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duration before the introduction of matter into the universe, or the conversion of a vacuum into a plenum, the great Mind of the universe must have been without employment, at least such as we have any conception of. Mind and inatter must have always existed. The universe is subject to continual change, but there is neither creation nor annihilation.
But how different is the story which science tells us ! A close scrutiny of the phenomena of nature, as they are successively produced by the changes which take place in the physical world, reveals to us immutable laws which control the operations of the material universe, and even the phenomena of mind, so far as we see it manifested in this world. As yet, at least, we have not been able to discover any exception to this general truth. By observing the facts which the material world presents to us, and comparing them through the medium of the fixed laws of nature, we are carried back to a beginning of the present order of things in our world ; but all the facts that we have yet been able to gather make known to us but very little in relation to the amount of time which has been necessary to work all the changes that have taken place, and absolutely nothing respecting the date of the origin of this world. As we have said, science teaches us that there was a beginning, and we may truthfully say that we have learned very little respecting the nature of the present order of things.
We might perhaps show with a good degree of reason that what is known as the Nebular Hypothesis has a foundation in the actual operations of nature.* The examination and coordination of the facts observed on the earth and in the heavens, teach us that some process like that implied in the nebular hypothesis has been followed by nature in her method of world-making. Although it cannot be claimed that this is universally admitted among men of science, yet they who view it as a very probable hypothesis are by no means isolated examples. This hypothesis requires that a former condition of the earth was one of fluidity, and that the in
* See National Quarterly Review, No. XXXII, art. vi.
ternal parts may yet be in a highly heated state, if not actually molten. Indeed, until quite recently, the belief has been nearly general that the earth is composed of a solid crust of no great thickness, enclosing a molten interior, which is still the source of the internal fire of the earth. But whether we assume this to be true or not, it is demonstratively certain that portions of the interior of the earth, if not the whole interior, are highly heated, and this heat may properly be spoken of as the internal fire of our globe. In relation to the elevated temperature, or even molten state of internal parts of the earth, there is no disagreement; the question in dispute is whether all or only portions are in a fluid condition. But with this part of geological theory, we have but little concern at this time.* Indeed, the proper task of the geologist begins after the earth's crust has assumed its present condition, or one that is in many respects similar. The earliest processes in the formation of a crust, supposing the solidifying process to begin at the surface, could scarcely have left their impress on the existing strata.
M. Poisson, in his work on heat, has controverted the doctrines of a high internal temperature. His opinion is, that if the earth ever cooled down from a liquid state to its present condition, the consolidation would have begun at the centre, and not at the surface, as it is generally considered to have done. The temperature which the earth at present exhibits, M. Poisson thinks, is not inherent; that is, it did not originally possess an elevated temperature, but in passing through space, by the motion of the solar sytem, it somewhere reached a position where its temperature was much increased, and the effects of that is what we now observe. This is not only a gratuitous hypothesis, but one which a little consideration will show to be highly improbable. If it should pass sufficiently near a star, or a cluster of stars, to receive heat enough to be so marked in its influence, the mechanical motions communicated to the planets would derange their rel
* For a sketch of the early history of geology, and a general view of the formation of the earth, see N. Q. R., No. XLIII., Art. V.
VOL. XXVI. ---NO. LII.
ative motions about the sun to such an extent that they could be easily traced by mathematical analysis. Besides, the heat would be much more intense than that derived from the sun at present.* The conservation and correlation of force enable us to render a much more satisfactory account of the origin of internal terrestrial temperature.
It seems probable that the liquid condition of the earth is at present exhibited approximately by the sun. The formation of a crust would, perhaps, begin in spots, which would many times be re-melted before the existence of a crust would become permanent; and the contraction of the earth, in consequence of cooling, would cause the shell to wrinkle and produce hills and valleys, which would often be destroyed, because it can be shown that such a condition of surface would not be permanent.t Many changes must have taken place before any revolutions could have left their impress so as to be visible in the formations as they now exist. The earth's crust would not be stable, if its thickness were only a few miles, as has heretofore been supposed.
