tain relations of a symmetrical character between so many widely distributed phenomena of elevation, belonging to the same or to successive geological epochs, as would indicate the operation of general and wide-spread disturbing agencies beneath the solid crust of our planet. the of being actions. If some parts of an elevated district were fractured, Foldings while its other parts were still entire, it is easy to per- and ceive how foldings and ridges would be produced, as well ridges of as in the system of subsidence. This view is further Earth's confirmed, by considering that the thickness of the solid crust shell of the Earth, although small compared to the capable Earth's radius, would soon be sufficiently great to enable produced the shell to stand of itself, independently of the support of by elethe fluid nucleus beneath. It is true, that if both surfaces of vatory the shell possessed ellipticities, such as they had when the matter composing them was in a fluid state, its smallest limiting thickness, at the present day, might be an evanescent quantity; for, with any thickness, the phenomena of the variation of gravity, of the inequality in the Moon's motion, and of precession and nutation, would be precisely the same, whether the Earth was fluid to its surface, or solid to its centre; but, as it already appears that if we admit a change of position in the particles composing the fluid in their passage to the solid state, the ellipticities of the inner and outer surface of the shell will follow a different law from that of the original fluid strata of equal pressure, we cannot immediately conclude that the limiting value of the present thickness of the Earth's crust is evanescent. I have attempted to assign a limiting thickness, which is also very small, but I admit that the calculations on this point require revision, for as yet we want some of the most important data. It will not suffice to take, as is usually done, the surface of equilibrium of face of the watery covering of the Earth, as the Earth's true sur- equiliface, for by so doing, we would be already begging the brium of question to be decided; we would be thereby tacitly as- watery suming that the Earth's surface is perpendicular to gravity, coating an assumption from which the evanescent value of the of the thickness of its crust would immediately follow. The not the elegant manner in which Professor Stokes has deduced Earth's Clairault's theorem, shows, with remarkable clearness, that true if we assume for the Earth's surface the characteristic property of a fluid surface, the variation of gravity and other statical and dynamical results of the Earth's figure and structure, will be the same, whatever may be its internal The sur the Earth surface. constitution. But if we consider the surface of the Earth as that surface which would be exposed to view by stripping off, not only its liquid covering, but the strata of sand, clay, and rock, which had evidently been deposited from water in a sedimentary condition on the outer surface of the solidified shell, we shall have a surface differing from that of the water covering of the spheroid. The immense elevated table-lands of central Asia and of the the New World, and still further, the great depressions of the bed of the ocean, revealed by recent soundings and tidal phenomena, sufficiently prove that the water surface is far from representing with certainty the true surface of the solid spheroid; and although the former is necessarily perpendicular to gravity, so far from being entitled to infer that the latter possesses the same property, we might very safely assume the reverse." We may therefore fairly infer provisionally, that the thickness of the shell is not necessarily so extremely small as to be a negligible fraction of the Earth's radius, when considering the great statical and dynamical conditions of the Earth. The results obtained by Mr. Hopkins regarding the miclusive- nimum thickness of the Earth's crust, which would be ness of consistent with the observed amount of precession and nutation, having been sometimes referred to by geologists, kins's es- it is right to add that these results are not at all conclusive. timate They are derived from a discussion of the values of the thickness fraction contained in the expression already quoted. Incon Mr. of the of the crust. Earth's In order to estimate the value of this fraction, Mr. Hopkins tacitly assumes that the forms of the strata of equal density in the solid crust, including that of its inner and outer surfaces, are precisely the same as those of the fluid mass from which the strata had solidified; and in short, that the process of solidification of the globe was accompanied by no change whatever in the geometrical distribution of its particles. This assumption can now no longer be considered as tenable, consequently no conclusions as to the thickness of the Earth's crust can be derived from considerations of which it forms the essential foundation. With any but a very small thickness, it appears impossible that subsidence of the shell could take place in such a way as to account for the phenomena of elevation Diffi culty of account ing for These views have been further developed by the author in a paper read at the meeting of the British Association in Dublin. tains, by of the of mountains or plateaus on their edges. If any portion the proof the crust were unsupported by the nucleus, its ten- duction dency would be to support itself on the principle of the of mounarch. We cannot compare its condition to that of a thin, subsiunsupported, and brittle egg-shell, as has been done by dence of M. Elie de Beaumont, for the attractions to which the solid portions shell of the Earth is subjected, acting very nearly perpendi- Earth's cularly to the tangent plane at any point of its inner sur- crust. face, acts precisely in the direction best adapted for securing its stability. On the contrary, a