Illustration: Sketch of River Stratification
The period at which these phenomena took place cannot be fully determined, nor is it easy to ascertain whether allglacial drift is contemporaneous. It would seem, however, as if the extensive accumulation of drift all around the northern pole in Europe, Asia, and America was of the same age as the erratic of the Alps. The climatic circumstances capable of accumulating such large masses of ice around the north pole, having no doubt extended their influence over the temperate zone, and probably produced, in high mountain chains, as the Alps, the Pyrenees, the Black Forest, and the Vosges, such accumulations of snow and ice as may have produced the erratic phenomena of those districts. But extensive changes must have taken place in the appearance of the continents over which we trace erratic phenomena, since we observe in the Old World, as well as in North America, extensive stratified deposits containing fossils which rest upon the erratics; and as we have all possible good reasons and satisfactory evidence for admitting that the erratics were transported by the agency of terrestrial glaciers, and that, therefore, the tracts of land over which they occur stood at that time above the level of the sea, we are led to the conclusion that these continents have subsided since that period below the level of the sea, and that over their inundated portions, animal life has spread, remains of organized beings have been accumulated, which are now found in a fossil state in the deposits formed under those sheets of water.
Such deposits occur at various levels in different parts of North America. They have been noticed about Montreal, on the shores of Lake Champlain, in Maine, and also in Sweden and Russia; and what is most important, they are not everywhere at the same absolute level above the surface of the ocean, shewing that both the subsidence and the subsequent upheaval which has again brought them above the level of the sea, have been unequal; and that we should therefore be very cautious in our inferences respecting both the continental circumstances under which the ancient glaciers were formed, and also the extent of the sea afterward, as compared with its present limits.
The contrast between the unstratified drift and the subsequentlystratified deposits is so great, that they rest everywhere unconformably upon each other, shewing distinctly the difference of the agency under which they were accumulated. This unconformable superposition of marine drift upon glacial drift is so beautifully shewn at the above-mentioned locality near Cambridge (see diagram,p.114.) In this case the action of tides in the accumulation of the stratified materials is plainly seen.
The various heights at which these stratified deposits occur, above the level of the sea, shew plainly, that since their accumulation the main land has been lifted above the ocean at different rates in different parts of the country; and it would be a most important investigation to have their absolute level, in order more fully to ascertain the last changes which our continents have undergone.
From the above mentioned facts, it must be at once obvious that the various kinds of loose materials all over the northern hemisphere, have been accumulated, not only under different circumstances, but during long-continued subsequent distinct periods, and that great changes have taken place since their deposition, before the present state of things was fully established.
To the first period,—the ice period, as I have called it,—belong all the phenomena connected with the transportation of erratic boulders, the polishing, scratching, and furrowing of the rocks, and the accumulation of unstratified, scratched, and loamy drift. During that period the mainland seems to have been, to some extent at least, higher above the level of the sea than now; as we observe, on the shores of Great Britain, Norway, and Sweden, as well as on the eastern shores of North America, the polished surfaces dipping under the level of the ocean, which encroaches everywhere upon the erratics proper, effaces the polished surfaces, and remodels the glacial drift. During these periods, large terrestrial animals lived upon both continents, the fossil remains of which are found in the drift of Siberia, as well as of this continent. A fossil elephant, recently discovered in Vermont, adds to the resemblance, already pointed out, between thenorthern drift of Europe and that of North America; for fossils of that genus are now known to occur upon the northern-most point of the western extremity of North America, in New England, in Northern Europe, as well as all over Siberia.
To the second period we would refer the stratified deposits resting upon drift, which indicate, that during their deposition the northern continent had again extensively subsided under the surface of the ocean.
During this period, animals, identical with those which occur in the northern seas, spread widely over parts of the globe which are now again above the level of the ocean. But, as this last elevation seems to have been gradual, and is even still going on in our day, there is no possibility of tracing more precisely, at least for the present, the limit between that epoch and the present state of things. Their continuity seems almost demonstrated by the identity of fossil-shells found in these stratified deposits, with those now living along the present shores of the same continent, and by the fact, that changes in the relative level between sea and mainland are still going on in our day.
