« AnteriorContinuar »
The above table shows that before the mass has descended 31-85 km. (1/200th of the radius) the pressure about it would have become more than 8000 atmospheres, which would force the molten fluid deep into the heated rock. the rising temperature at that depth would also rapidly dissolve the mass, and before the solid has sunk through another equal space in the viscid liquid, and thus reached a depth of 63.7 km., it seems almost certain that it would the completely dissolved.
It must be borne in mind that the solid is not much denser than the liquid; and as the liquid is highly viscous the mass would sink slowly, while the increase of temperature and pressure would conspire together in the most powerful manner to dissolve the mass and reduce it to the same temperature and density as the enclosing liquid, which would be forced into it on all sides by a pressure vastly greater than any known in our laboratories.
Even if we make the violent assumption that the sinking mass is a kilometre, or several kilometres, thick, it is diflicult to see how it could continue its downward course, undissolved by temperature and pressure, below a depth approximating one-tenth of the radius, or 637 kilometres. The sinking would be quite slow, owing to stiffness of the fluid, and could hardly be accomplished to this depth inside of several days, or more probably weeks.
Moreover, before the mass reached a depth of 260 kilometres, or less than one-twentieth of the radius, the density of the molten fluid would become 20 per cent. greater than it was at the surface, owing to pressure; and when the solid mass was no denser than the surrounding fluid it would cease to sink. Or, if it had acquired a small velocity downward in the fall from the surface against the viscous resistance of the fluid, which is enormously increased by the eddy arising from the condition of continuity, it might go down a little lower until the motion was overcome by the buoyancy of the denser fluid below. Accordingly, so far as one can see, solidified crust in sinking could by no possibility go lower than one-tenth of the radius, which would hardly accomplish the building up of a solid nucleus.
In considering the effects of pressure in forcing molten fluid into the sinking solid, we have not assumed that the density would thereby be increased; for at the great temperature of the fluid it is obvious that the solid into which the hot liquid entered would be dissolved, and heat from the fluid would be conducted rapidly through the solid mass. Thus no cause seems to be overlooked which could invalidate our conclusion.
It rests primarily upon the enormous pressures known to exist at great depths in the earth, and their undeniable effect in forcing the molten fluid into any possible solid body, so as to prevent it attaining any considerable depth without dissolving; and upon the assumption that even molten rock under such forces would take approximately the density given by Laplace's law, which hardly admits of reasonable doubt.
In considering these questions heretofore, the hypothesis of incompressibility for the molten fluid has been tacitly implied or assumed. Whether such an hypothesis is justified will appear differently to different minds, but for our
part we cannot hesitate in rejecting it on account of the known porosity of all matter, and its observed yielding and condensation under great forces.
On account of the difficulty in handling liquids, especially when at high temperatures, they have not been so carefully investigated in the laboratory as solids; but there remains scarcely any doubt that under planetary pressure they would all yield like sponges.
In indicating his interest in the paper on planetary pressures (Astronomische Nachrichten, No. 3992), one of the most eminent British mathematical physicists has pointed out that to his mind the present writer has underestimated the probability that the earth has a metallic nucleus. I have since pointed out in a letter to the editor of NATURE (April 13, p. 559) that pressure, and not metallic of the earth's constitution, is the true physical cause rigidity; for under such pressure any kind of matter would assume a hardness greater than that of steel; and as the material is above the critical temperature of every substance it is really gaseous, and would expand with incredible violence if the pressure could only be relieved.
In the Astronomische Nachrichten, No. 3992, I have shown that in any mass of considerable size, so condensed that the pressure amounts to millions of atmospheres, circulation at great depth becomes practically impossible, on account of the friction due to the increasing pressure as we descend within the mass. The pressure and friction which prevent circulation also prevent separation of the elements according to their densities. While it may not be possible to say that there is not an increasing amount of metals, such as iron, towards the centre of the earth, it is, I think, clear that there is no distinctively iron nucleus; for the existence of such a nucleus would imply that the earth's mass had unimpeded circulation when in a fluid state, all of which is to the last degree improbable.
When the earth was less condensed it was at lower temperature, and the elements may not have been fused; and as condensation advanced, and the temperature rose, the friction due to pressure operated with increasing intensity to destroy circulation, which would thus be restricted to the subsidence of compact masses decidedly denser than the surrounding fluid. As the fluid was necessarily at high temperature, a compact mass would soon be dissolved, and further circulation of its elements practically cease.