small, round object like an egg-shell, at the Earth's surface, is subjected to parallel pressures, and is thus placed under more unfavourable conditions for stability. If we consider two arches of equal dimensions and Illustrastrength, one with a mass of fluid pressing down on its tion extrados, the other with a mass of fluid pressing upwards from the on its intrados, the head of fluid producing pressure equiliin brium of both cases being equal to the depth of the fluid over the arches. first arch, it is manifest that the second arch would be far more readily burst upwards than the other would be crushed downwards. It is well known to engineers that arches made to sustain incredible pressures from above, may be easily "blown up" by a comparatively moderate pressure from below. a by the against adequate The forces resulting from the expansion of the nu- The cleus, and its pressure against the shell, are, as well pressures as the action of gravitation, perpendicular to the tan- exerted gent planes of the shell, but while the latter acts in fluid nuthe direction most favourable to stability, the former act cleus of in the direction most favourable to rupture, and would, the Earth therefore, be far more likely to be effective in producing the solid disturbances of the Earth's crust, and above all the ele- crust, vation of the lines of mountains, which impart such present peculiar character to its general configuration. In my Re- causes searches on Terrestrial Physics, I have in some measure for pheconsidered the action of such a pressure, combined with nomena another that would result from a tendency in the nucleus tion. to change its figure, and I have shown that if the former happened to be small compared to the latter, a zone of least disturbance might exist on the Earth's surface, for the position of the boundaries of which formulæ are assigned. As no trace scems to exist of such a zone from geodesical measurements, I was led to infer that the general pressure predominated over the variable pressure, and, therefore, that lines of elevation on the Earth's surface of eleva should not present any marked relation of parallellism, either to the equator or to the meridians. It is satisfactory to find that this theoretical inference is confirmed by a conclusion of M. Elie de Beaumont, in the work already quoted. If the lines of elevation of the Earth's surface are grouped, so as to form, for the most part, a series of diametral lines to each figure of a network of regular pentagons, I cannot see any reason why such a symmetrical network might not be formed far more readily by the pressure of the nucleus acting outwardly against the shell, than by the subsidence of the latter inwards. The more regular and symmetrical the arrangements of the mountain systems, the more difficult it appears to reconcile them with mere subsidence, and the more easily do these arrangements seem to admit of explanation by the action of purely elevatory forces. The analogy between an interior, expanding, elevatory force, which separates the parts of a mass, and the molecular forces, which cause portions of certain rocks-for instance, basalt-to split into polygonal prisms, is far more clearly manifest, than between these phenomena and the crushing force which would accompany an action of subsidence. Lines of least resistance to separation or simple fracture, are more easily determined by the action of these forces, than lines of easiest crushing or squeezing, and greater symmetry might be fairly expected in the distribution of the former than in that of the latter. ART. III.-Note on the differences of level (seiches) observed by M. Stabrowski on Lake Onéga in Russia. By HENRY HENNESSY. HE phenomena briefly described by M. Stabrowski in the Comptes rendus of the French Academy for last July, present some relations of resemblance to those occurring on the surface of Lough Erne, the physical explanation of which is contained in a letter addressed by me to the President of the Royal Irish Academy, which appears in the Proceedings of that body.' Both at Lough Erne and Lake Onéga, the abnormal 1 Vol. vi, p. 279. condition of the surface of the water is due to atmospheric disturbance; but while in the former the action of the air seems to be entirely dynamical, in the latter its mode of action, and the resulting effects, present a statical character. The transitory wave of translation, which sometimes unexpectedly beats against one of the shores of our Irish lake, is due, as I have shown, to the action of descending currents of air from the hills at the opposite side; but Lake Onéga appears to act under changes of atmospheric pressure, like a differential barometer. It possesses all the conditions essential for this purpose, being long and narrow. The result is, that accidental differences of atmospheric pressure at its extremities would produce very observable changes in the water level. The rising of the water at one side of the lake is usually accompanied by a fall in the barometer, and vice versa. The seiche is always the precursor of wind [horizontal currents], and the oscillations of the surface of the lake enable the natives to foretell the direction and force of the winds. ART. IV. On the formation of several Acids of the W animal HENEVER we submit animal or vegetable sub- Action of stances to the action of heat in close vessels, we heat obtain three classes of products-gaseous, liquid, and upon solid. The gaseous products consist chiefly of carbonic and veacid, carbonic oxide, olefiant gas, and marsh gas. The getable liquid products consist of water holding certain liquid and some few solid bodies in solution. Another portion in close of the liquid products insoluble in water, and holding vessels. the chief part of the solid bodies in solution, forms a mass An abstract of the chief results contained in this paper was read at the Dublin meeting of the British Association, August 27, 1857. sub stances |