tion. In a portion of my work, which treats of volcanic phenoniena, I cannot wholly pass in silence those problems, which have been suggested by other inquirers in reference to the currents pervading the general fluid in the interior of our planet, or the probable or improbable periodically ebbing and flowing movement in individual and imperfectly filled basins, or the existence of portions of space, having a very low specific gravity and underlying the upheaved mountain chains 29 In a work devoted to cosmical phenomena no question should be overlooked on which actual observations have been instituted, or which may seem to be elucidated by close analogies.

b. The Existence and Distribution of Heat in the interior of our Globe.

(Expansion of the Delineation of Nature,
Cosmos, vol. i, pp. 160—168.)

Considerations regarding the internal heat of our earth, the importance of which has been greatly augmented by the connection which is now generally recognised to exist between it and phenomena of upheavals and of volcanic action, are based partly upon direct, and therefore incontrovertible measurements of temperature in springs, borings, and subterranean mines, and partly upon analytical combinations regarding the gradual cooling of our planet, and the influence which the decrease of heat may have exercised in primeval ages upon the velocity of rotation and upon the direction of the currents of internal heat.30 The figure of the compressed terrestrial spheroid is further dependent upon the law, according to which density increases in concentric superimposed non-homogeneous strata. The first or experimental, and therefore the more certain portion of the investigation to which we shall limit ourselves in the present place, throws light only upon the accessible crust of the earth, which is of very inconsiderable thickness, whilst the

29 See Petit sur la latitude de l'Observatoire de Toulouse, la densité moyenne de la chaine des Pyrénées, et la probabilité qu'il existe un vide sous cette chaine, in the Comptes rendus de l'Acad. des Sc,, t. xxix, 1849, p. 730.

30 Cosmos, vol. i, p. 169.

31

second or mathematical part, in accordance with the nature of its applications, yields rather negative than positive results. This method of enquiry, which possesses all the charm of ingenious and intellectual combinations of thought, leads to problems, which cannot be wholly overlooked when we touch upon conjectures regarding the origin of volcanic forces, and the reaction of the fused interior upon the solid éxternal crust of our earth. Plato's geognostic myth of the Pyriphlegethon, as the origin of all thermic springs as well as of volcanic igneous currents, emanated from the early and generally felt requirement of discovering some common cause for a great and complicated series of phenomena.

32

Amid the multiplicity of relations presented by the earth's surface, in respect to insolation (solar action) and its capacity of radiating heat, and amid the great differences in the capacity for conducting heat, which varies in accordance with the composition and density of heterogeneous rocks, it is worthy of notice, that wherever the observations have been conducted with care, and under favourable circumstances, the increase of the temperature with the depth has been found to present for the most part very closely coinciding results, even at very different localities. For very great depths we obtain the most certain results from Artesian wells, especially when they are filled with fluids that have been rendered turbid by the admixture of clay, and are therefore less favourable to the passage of internal currents, and when they do not receive many lateral affluents flowing into them at different elevations through transverse fissures. On account of their depth, we will begin with two of the most remarkable Artesian wells, namely that of Grenelle, near Paris, and that of the New Salt Works at Oeynhausen, near Minden. We will proceed in the following paragraph to give some of the most accurate results which they have yielded.

33

According to the ingenious measurements of Walferdin,3 31 Hopkins, Physical Geology, in the Report of the British Association for 1838, p. 92; Philos. Transact., 1839, pt. ii, p. 381, and 1840, pt. i, p. 193; Hennessey (Terrestrial Physics) in the Philos. Transact., 1851, pt. ii, pp. 504-525.

32 Cosmos, vol. i, p. 235.

33 The observations of Walferdin were made in the autumn of 1847, and deviate very slightly from the results obtained with the same appa

to whom we are indebted for a complete series of very delicate apparatus for determinations of temperature at great depths in the sea and in springs, the surface of the basin of the well at Grenelle lies at an elevation of 36.24 metres or 119 feet above the level of the sea.

The upper outlet

of the ascending spring is 33.33 metres or 109.3 feet higher. This total elevation of the ascending water (69.57 metres or 228.2 feet) is, when compared with the level of the sea about 196.8 feet lower than the outbreak of the green sandstone strata in the hills near Lusigny, south-east of Paris, to whose infiltrations the rise of the waters in the Artesian wells at Crenelle have been ascribed. The borings extend to a depth of 547 metres or 1794.6 feet below the base of the Grenelle basin, or about 510.76 metres or 1675 feet below the level of the sea; the waters consequently rise to a total height of 580.33 metres or 1904 feet. The temperature of the spring is 81°.95 F.; consequently the increase of heat marks 1° F. for about every 59 feet.