It seems, therefore, very difficult to escape the conclusion that the earth's interior is a magma of all the elements, the increasing density towards the centre being due primarily to pressure. If any separation of the metals from the rocks took place, it could only be near the surface where the pressure is slight; but because the rocks predominate at the surface, we must not conclude the same material does not exist abundantly in the great central nucleus of the globe.
The difference in the point of view here adopted and that held by the older school of physicists is based primarily upon the effects of pressure. While there is a certain disappointment in negative results, they are sometimes useful in leading us to new conceptions, and perhaps we may hope that further study of these difficult questions will produce results admitting of general acceptance. It should be added that the pressures for the interior of the earth, calculated in the Astronomische Nachrichten, No. 3902, would not be very greatly modified by any other admissible law of density.
The researches of Radau and Darwin (cf. Monthly Notices, Roy. Astron. Soc., December, 1899, pp. 122-3) have shown that, so far as the mathematical conditions are concerned, the law of density within the earth might depart considerably from that of Laplace. But on physical grounds, including the incontestably steady rise of pressure towards the earth's centre, whatever be the exact law of density, and especially the observed yielding and condensation of all matter under such forces, I hold that the true law is essentially that of Laplace, and any departure from it in the actual arrangement of the matter of the globe is likely to be extremely small and unimportant. T. J. J. SEE. U.S. Naval Observatory, Mare Island, California, March 31.
NOTES ON STONEHENGE.'
VI. ON THE SOLAR OBSERVATIONS MADE IN BRITISH STONE CIRCLES.
IN my last notes I referred to the star observations which might be made by means of stone circles. I now pass to solar observations.
I have already pointed out that much time has been lost in the investigation of our stone circles, for the reason that in many cases the exact relations of the monuments to the chief points of the horizon, and therefore to the place of sunrise at different times of the year, have not been considered; and when they were, the observations were made only with reference to the magnetic north, which is different at different places, and besides is always varying; few indeed have tried to get at the real astronomical conditions of the problem.
The first, I think, was Mr. Jonathan Otley, who in 1849 showed the "orientation" of the Keswick circle "according to the solar meridian," giving true solar bearings throughout the year.
and alignments in 1901, but other pressing calls on my time then caused me to break off the inquiry. Quite recently it occurred to me that a complete study of the Stenness circles might throw light on the question of an earlier Stonehenge, so I have gone over the old papers, plotting the results on the Ordnance map.
Now that the inquiry is as complete as I can make it without spending some time in Orkney with a theodolite, I may say that in my opinion Mr. Spence's contention in his pamphlet on Maeshowe is confirmed. although many of the alignments to which he refers in support of it prove to be very different from those he supposed and drew on the map which accompanies his paper.
The alignments on which he chiefly depended were two, one running from the stone circle past the entrance of Maeshowe to the place of sunrise at Halloween (November 1), another from the same circle by the Barnhouse standing stone to the mid-winter sunrise at the solstice.
I give a copy of the Ordnance map showing the true orientation of these and of other sight lines I
FIG. 14.-Maeshowe, in the foreground, and the Stones of Stenness.
I wrote a good deal in NATURE 2 on sun and star temples in 1891, and Mr. Lewis the next year expressed the opinion that the British stone monuments, or some of them, were sun and star temples.
Mr. Magnus Spence, of Deerness, in Orkney, published a pamphlet, "Standing Stones and Maeshowe of Stenness," in 1894; it is a reprint of an article in the Scottish Review, October, 1893. Mr. Cursiter, F.S.A., of Kirkwall, in a letter to me dated March 15, 1894, a letter suggested by my Dawn of Astronomy," which appeared in that year, and in which the articles which had been published in NATURE in 1891 had been expanded, directed my attention to the pamphlet; the observations had no pretension to scientific accuracy, and some of the alignments are wrongly stated, but a possible solar connection was pointed out.
I began the consideration of the Stenness circles
1 Continued from vol. lxxi. p. 538.
2 See especially NATURE, July 2, 1891, p. 201.
3 Gardner: Paisley and London.
From "Notice of Runic Inscriptions," by James Farrer, M.P. (1862). have made out. From this it will be seen that observations of the sun were provided for on the days in question, and that the circles and outstanding stones were undoubtedly set up to guide astronomical observations relating to the different times of the year. Of course, as I have shown elsewhere, such astronomical observations were always associated with religious celebrations of one kind or another, as the astronomer and the priest were one.