Indications of such relative changes between the level of the waters and the land are also observed about Lake Superior. And here they assume a very peculiar character, as the level of the lake itself, in its relation to its shores, is extensively changed.[48]
[45]VideLake Superior, its physical character, vegetation, and animals. By Professor Louis Agassiz. 1850.
[46]A comparison of the maps, shewing the arrangement of the moraines upon the glacier of the Aar, in mySystème Glaciaire, with the maps which Professor Guyot is about to publish of the distribution of the erratic boulders in Switzerland, will shew more fully the identity of the two phenomena.
[47]Berlin Academy, 1846.
[48]An interesting account of the natural terraces around Lake Superior is given atp.413-416 of“Lake Superior.”
ByJohn Adie,F.R.S.E., F.R.S.S.A.Communicated by the Author.
The instrument which has been popularly named the Water, or Marine Telescope, from the power given by its use to see into the water, consists of a tube of metal or wood, of a convenient length, to enable a person looking over the gunnel of a boat to rest the head on the one end, while the other is below the surface of the water; the upper end is so formed, that the head may rest on it, both eyes seeing freely into the tube. Into the lower end is fixed (water-tight) aplate of glass, which, when used, is to be kept under the surface of the water.
Illustration: Sketch of Marine Telescope
A very convenient size for the instrument represented in the above figure, is to make the length AC, 3 feet, and the mouth A, where the face is applied, of an irregular oval form, that both eyes may see freely into the tube, with an indentation on one side, that the nose may breathe freely, not throwing the moisture of the breath into the tube. B is a round plate of glass, 8 inches diameter, over which is the rim or edge C; this rim is best formed of lead, ¼ of an inch thick, and 3 inches deep; the weight of the lead serves to sink the tube a little into the water. Holes must be provided at the junction of B to C, for the purpose of allowing the air to escape, and bring the water into contact with the glass; on each side there is a handle for holding the instrument. This size and form is very much that of the instrument brought from Norway by John Mitchell,Esq., Belgian Consul, of Mayville, with the improvement for excluding the breath, and allowing the water to get into contact with the glass, which was not provided for in that instrument.
The reason why we so seldom see the bottom of the sea, or of a pure lake, where the depth is not beyond the powers of natural vision, is not that the rays of light reflected from the objects at the bottom are so feeble as to be imperceptibleto our sense, from their passage through the denser medium of the water, but from the irregular refractions given to the rays in passing out of the water into the air, caused by the constant ripple or motion of the surface of the water, where that refraction takes place. Reflections of light from the surface also add to the difficulty; and before we can with any just hope expect to see the objects distinctly at the bottom, these obstructions must be removed.
This is done to a very great extent by the use of the instrument which forms the subject of this notice; the tube serves to screen the eyes from reflections, and the water being in contact with the glass plate, all ripple is got rid of, so that the spectator, looking down the tube, sees all objects at the bottom, whose reflective powers are able to send off rays of sufficient intensity to be impressed on the retina, after suffering the loss of light caused by the absorbing power of the water, which obeys certain fixed laws, proportionate to the depth of water passed through; for as light passing through pure sea-water loses half its intensity for each 15 feet through which it passes,[49]we must, from this cause alone, at a certain depth lose sight of objects of the brightest lustre. The perfect purity of the water, and its freedom from all muddy particles floating in it, form an important element in the effective use of the water-telescope; for example, in the Frith of Forth, and similar estuaries, where the influx and reflux of the tide keep particles of mud in constant motion, the instrument is of little or no use; for these act in exactly the same way in limiting our vision through water, as a fog does through the air: it is therefore only in the pure waters of our northern and western shores that this contrivance is applied with any advantage; and in such situations we can speak of its powers with confidence. In a trial made with the instrument last autumn on the west coast of Scotland, the bottom was distinctly seen (a white bottom) at a depth of 12 fathoms; and on a black, rocky bottom, at 5 fathoms under water, objects were so distinctly seen that the parts of a wreck were taken up—theexact place of which was not known previous to its use. In these experiments a lenticular form of glass was