b. Thermal Springs. (Amplification of the Representation of Nature. As a consequence of the vital activity of the interior of our planet, evidenced in irregularly repeated and often fearfully destructive phenomena, we have described the 14th of December, 1796. Earthquake and destruction of the city of Cumana; 4th of February, 1797. Earthquake and destruction of Riobamba. On the same morning the columns of smoke of the volcano of Pasto, at a distance of at least 200 geographical miles from Riobamba, disappeared suddenly, and never reappeared; no commotion was felt in its vicinity. The ele 30th of January, 1811. First appearance of the island of Sabrina, in the group of the Azores, near the island of St. Michael. vation preceded the eruption of fire, as in the case of the little Kameni (Santorin) and that of the volcano of Jorullo. After an eruption of cinders, lasting for six days, the island rose to a height of 320 feet above the surface of the sea. It was the third appearance and disappearance of the island nearly at the same point, at intervals of 91 and 92 years. May, 1811. More than 200 shocks of earthquake on the island of St. Vincent up to April, 1812. December, 1811. Innumerable shocks in the river-valleys of the Ohio, Mississippi, and Arkansas up to 1813. Between New Madrid, Little Prairie, and La Saline, to the north of Cincinnati, the earthquakes occurred almost every hour for months together. December, 1811. A single shock in Caraccas. 26th of March, 1812. Earthquake and destruction of the town of Caraccas. The circle of commotion extended over Santa Marta, the town of Honda, and the elevated plateau of Bogotá, to a distance of 540 miles from Caraccas. The motion continued until the middle of the year 1813. 30th of April, 1812. Eruption of the volcano of St. Vincent; and on the same day, about 2 o'clock in the morning, a fearful subterranean noise, like the roar of artillery, was heard at the same time and with equal distinctness on the shores of Caraccas, in the Llanos of Calabazo and of the Rio Apure, without being accompanied by any concussion of the earth (see ante, p. 178). The subterranean noise was also heard upon the Island of St. Vincent, but, and this is very remarkable, it was stronger at some distance upon the sea. earthquake. In this, there prevails a volcanic power, which in its essential nature only acts dynamically, producing movement and commotion, but when it is favoured at particular points by the fulfilment of subsidiary conditions, it is capable of bringing to the surface material products, although not of generating them like true volcanoes. Just as water, vapours, petroleum, mixtures of gases, or pasty masses (mud and moya) are thrown out, through fissures suddenly opened in earthquakes sometimes of short duration, so do liquid and aerial fluids flow permanently from the bosom of the earth through the universally diffused network of communicating fissures. The brief and impetuous eruptive phenomena are here placed beside the great peaceful spring-system of the crust of the earth, which beneficently refreshes and supports organic life. For thousands of years it returns to organized nature the moisture which has been drawn from the atmosphere by falling rain. Analogous phenomena are mutually illustrative in the eternal economy of nature; and wherever an attempt is made at the generalisation of ideas, the intimate concatenation of that which is recognized as allied must not remain unnoticed. The widely disseminated classification of springs, into cold and hot, which appears so natural in ordinary conversation, has but a very indefinite foundation when reduced to numerical data of temperature. If the temperature of springs be compared with the internal heat of man (found, with thermo-electrical apparatus, to be 980-98°.6 F. according to Brechet and Becquerel), the degree of the thermometer at which a fluid is called cold, warm, or hot, when in contact with parts of the human body, is very different according to individual sensations. No absolute degree of temperature can be established, above which a spring should be designated warm. The proposition to call a spring cold in any climatic zone, when its average annual temperature does not exceed the average annual temperature of the air in the same zone, at least presents a scientific exactitude, by affording a comparison of definite numbers. It has the advantage of leading to considerations upon the different origin of springs, as the ascertained agreement of their temperature with the annual temperature of the air is recognized directly in unchangeable springs; and in changeable ones, as has been shown by Wahlenberg and Erman the elder, in the averages of the summer and winter months. But in accordance with the criterion here indicated, a spring in one zone must be denominated warm, which hardly attains the seventh or eighth part of temperature of one which in another zone, near the equator, will be called cold. I may mention the differences between the average temperature of St. Petersburg (38°.12 F.) and of the shores of the Orinoco. The purest spring water which I drank in the vicinity of the cataracts of Atures 3 and Maypures (81°.14 F.) or in the forest of Atabapo, had a temperature of more than 79° F.; even the temperature of the great rivers in tropical South America, corresponds with the high degrees of heat of such cold springs. 31 Humboldt, Voyage aux Régions Equinoxiales, t. ii, p. 376. 