I shall not refer to all the sight lines indicated, but deal only with those, bearing upon the Stonehenge question, which I have without local knowledge been able to test and justify.
But first we must consider the astronomical differences between the rising of a star and of the sun, by which we mean that small part of the sun's limb first visible.
It is too frequently imagined that for determining the exact place of sunrise or sunset in connection with these ancient monuments we have to deal with the
FIG. 15.-The Azimuths of the Sunrise (upper limb) at the Summer Solstice. The Values given in the table have been plotted, and the effect of the height of hills on the azimuth is shown.
To make this quite clear I give a table which has been computed by Mr. Rolston, of the Solar Physics Observatory, showing the true azimuth with hills up to high for lat. 59° N., the latitude of Stenness, and 51°, nearly the latitude of Stonehenge, of the sun's upper limb for the solstitial year.
Now the most interesting and best defined line with this azimuth on the Ordnance map is the one stretching S.E. from the centre of the Stenness circle to the Barnstone, with an azimuth of 57° 15'. The line contains between the two points I have named another stone, the Watchstone, 18 feet high, in the SOLAR AZIMUTHS
Lat. 51° Rising N. of E. or Setting N. of W.
The first important thing we learn from the table is that although at any solstice the azimuths of the rising and setting of the sun's centre are the same, the azimuths of the upper limb at the summer and winter solstices differ in a high northern latitude by some 5°. The difference arises, of course, from the fact that the limb is some 16' from the sun's centre, so that considering the sun's centre as a star with fixed declination, at rising the limb appears before the centre, and at setting it lags behind it.
It will also be seen that at sunrise hills increase the azimuth from N., and refraction reduces it; while at setting, hills reduce the azimuth from S. and refraction increases it.
Not only does calculation prove the worship of the May and June years, but I think the facts now before us really go to show that in Orkney the May | year was the first established, and that the solstitial (June) year came afterwards, and this was the chief question I had in view.
I will begin with the May year. I have already
and hill high
and hill high
| precise alignment; and from the statements made and measures given it is to be inferred that a still more famous and perforated stone, the "Stone of Odin," demolished seventy years since, was also in the same line within the extremities named.
If we may accept this we learn something about perforated stones, and can understand most of the folk lore associated with them, and few have more connected with them than the one at Stenness. I suggest that the perforation, which was in this case 5 feet from the ground, was used by the astronomerpriest to view the sunrise in November over the Barnhouse stone in one direction, and the sunset in May over the circle in the other.
There is another echo of this fundamental line; that joining the Ring of Bookan and the Stones of Via has the same azimuth and doubtless served the same purpose for the May year.
But this line, giving us the May sunset and November sunrise, not the December solstitial sunrise as Mr. Spence shows it, is not the only orienta
FIG. 16.-Copy of Orduance Map showing chief sight lines from the Stones of Stenness.
ascending to the E., vertical angle = 1° 36' 30". The near alignment is on and over the centre of Maeshowe. Colonel Johnston, the Director-general of the Ordnance Survey, has informed me that the true azimuth of this bearing is N. 41° 16' E., and in all probability it represents the place of sunrise as seen from the Barnstone when Maeshowe was erected. What is most required in Orkney now is that some one with a good 6-inch theodolite should observe the sun's place of rising and the angular height of the hills at the next summer solstice in order to determine the date of the erection of Maeshowe. Mr. Spence and others made an attempt to determine this value with a sextant in 1899, but not from the Barnstone.
The Ordnance maps give no indication of stones, &c., by which the direction of the midsummer setting or the midwinter rising and setting might have been indicated from either the Maeshowe or the Barnstone. To sum up the solar alignments from the circle. We have the May sunrise marked by the top of Burrien Hill, from 600 to 700 feet high, Az. 59° 30′.
was an after structure to use the Barnstone for the summer solstice rising; then these two other tumuli, to deal with the winter solstice at Stenness circle, may have been added at the same time. All these provided for a new cult.
There are also tumuli near the line (which cannot be exactly determined because the heights of the hills are unknown) of the summer solstice setting; none was required for the sunrise at this date, as the line passes over the highest point of Hindera fiold, a natural tumulus more than 500 feet high, and on that account a triangulation station.