made use of at the bottom of the tube, having a plane surface to the water, but no great or marked advantage was observable from this construction. With respect to the history of this contrivance for viewing the bottom of the sea, we are unable to assign any particular date: so far as our information goes, it has been in use from a very remote period. We are informed that it is in general use in seal-shooting along our northern and western islands, where, sometimes in the form of an ordinary washing-tub, with a piece of glass fixed in its bottom, the shot-seal was looked for, and the grappling-hook let down to bring him to the surface. It may not be generally known, that in seal-shooting, the shot or wounded seal always seeks the bottom, from which he never rises after death, till washed ashore by the action of the sea: it is only when the fatal ball deprives him of the power of diving that he is ever found at the surface. In such employments, therefore, the use of this instrument, however modified, must form an important auxiliary to the best rifle. Throwing oil over the surface of the water is used in the same pursuits; but this only so far stills the ripple, leaving the reflections. Our eminent engineer, Mr Robert Stevenson, made use of the water-telescope more than 30 years ago, in works connected with harbour improvement in the north of Scotland; it has also been used to examine the sand-banks,&c., at the bottom of the River Tay, but in this case the mud prevented its use in any considerable depth of water. To obviate this difficulty, the construction was modified thus: by making the tube of considerable length, and placing the glass at the lower end, this tube was thrust through the water till within a few feet of the bottom, acting as a cofferdam to set aside the dirty water, and enable the bottom to be seen; but in this method of application it was found very difficult to hold the tube down in the water from its buoyant power, and we are informed by Mr Thomas Stevenson, C. E., that, he understood from this cause its use had been discontinued. He suggested a simple remedy;viz., to fill up the empty tube with pure water. We are indebted to Mr Mitchell, the gentlemanalready mentioned, for having brought this instrument into notice in the public prints, under the name of Norwegian water-telescope, on the shores of which country it is stated to be much used in fishing—in particular, that of the herring; but the herring-fishers on the east coast of Scotland inform us, that they require no such auxiliary, as, from the surrounding elevated grounds, they can tell the position of the shoal, and, from their motions seen from such situations, they know where they are to be found when they go out a-fishing.[50]
[49]Leslie's Elements ofNat. Phil., p.19.
[50]Norwegian Water-Telescope.
The water-telescope is thus noticed in a very promising periodical, the American Annual of Scientific Discovery, just published, of which a copy reached us a few days ago.—Ed.Phil.Journal.
The water-telescope is an instrument which the people of Norway have found of so great utility, that there is scarcely a single fishing-boat without one of three or four feet in length, which they carry in their boats with them when they go a-fishing. When they reach the fishing-grounds, they immerse one end of this telescope in the water, and look through the glass, which shews objects some ten or fifteen fathoms deep as distinctly as if they were within a foot of the surface. When a shoal of fish comes into their bays, the Norwegians instantly prepare their nets, man their boats, and go out in pursuit. The first process is minutely to survey the ground with their glasses, and where they find the fish swarming about in great numbers, they give the signal, and surround the fish with their large draught-nets, and often catch them in hundreds at a time. Without these telescopes their business would often prove precarious and unprofitable; as the fish, by these glasses, are as distinctly seen in the deep, clear sea of Norway, as gold-fish in a crystal jar. This instrument is not only used by the fishermen, but is also found aboard the navy and coasting-vessels of Norway. When their anchors get into foul ground, or their cables warped on a roadstead, they immediately apply the glass, and, guided by it, take steps to put all to rights, which they could not do so well without the aid of the rude and simple instrument, which the meanest fisherman can make up with his own hands, without the aid of a craftsman. This instrument has been lately adopted by the Scotch fishermen on the Tay, and, by its assistance, they have been enabled to discover stones, holes, and uneven ground, over which their nets travel, and have found the telescope answer to admiration, the minutest object in twelve feet of water being as clearly seen as on the surface. We see no reason why it could not be used with advantage in the rivers and bays of the United States.
ByJohn Adie,F.R.S.E., F.R.S.S.A.Communicated by the Author.
