INTERNAL TEMPERATURE OF THE EARTH. 



fit 



ence to particular portions of the surface, that 

 nothing like a regular figure can be inferred 

 which would accord with the whole of the re- 

 sults hitherto obtained in these ways. The 

 true figure of the' earth stands in the same re- 

 lation to a regular figure. " as the uneven sur- 

 face of ruffled stands to the even surface of 

 unruffled water." 



After the earth has been measured, it must 

 be WEIGHED. Pendulum vibrations and the 

 plumb-line have alike served to determine the 

 nean density of the earth — whether the rela- 

 tive density was investigated by a combina- 

 tion of astronomical and geodetical operations, 

 through the deflection of a plumb-line from the 

 perpendicular in the vicinity of a mountain, or 

 by contrasting the length of the pendulum beat- 

 ing seconds on a plain and on the summit of a 

 neighbouring height, or, finally, by the applica- 

 tion of the torsion-balance, which may be re- 

 garded as a delicate horizontally swinging pen- 

 dulum. Of these three methodsC^^**), the last 

 is the safest, inasmuch as it is independent of 

 the difficult determination of the density of the 

 minerals composing the spherical segment of a 

 mountain in the neighbourhood of which the 

 observations are made. The latest research- 

 es, which are those of Reich, give 5-44 as the 

 mean density of the whole earth ; that is to 

 say, the earth is nearly 5i^ times more dense 

 than pure water. But as the mineral species 

 which constitute the dry land have a mean 

 density of no more than about 2-7, and the dry 

 land and the ocean together a density of but 

 1-6, it foUow^s from this assumption how much 

 the elliptical unequally oblated strata of the in- 

 terior must increase in density through pres- 

 sure, or through heterogeneousness of material 

 towards the centre. And here we see, again, 

 with what propriety the pendulum, both that 

 which swings perpendicularly and that which 

 swings horizontally, has been designated a ge- 

 ognostical instrument. 



But the conclusions to which the use of such 

 an instrument leads, have induced distinguish- 

 ed natural philosophers to take entirely oppo- 

 site views of the constitution of the earth's in- 

 terior. It has been calculated at what depth 

 liquid, and even aeriform bodies, would come 

 to surpass platinum, and even iridium, in den- 

 sity, through the proper pressure of their own 

 superimposed strata ; and in order to bring the 

 oblateness of the earth's spheroid, known with- 

 in a very small quantity, into harmony with 

 the assumption of a single and infinitely com- 

 pressible substance, the acute Leslie has gone 

 so far as to have described the nucleus of the 

 earth as a hollow sphere, filled with " impon- 

 derable matter of enormous repulsive powers." 

 These daring and arbitrary conjectures have 

 given rise to still more fantastical dreams in 

 non-scientific circles. The hollow sphere has, 

 by degrees, been peopled with plants and ani- 

 mals, and furnished, moreover, with a couple 

 of small subterranean planets — Pluto and Pros- 

 erpine, which there dispense their gentle light. 

 An unvarying temperature reigns in this inter- 

 nal space, and the air, self-luminous by com- 

 pression, might well make the presence of the 

 subterraneous planets, Pluto and Proserpine, 

 unnecessary. Near the north-pole, under the 



82d parallel of latitude, where the aurora bo- 

 realis streams up into the sky, there is an en- 

 ormous opening, through which it were easy to 

 descend into the hollow sphere. To such a 

 subterranean expedition the late Sir Humphry 

 Davy and I were repeatedly and publicly invi- 

 ted by Captain Symmes. So strongly is the 

 morbid disposition of man inclined, unencum- 

 bered with the contradictory testimony of well- 

 established facts or generally admitted natural 

 laws, to fill unseen space with marvellous 

 forms ! But the celebrated Halley himself, at 

 the end of the 17th century, had hollowed out 

 the earth in the course of his magnetical spec- 

 ulations : a subterraneous freely rotating nu- 

 cleus, by its varying position, occasions the 

 diurnal and annual variations of the magnetical 

 declination ! What was a mere lively fiction 

 with the clever Holberg, has, in our days, with 

 tedious solemnity, been attempted to be decked 

 out in a scientific garb. 



The figure of the earth, and the degree of 

 solidity or density which it possesses, stand in 

 intimate connection with the forces which an- 

 imate our globe, in so far, namely, as these 

 forces are not excited or awakened from with- 

 out by our planetary position opposite to a self- 

 luminous central body. The oblateness, a con- 

 sequence of the operation of the centrifugal 

 force upon a rotating mass, reveals the pristine 

 or former state of fluidity of our planet. On 

 the setting or solidification of this fluid, which 

 we are accustomed to conjecture as existing in 

 the shape of a vaporiform matter, originally 

 heated to a very high temperature, an enormous 

 amount of latent caloric became free. If the 

 process of consolidation began in the way 

 Fourier will have it, by radiation from the sur- 

 face into celestial space, the parts of the earth 

 which are situated towards the centre must 

 still be hot and molten. While, after long ra- 

 diation of the heat of the central parts towards 

 the surface, a state of stability in the tempera- 

 ture of the earth is finally attained, it is at the 

 same time assumed that, with an increase in 

 depth, there will also be a regular progressive 

 increase of temperature. The temperature of 

 the water which flows from bores of great 

 depth into the bowels of the earth (Artesian 

 wells), immediate experiments on the temper- 

 ature of the rocks in mines, above all, however, 

 the volcanic activity of the earth, in other words, 

 the discharge of molten mineral streams through 

 fissures in the surface, bear testimony in the 

 most incontestable manner to this increase of 

 temperature in the upper strata of the earth at 

 considerable depths. From conclusions which, 

 it is true, are only founded on analogy, it is 

 more than probable that the temperature goes 

 on increasing in a still greater degree towards 

 the centre. 



The conclusions which have been presented 

 to us by an ingenious, and, for this class of in- 

 quiries, singularly perfect analytical calculus, 

 on the motion of heat in homogeneous metallic 

 spheroids(^"), can only be applied, with many 

 precautions, to the actual constitution of our 

 planet, in consequence of our ignorance of the 

 matter of which the earth is composed, of the 

 various capacities for heat and powers of con- 

 duction inherent in the superimposed masses, 