The boring at the New Salt Works at Rehme is situated 231 feet above the level of the sea (above the watermark at Amsterdam). It has penetrated to an absolute depth of 2281 feet below the surface of the earth, measuring from the point where the operations were begun. The salt spring which, when it bursts forth, is impregnated with a large quantity of carbonic acid, lies therefore 2052 feet below the level of the sea, a relative depth which is perhaps the greatest that has ever been reached by man in the interior of the earth. The temperature of the salt spring at the New Salt Works of Oeynhausen is 91°04 F., and as the mean annual temperature of the air at these works is about 49°.3 F., we may assume that there is an increase of temperature of 1° F'. for every 54.68 feet. The boring at these Salt Works 34 is therefore 491 feet absolutely deeper than the boring at

ratus, by Arago, in 1840, at a depth of 1657 feet, when the borer had left the chalk and was beginning to penetrate through the gault. See Cosmos, vol. i, p. 167, and Comptes rendus, t. xi, 1840, p. 707.

34 According to the manuscript results given by the superintendent of the mines of Oeynhausen. See Cosmos, vol. i, pp. 148, 166; and Bischof, Lehrbuch der Chem. und Phys. Geologie, Bd. i, Abth. 1, s. 154 -163. In regard to absolute depth, the borings at Mondorf, in the Grand Duchy of Luxemburg (2202 feet), approach most nearly to those at the new salt works at Oeynhausen.

Grenelle; it sinks 377 feet deeper below the surface of the sea, and the temperature of its waters is 9°.18 F. higher. The increase of the heat at Paris, is about 1° F. for 59 feet, and therefore scarcelyth greater. I have already elsewhere drawn attention to the fact that a similar result was obtained by Auguste de la Rive and Marcet, at Brégny, near Geneva, in investigating a boring which was only 725 feet in depth, although it was situated at an elevation of more than 1600 feet above the Mediterranean Sea.35

If to these three springs, which possess an absolute depth varying between 725 feet and 2285 feet, we add another, that of Monkwearmouth, near Newcastle, (the water rising through a coal mine which, according to Phillips is worked at a depth of 1496 feet below the level of the sea,) we shall find this remarkable result, that at four places widely separated from one another an increase of heat of 1° F. varies only between 54 and 58.6 feet; such a coincidence in the results cannot, however, be always expected to occur when we consider the nature of the means which are employed for determining the internal heat of the earth at definite depths. Although we may assume that the water which is infiltrated in elevated positions through hydrostatic pressure as in connected tubes, may influence the rising of springs at points of great depth, and that the subterranean

35 Cosmos, vol. i, p. 166, and Mémoires de la Société d'Hist. Naturelle de Genève, t. vi, 1833, p. 243. The comparison of a number of Artesian wells in the neighbourhood of Lille, with those of Saint Ouen and Geneva would, indeed, lead us to assume, if we were quite certain as to the accuracy of the numerical data, that the different conductive powers of terrestrial and rocky strata exert a more considerable influence than has generally been supposed (Poisson, Théorie Mathématique de la Chaleur, p. 421).

36 In a table of fourteen borings, which were more than one hundred yards in depth, and which were situated in various parts of France, Bravais, in his very instructive encyclopædic memoir in the Patria, 1847, p. 145, indicates nine in which an increase of temperature of 1° F. is found to occur for every 50-70 feet of depth, which would give a deviation of about 10 feet in either direction from the mean value given in the text. See also Magnus in Poggen. Ann. Bd. xxii, 1831, s. 146. It would appear, on the whole, that the increase of temperature is most rapid in Artesian wells of very inconsiderable depth, although the very deep wells of Monte Massi in Tuscany, and Neuffen on the north-west part of the Swabian Alps, present a remarkable exception to this rule.

waters acquire the temperature of the terrestrial strata with which they are brought in contact, the water that is obtained through borings may, in certain cases, when communicating with vertically descending fissures, obtain some augmentation of heat from an inaccessible depth. An influence of this kind, which is very different from that of the varying conductive power of different rocks, may occur at individual points widely distant from the original boring. It is probable that the waters in the interior of our earth move in some cases within limited spaces, flowing either in streams through fissures (on which account it is not unusual to find that a few only of a large number of contiguous borings prove successful), or else follow a horizontal direction, and thus form extensive basins-a relation which greatly favours the labour of boring, and in some rare cases betrays, by the presence of eels, mussels, or vegetable remains, a connection with the earth's surface. Although from the causes which we have already indicated, the ascending springs are sometimes warmer than the slight depth of the boring would lead us to anticipate, the afflux of colder water which flows laterally through transverse fissures leads to an opposite result.

It has already been observed that points situated on the same vertical line at an inconsiderable depth within the interior of our earth, experience at very different times the maximum and minimum of atmospheric temperature, which is modified by the sun's place, and by the seasons of the year. According to the very accurate observations of Quetelet, daily variations of temperature are not percep tible at depths of 34ths feet below the surface; 37 and at Brussels, the highest temperature was not indicated until the 10th of December in a thermometer which had been sunk to a depth of more than 25 feet, whilst the lowest temperature was observed on the 15th of June. In like manner, in the admirable experiments made by Professor Forbes, in the neighbourhood of Edinburgh, on the conductive power of different rocks, the maximum of heat was not observed until the 8th of January in the basaltic trap of Calton Hill at a depth of 24 feet below the surface.38 It would appear

37 Quetelet, in the Bulletin de l'Acad. de Bruxelles, 1836, p. 75. 38 Forbes, Exper. on the temperature of the earth at different depths