It has long been known to experimentalists that, in thermometers constructed with the greatest care, a change takes place after a lapse of time in the standard points, as given by the melting of ice and boiling of water under a fixed pressure; on this account it has been recommended by most writers, where the employment of thermometers is treated of, that they should from time to time be compared one with another, and also at the freezing point. This change is a rising of the mercury in the tube, so that, after a length of time, the mercury will not sink to the point laid off in the construction of the instrument. To investigate to what cause this change was due, formed the object of my experiments: Was it a change in the glass of which the bulbs are formed, or in the mercury with which they are filled? I was aware that thermometers filled with alcohol were not subject to this change, which would lead to the inference, that the change was in the mercury and not the glass; but then, in the spirit-thermometer, air is left above the column of spirit, whereas, in those constructed with mercury, the air is expelled, and there is a vacuum above the column; consequently, the bulb is pressed together with the force of an atmosphere on all sides; might not this force, acting for a length of time, cause some small alteration in the arrangement of the particles forming the glass of the bulb?
This is the explanation accepted by most of the Italian and French writers on this subject. Some suppose that the mercury may contain air and moisture within its particles; but such a hypothesis I think inadmissible, as in the case of a vacuum over the mercury, these particles would seek the void, and cause rather a depression than a rising of the freezing point. Mr Daniell, in his Essay on Climate, adopts the same view; and Sir John Herschel, in his article“Heat,”in the Encyclopædia Metropolitana, says:“The freezing point upon the mercurial thermometer has been supposed to undergosome slight variation, so as to appear too low upon the scales of those instruments which have been long made; and it is said that, in such cases, the just indication was again recovered by breaking off the end of the stem, so as to admit atmospheric air.”But, as I had observed that the change went on for a time only, after which it ceased, and that it affected thermometers sealed with air over the mercury, as well as those with a vacuum, I undertook the following experiments:—
In September 1848 I made four thermometers having long degrees,—such that 1/10° might be easily noted, constructed of the same draft of glass tube; two of these I placed in boiling water, and kept them at that temperature for a week: my object in this was to learn if any change in the form of the bulb would take place from this slow process of annealing, as glass is known to undergo some change from such exposure.
The four thermometers were now filled with pure mercury: two of these were sealed with a vacuum over the mercury; one tube that had been boiled, and the other not: the other two tubes were sealed with air over their columns, and the freezing points of all were marked on the tubes; after which they were placed in a window freely exposed to light, where they were left till January 1849—a space of four months—when they were again placed in melting ice, and the freezing points marked; they had risen ·24°, ·24°, ·20°, ·06° parts of a degree. The whole four thermometers were now placed in boiling water, and kept there for a week, when the freezing points were again observed to have risen respectively ·48°, ·41°, ·50°, ·45°.
The instruments were now left exposed to light as at first; and, in January 1850, the freezing points were again observed, when they were found to have farther risen ·12°, ·18°, ·20°, ·13°; and, lastly, they were observed in May 1850, when no change from last observation was notable.
The whole amount of rising of the freezing point in these four thermometers, after a lapse of eighteen months, is respectively ·84°, ·83°, ·90°, ·65°; and these changes may be the full amount that would take place were the instruments observed after a greater lapse of time. From my experience, I know that there is a period after which no change takesplace; but, from the method in which these experiments have been conducted, I am not at present in a condition to assign a time; moreover, it is evident that this period will be much modified by circumstances. The results above stated form the following Table:—
From inspection of the Table, no very remarkable difference is observable in the rising of these four instruments. No. 4 appears to have risen less during the first period, but goes along with the others afterwards. The effect of exposure to the temperature of boiling water shews that, under high temperature, the change goes on much faster than at the ordinary temperature of the air; from the Table it will be observed, that about twice the amount of change was caused by the boiling of the thermometers for a week, than had taken place between the first and second observations, a period of four months.
It does not appear that the boiling of the thermometer tubes for eight days, previous to their being filled with mercury, had produced any change on the form of the bulbs; we should at least infer this from the change in their freezing points keeping pace so nearly with those which had not been boiled.