Another argument in favour of the tumuli being additions to the original design is that the place of the November setting from the Stenness circle is marked, not by a tumulus, but by a standing stone. As the stone near Deepdale and the tumulus at Onston are only about 1200 yards apart, the sugges tion may be made that in later times tumuli in some cases replaced stones as collimation marks.
SOUTH AFRICAN GEOLOGY.
MR. ROGERS has produced a handbook to the geology of Cape Colony which is sure to remain a standard treatise. New observations will be recorded in future editions, as the work of his survey is carried on; but results made public as recently as 1904 are included in the present volume. The book appears with especial appropriateness, now that the visit of the British Association to South Africa has been officially organised; and the included geological map, on the scale of about one inch to ninety miles, gives an admirable impression of the country. In it we see the huge Karroo synclinal, occupying almost all the colony, and lying between the pre-Devonian masses that crop out upon the north and the closely folded rocks of the Cape system along the south; while Mr. Rogers's introduction connects the scenic features with the geological structure in a manner that attracts us at the outset.
It is unfortunate that the names chosen for the colonial systems of rocks are not readily represented by adjectives. Hence such ungrammatical expressions as "preCape" and "pre-Karroo" have been received indelibly into literature. Even the International Congress may hesitate to speak of an "étage bokkeveldien," though we have, to be sure, purbeckien" and "bathonien" in Europe. This use of local names is, of course, greatly to be commended, in view of the scarcity of fossils in the great majority of the series.
The invasion of the old Malmesbury beds in the west of the colony by granite is concisely described on p. 38; and it is interesting to note how gneissic structures have arisen in the granite, as in so many other instances, without "evidence of a great amount of crushing or rearrangement of its component minerals after it solidified." The foliation-planes in the gneissoid granite are parallel with the strike and cleavage of the adjacent sedimentary rocks. and the whole structure seems one of subterranean flow. The granulites of the Darling area will clearly bear comparison with those that have been so much discussed in Saxony. The intercalation of orthoclase crystals from the granite in lenticular areas between laminæ of slate (p. 43) reminds us, again, of the composite rocks of Donegal.
beds, and are regarded as the first evidence of a neighbouring highland on which glaciers gathered. The Devonian Bokkeveld beds follow, and the still higher and famous Dwyka conglomerate is, as all geologists know, of Permo-Carboniferous age. It is somewhat fascinating to conceive the growth of glacial conditions through at least two long geological periods, until the flood of ice at last spread southward from the Transvaal territories, and scored and rounded all the preceding rock-masses down to the region of the Cape itself.
The Dwyka beds, a facies of the Kimberley-Ecca series, and long regarded as volcanic tuffs, are here
FIG. 1.-Overfo'ded quartzites of the Table Mountain Series, Meiring's Poort, representative of the great upheaval, which probably took place in early Jurassic times. From Rogers's "Geology of Cape Colony."
Mr. Rogers gives an interesting account of the stages in the passage from the well known blue crocidolite to the more siliceous yellow "griqualandite " in the slates of the Griquatown series. The slates themselves are converted into into jasper-rocks where the most altered amphibole occurs; and the crests and troughs of the folds have afforded hollows in which the fibres of amphibole have crystallised across from one surface to another.
The Cape system, including the Table Mountain series at its base, has been greatly contorted and overfolded in the south; but the southern edge of the Karroo beds is also involved (p. 407), and the great east-and-west ridges of the continental margin date from somewhere about Jurassic times. Flattened and striated pebbles occur in the Table Mountain
1 "An Introduction to the Geology of Cape Colony." By A. W. Rogers, M.A, F.G.S., Director of the Geological Survey of Cape Colony. Pp. TI-463. (London: Longmans, Green and Co., 1905.) Price gs. net.
very adequately described, with several effective illustrations. The glacial series at Vereeniging is associated with beds containing the Glossopteris flora, and also Sigillaria and other northern forms; and Mr. Rogers points out that the cold cannot have been responsible for preventing a more frequent mingling of these two well marked floras. The most novel portion of the account of the reptiliferous Beaufort beds of the " Karroo system" is the strong hint (p. 198) that they should be regarded as Permian rather than Triassic. This view, based on Amalitzky's work in Russia, would lead to a reconsideration of the Elgin Sandstone also, and to the acceptance of a development of reptilian life in Permian