32 For the sake of comparing the temperature of springs where they break fort directly from the earth, with that of large rivers flowing through open channels, I here bring together the following average numbers from my journals : : Rio Apure, lat. 73°; temperature, 81o. Orinoco, between 4° and 8° of latitude; 81°.5--85°.3. Springs in the forest, near the cataract of Maypures, breaking forth from the granite, 82o. Cassiquiare, the branch of the Upper Orinoco, which forms the union with the Amazon; only 75°.7. Rio Negro, above San Carlos (scarcely 1° 53′ to the north of the equator); only 740.8. Rio Atabapo, 79°.2 (lat. 3o 509. Orinoco, near the entrance of the Atabapo, 82o. Rio Grande de la Magdalena (lat. 5° 12′ to 9° 56′), 79° 9′. Amazon, 5° 31′ south latitude, opposite to the Pongo of Rentema (Provincia Jaen de Bracamoros), scarcely 1300 fet above the South Sea, only 72°.5. The great mass of water of the Orinoco consequently pproaches the average temperature of the air of the vicinity. During great inundations of the Savannahs, the yellowish brown waters, which smell of sulphuretted hydrogen, acquire a temperature of 92°.8; this I found to be the temperature in the Lagartero,to the east of Guayaquil, which swarmed with crocodiles. The soil there becomes heated, as in shallow rivers, by the warmth produced in it by the sun's rays falling upon it. With regard to the multifarious causes of the low temperature of the water of the Rio Negro, which is of a coffee-brown colour by reflected light, and of the white waters of the Cassiquiare (a constantly clouded sky, the quantity of rain, the evaporation from the dense forests, and The breaking out of springs, effected by multifarious causes of pressure and by the communication of fissures containing water, is such a universal phenomenon of the surface of the earth, that waters flow forth at some points from the most elevated mountain strata, and at others from the bottom of the sea. In the first quarter of this century numerous results were collected by Leopold von Buch, Wahlenberg and myself, with regard to the temperature of springs and the diffusion of heat in the interior of the earth in both hemispheres, from 12° S. lat. to 71° N.33 The springs which have an unchangeable temperature were carefully separated from those which vary with the seasons; and Leopold von Buch ascertained the powerful influence of the distribution of rain in the course of the year, that is to say, the influence of the proportion between the relative abundance of winter and summer rain upon the temperature of the variable springs, which, as regards number, are the most widely distributed. More recently some very ingenious the want of hot sandy tracts upon the banks), see my river voyage, in the Relation Historique, t. ii, pp. 463 and 509. In the Rio Guancabamba or Chamaya, which falls into the Amazon, near the Pongo de Rentema, I found the temperature of the water to be only 67°.6, as its waters come with prodigious swiftness from the elevated lake Simicocha on the Cordillera. On my voyage of 52 days up the river Magdalena, from Mahates to Honda, I perceived most distinctly, from numerous observations, that a rise in the level of the water was indicated for hours previously by a diminution of the temperature of the river. The refrigeration of the stream occurred before the cold mountain waters from the Paramos near the source caine down. Heat and water move, so to speak, in opposite directions and with very unequal velocities. When the water near Badillas rose suddenly, the temperature fell long before from 80°.6 to 74°.3. As, during the night, when one is established upon a low sandy islet, or upon the bank, with bag and baggage, a rapid rise of the river may be dangerous, the discovery of a prognostic of the approaching rise (the avenida) is of some importance. 33 Leopold von Buch, Physicalische Beschreibung der canarischen Inseln, s. 8: Poggend. Annalen, Bd. xii, s. 403; Bibliothèque Britannique, Sciences et Arts, t. xix, 1802, p. 263; Wahlenberg, De Veget. et Clim. in Helvetia Septentrionali Observatis, pp. lxxviii and lxxxiv; Wahlenberg, Flora Carpathica, p. xciv, and in Gilbert's Annalen, Bd. xli, s. 115; Humboldt, in the Mém. de la Soc. d'Arcueil, t. iii (1817) p. 599. 34 De Gasparin, in the Bibliothèque Univ. Sciences et Arts, t. xxxviii, 1828, pp. 54, 113 and 264; Mém. de la Soc. Centrale d'Agriculture, 35 comparative observations by De Gasparin, Schouw and Thurmann have thrown considerable light, in a geographical and hypsometrical point of view, in accordance with latitude and elevation upon this influence. Wahlenberg asserted that in very high latitudes the average temperature of variable springs is rather higher than that of the atmosphere; he sought the cause of this, not in the dryness of a very cold atmosphere and in the less abundant winter rain caused thereby, but in the snowy covering diminishing the radiation of heat from the soil. In those parts of the plain of Northern Asia, in which a perpetual icy stratum, or at least a frozen alluvial soil mixed with fragments of ice is found at a depth of a few feet, the temperature of springs can only be employed with great caution for the investigation of Kupffer's important theory of the isogeothermal lines. A two-fold radiation of heat is then produced in the upper stratum of the earth: one upwards towards the atmosphere, and another downwards towards the icy stratum. A long series of valuable observations made by my friend and companion, Gustav Rose, during our Siberian expedition in the heat of summer (often in springs still surrounded by ice) between the Irtysch, the Obi, and the Caspian Sea, revealed a great complication of local disturbances. Those which present themselves from perfectly different causes in the tropical zone, in places where 1826, p. 178; Schouw, Tableau du Climat et de la Végétation de l'Italie vol. i, 1839, pp. 133-195; Thurmann, Sur la température des sources de la chaîne du Jura, comparée à celle des sources de la plaine Suisse, des Alpes et des Vosges, in the Annuaire Météorologique de la France, 1850, pp. 258-268. As regards the frequency of the summer and autumn rains, De Gasparin divides Europe into two strongly contrasted regions. Valuable materials are contained in Kämtz, Lehrbuch der Meteorologie, Bd. i, s. 448-506. According to Dove (Poggend. Annalen, Bd. XXXV, s. 376) in Italy, "at places to the north of which a chain of mountains is situated, the maxima of the curves of monthly quantities of rain fall in March and September; and where the mountains lie to the south, in April and October." The totality of the proportions of rain in the temperate zones may be comprehended under the following general point of view :-"The period of winter rain in the borders of the tropics constantly divides, the further we depart from these, into two maxima united by slighter falls, and these again unite into a summer-maximum in Germany; where, therefore, a temporary want of rain ceases altogether." See the section "Geothermik" in the excellent Lehrbuch der Geognosie, by Naumann, Bd. i, (1850), s. 41-73. 35 See above, p. 45. |