I now come to the concluding experiment with these instruments, and, it appears to me most interesting and anomalous. The four tubes being placed in pounded ice, the columns stood at the points indicated in the last column of the Table; in this situation the tops of the tubes were broken off, so as to admit the free pressure of the air, and instantly the thermometers fell, in the order of their numbers, ·54, ·43, ·40, ·35 of a degree, now indicating on their scales +·30, +·40, +·50, +·35. The remarkable features shewn by this experiment are; first, that the two thermometers sealed with vacuum, and the two having air over their columns, should have risen nearly equally, when two had their bulbs pressed with the whole force of an atmosphere, while the other two had no pressure externally, farther than that caused from changes in the pressure of the atmosphere. Next, that on being opened, those with air over them should have started down nearly as much as those with a vacuum; and on all these appears a permanent change from three to five-tenths of a degree. I confess that I am very much at a loss to account for these singular changes; atmospheric pressure on the bulbs would account for the change in those sealed with a vacuum; for we can easily suppose that a permanent form had been taken from long exposure to that pressure by the glass forming the bulbs: besides this permanent form, there appears to have been a spring inwards, which instantly sprung out on removal of the pressure by the admission of air over the mercury; but the same reasoning will not apply to the thermometers having air over the mercury; and before I attempt to make any suggestions as to the cause of these changes, I propose to institute the following experiments. Having had three thermometers blown and filled with mercury, I shall make one with a perfect vacuum over the mercury, the next with air over it, and the third with air condensed over it; and, noting the changes that may go on in these, I hope to be able to assign a cause or causes for the change. It is argued by some continental writers on this subject that the reason why we do not perceive any change in the freezing point in spirit-thermometers is from the great expansion of spirit above mercury, volume for volume, therebyrequiring a much smaller mass of fluid to give the same length of a degree: this I propose to test by making a thermometer with the same size of tube and bulb as those to be experimented on with mercury. In mentioning these experiments to Professor Forbes, he kindly put me in possession of some spirit-thermometers, one of these, made in 1837, having a very large bulb—this, with three others, shewed no change in the places of their freezing points.
ByLeonard Horner,Esq., F.R.S.S. L. & E., F.G.S.,&c.Communicated by the Author. With a Plate.
The recent archaeological[N7]researches of Professor Lepsius in Egypt, and the Valley of the Nile, in Nubia, have given a deserved celebrity and authority to his name, among all who take an interest in the early history of that remarkable portion of the Old World. While examining the ruins of a fortress, and of two temples of high antiquity at Semne, in Nubia, he discovered marks cut in the solid rocks, and in the foundation-stones of the fortress, indicating that, at a very remote period in the annals of the country, the Nile must have flowed at a level considerably above the highest point which it has ever reached during the greatest inundations in modern times. This remarkable fact would possess much geological interest with respect to any great river, but it does so especially in the case of the Nile. Its annual inundations, and the uniformity in the periods of its rise and fall, have been recorded with considerable accuracy for many centuries; the solid matter held in suspension in its waters, slowly deposited on the land overflowed, has been productive of changes in the configuration of the country, not only in times long antecedent to history, but throughout all history, down to the present day. Of no other river on the earth's surface do we possess such or similar records; and moreover, the Nile, and the changes it has produced on the physical character of Egypt, are intimately associated with the earliest records and traditions of the human race. Everything, therefore, relating to the physical history of the Nile Valley must always be an object of interest; but the discovery of Professor Lepsiusis one peculiarly deserving the attention of the geologist; for he does not merely record the facts of the markings of the former high level of the river, but he infers from these marks, that since the reign of Mœris, about 2200 years before our era, the entire bed of the Nile, in Lower Nubia, must have been excavated to a depth of about 27 feet; and he further speculates as to the process by which he believes the excavation to have been effected.
