 degrees, and during the rain sank to 74 degrees, see my 'Relat. Hist.', t. ii., p. 22. The rain-drops, while falling, change the normal temperature they originally possessed, which depends on the height of the clouds from which they fell, and their heating on their upper surface by the solar rays. The rain-drops, on their first production, have a higher temperature than the surrounding medium in the superior strata of our atmosphere, in consequence of the liberation of their latent heat; and they continue to rise in temperature, since, in falling through lower and warmer strata, vapor is precipitated on them, and they thus increase in size (Bischof, 'Warmelehre des inneren Erdkorpers' s. 73); but this additional heating is compensated for by evaporation. The cooling of the air by rain (putting out of the question what probably belongs to the electric process in storms) is effected by the drops, which are themselves of lower temperature, in consequence of the cold situation in which they were formed, and bring down with them a portion of the higher colder air, and which finally, by moistening the ground, give rise to evaporation. The cooling of the air by rain (putting out of the question what probably belongs to the electric process in storms) is effected by the drops, which are themselves of lower temperature, in consequence of the cold situation in which they were formed, and bringi down with them a portion of the higher colder air, and which finally, by moistening the ground, give rise to evaporation. These are the ordinary relations of the phenomenon. When, as occasionally happens, the rain-drops are warmer than the lower strata of the atmosphere (Humboldt, 'Rel. Hist.', t. iii., p. 513), the cause must probably be sought in higher warmer currents, or in a higher temperature of widely-extended and not very thick clouds, from the action of the sun's rays. How, moreover, the phenomenon of supplementary rainbows, which are explained by the interference of light, is connected with the original and increasing size of the falling drops, and how an optical phenomenon, if we know how to observe it accurately, may enlighten us regarding a meteorological process, according to diversity of zone, has been shown, with much talent and ingenuity, by Arago, in the 'Annuaire' for 1836, p. 300.
Cold springs can only indicate the mean atmospheric temperature p 221 when they are unmixed with the waters rising from great depths, or descending from considerable mountain elevations, and when they have passed through a long course at a depth from the surface of the earth which is equal in our latitudes to 40 or 60 feet, and according to Boussingault, to about one foot in the equinoctial regions,* these being the depths at which the invariability of the temperature begins in the temperate and torrid zones, that is to say, the depths at which horary, diurnal, and monthly changes of heat in the atmosphere cease to be perceived.
[footnote] * The profound investigations of Boussingault fully convince me, that in the tropics, the temperature of the ground, at a very slight depth, exactly corresponds with the mean temperature of the air. The following instances are sufficient to illustrate this fact:
________________________________________________________ Stations Temperature at Mean Height, in within 1 French foot Temperature English Tropic [1.006 of the of the feet, above Zones. English foot] air. the level below the of the sea. earth's surface. ________________________________________________________
Guayaquil 78.8 78.1 0
Anserma Nuevo 74.6 74.8 3444
Zupia 70.7 70.7 4018
Popayan 64.7 65.6 5929
Quito 59.9 59.9 9559
________________________________________________________
The doubts about the temperature of the earth within the tropics, of which I am probably, in some degree, the cause, by my observations on the Cave of Caripe (Cueva del Guacharo), 'Rel. Hist.', t. iii., p. 191-196), are resolved by the consideration that I compared the presumed mean temperature of the air of the convent of Caripe, 65.3 degrees, not with the temperature of the air of the cave, 65.6 degrees, but with the temperature of the subterranean stream, 62.3degrees, although I observed ('Rel. Hist.', t. iii., p. 146 and 195) that mountain water from a great height might probably be mixed with the water of the cave.
Hot springs issue from the most various kinds of rocks. The hottest permanent springs that have hitherto been observed are, as my own researches confirm, at a distance from all volcanoes. I will here advert to a notice in my journal of the Aguas Calientes de las Trincheras', in South America, between Porto Cabello and Nueva Valencia, and the 'Aguas de Comangillas', in the Mexican territory, near Guanaxuato; the former of these, which issued from granite, had a temperature of 194.5 degrees; the latter, issuing from basalt, 205.5degrees. The depth of the source from whence the water flowed with this temperature, judging from what we know of the law of the increase of heat in the interior of the earth, was probably 7140 feet, or above two miles. If the universally-diffused terrestrial heat be the cause of thermal springs, as of active volcanoes, the rocks can only exert an influence by the different capacities p 222 for heat and by their conducting powers. The hottest of all permanent springs (between 203 degrees and 209 degrees) are likewise, in a most remarkable degree, the purest, and such as hold in solution the smallest quantity of mineral substances. Their temperature appears, on the whole, to be less constant than that of springs between 122 degrees and 165 degrees, which in Europe, at least, have maintained, in a most remarkable manner, their 'invariability of heat and mineral contents' during the last fifty or sixty years, a period in which thermometrical measurements and chemical analyses have been applied with increasing exactness. Boussingault found in 1823 that the thermal springs of Las Tricheras had risen 12 degrees during the twenty-three years that had intervened since my travels in 1800.*