The astronomical phenomenon known as the precession of the equinoxes has, within a few years past, I been used to show that the earth must be nearly solid throughout. The precession for a homogeneous shell (which proves to be the same as that for a homogeneous solid earth) is compared with that for a heterogeneous shell, from which the minimum thickness of the shell is deduced, and it is found to be about one thousand miles. Archdeacon Pratt shows that the crushing effect of mountain masses like the Himalayas, and of the crust of the earth itself, is so great that a thickness much less than a thousand miles would not be able to resist it.
While we admit, however, that Mr. Hopkins' numerical results are the most definite of any that have hitherto been deduced, we must not fail to observe that they are based on a hypothetical law of density, namely, that the pressure varies
* See Whewell's Hist. Ind. Sciences, Vol. iii, p. 612.
+ Hopkins Philosophical Transactions, 1839-40-42. Pratt's Figure of the Earth, pp. 83-5.
| Figure of the Earth, p. 86.
with the square of the density. This law was assumed by Laplace, to enable him to integrate his formulæ for determining theoretically the figure of the earth. The probability that this law is nearly or quite the true law of nature, is very great, yet it would be well if it could be experimentally verified. It serves for determining very correctly, by calculation, the value of the precession of the equinoxes; but all the results are based on the continuity of the law of density, though there is a small probability, when we consider the facts presented by geology, that the real law of density is discontinnous somewhere between the surface and the centre, and yet such as to give the same value of the precession.
Many substances are of such a nature that pressure exerted upon them assists their solidification. The central portions of the earth may, therefore, be solid, even if the temperature be greater than what is required to fuse them at the surface. Mr. Hopkins imagines that the cooled masses - which ordinarilly would sink, owing to their greater specific gravitywould ultimately be prevented from sinking, by the imperfect liquidity of the material beneath. Thus, a solid surface shell would be formed, an interior highly heated solid nucleus, and a liquid stratum between, either continuous or discontinuous, according to circumstances, and of more or less perfect fluidity in different parts.* This seems to be a very probable hypothesis, and one perhaps that will account for all, or the most of the volcanic and earthquake phenomena of the present era, and of such as the earth's crust exhibits as having taken place in former times. * As an evidence that the interior of the earth has a much higher temperature than the surface, the geologist finds a gradual increase in the height of the thermometer as it is carried farther below the earth's surface. Very many experiments have been made on mines and artesian wells, and they all show an increase of temperature as we descend. Objections have been raised in relation to the results obtained in mines, since the breath of the miners, the heat given out by lamps, and the explosion of gunpowder, would necessarily add something to the natural heat in the mines; but, on the other hand, the circulation of cold air through the shafts of the mines would tend to counteract the effects of the above causes.
*We may add that the different rates of increase of temperature, presently to be noticed, in descending towards the centre, as observed in mines and artesian wells, seems to favor the nnequal distribution of heat and fusible and conducting material, in the interior portions of the earth, which lie about the liquid streams.
Solar heat is the principal cause of the superficial temperature of the earth, and we may readily conclude that its effects are felt to some distance beneath the terrestrial surface. Baron Fourier fixes 130 feet as the maximum depth, and M. Poisson gives 76 feet. The diurnal effect does not extend much below three or four feet. According to some experiments made by Professor Forbes, near Edinburgh, the annual oscillations of temperature would cease at the depth of 49 feet in trap tufa, 62 feet in incoherent sand, and 91 feet in compact sandstone.
From observations on springs in mines in Saxony, Brittany, Cornwall, and Mexico, the average number of feet of descent necessary to raise the thermometer (Fah.) one degree, varies from 45 to 351 feet, with a general average of fourteen localities of 92 feet. Observations on the temperature of the rocks in various localities in Europe give a rather more rapid increase, the numbers varying between 31 and 174 feet, for an increase of one degree. Observations on artesian wells give slightly different results.
Some very careful experiments made by Prof. Philips in two shafts, one near Durham and the other near Manchester, each of them 2,000 feet deep, give an increase of 1° Fahrenheit for every 65 to 70 feet.* Mr. Fox fixed a thermometer in the rock of the Dolcoath mine, in Cornwall, at the depth of 1,380 feet, and observed it frequently for a year and a half, and his result was that there is an increase of 1° for every 75 feet.t According to the result at which M. Cordier arrived in his numerous experiments and observations, there is
*Lyell's Principles of Geology, Vol. ii, p. 805.