It will be convenient, before entering upon the observations I have to offer upon the cause assigned by Professor Lepsius for the former higher levels of the Nile indicated by these marks, that I should give the description of the discovery itself, by translating Dr Lepsius's own account of it, in letters which he addressed to his friends, Professors Ehrenberg and Böckh of Berlin, from the island of Philæ, in September 1844.[51]
"You may probably remember, when travelling to Dongola on the Lybian side of the Nile, and in passing through the district of Batn el hagér, that one of the most considerable of the cataracts of the country occurs near Semne, a very old fortress, with a handsome temple, built of sandstone, in a good state of preservation; the track of the caravan passing close to it, partly over the 4000-year-old artificial road. The track on the eastern bank of the river is higher up, being carried through the hills; and you must turn off from it at this point in order to see the cataract. This Nile-pass, the narrowest with which I am acquainted, according to the measurement ofHr.Erbkam, is 380 metres (1247 English feet) broad;[52]and both in itself, and on account of the monuments existing there, is one of the most interesting localities in the country, and we passed twelve days in its examination.
"The river is here confined between steep rocky cliffs on both sides, whose summits are occupied by two fortresses of the most ancient and most massive construction, distinguishable at once from the numerous other forts, which, in the time of the Nubian power in this land of cliffs, were erected on most of the larger islands, and on the hills commanding the river. The cataract (or rapid) derives its name of Semne from that of the higher of the two fortresses on the western bank; that on the opposite bank, as well as a poor village lying somewhat south of it, is called Kumme. In both fortresses the highest and best position is occupied by a temple, built of huge blocks of sandstone, of two kinds, which must have been brought from a great distance through the rapids; for, southward, no sandstone is found nearer than Gebel Abir, in the neighbourhood of Amara and the island of Sai (between 80 and 90 English miles), and northward, there is none nearer than the great division of the district at Wadi Haifa (30 miles distant.)
"Both temples were built in the time of TutmosisIII., a king of the18th dynasty, about 1600 years before Christ; but the fortresses in which they stand are of a more ancient date. The foundations of these are granite blocks of Cyclopian dimensions, resting on the rock, and scarcely inferior to the rock itself in durability. They were erected by the first conqueror of the country, King SesuatesenIII., of the 12th dynasty, in order to command the river, so easily done in so narrow a gorge. The immediate successor of this king was AmenemhaIII., the Mœris of the Greeks: he who accomplished the gigantic work of forming the artificial lake of Mœris, in the Fayoum, and from whose time—the most flourishing of the whole of the old Egyptian kingdom—the risings of the Nile in successive years, doubtless by means of regular markings, as indeed Diodorus tells, remained so well known, that, according to Herodotus, they were recorded in distinct numbers from the time of Mœris. It appears that this provident king, occupied with great schemes for the welfare of his country, considered it of great importance that the rising of the Nile on the most southern border of his kingdom should be observed, and the results forthwith communicated widely in other parts of the land, to prepare the people for the inundations. The gorge at Semne offered greater advantages for this object than any other point; because the river was there securely confined by precipitous rocky cliffs on each side. With the same view he had doubtless caused Nilometers to be fixed at Assuan and other suitable places; for without a comparison with these, the observations at Semne could be of little use.
"The highest rise of the Nile in each year at Semne, was registered by a mark, indicating the year of the king's reign, cut in the granite, either on one of the blocks forming the foundation of the fortress, or on the cliff, and particularly on the east or right bank, as best adapted for the purpose. Of these markings eighteen still remain, thirteen of them having been made in the reign of Mœris, and five in the time of his two next successors. These last kings discontinued the observations; for, in the meantime, the irruption of the Asiatic pastoral tribes into Lower Egypt took place, and wellnigh brought the whole kingdom to ruin. The record is almost always in the same terms, short and simple:Ra en Hapi em renpe... mouth or gate of the Nile in the year.... And then follows the year of the reign, and the name of the king. It is written in a horizontal row of hieroglyphics, included within two lines—the upper line indicating the particular height of the water, as is often specially stated—
Illustration: Sketch of Heiroglyphics
"The earliest date preserved is that of the sixth year of the king's reign, and he reigned 42 years and some months. The next following dates are, the years 9, 14, 15, 20, 22, 23, 24, 30, 32, 37, 40, 41, and 43; and include, therefore, under this king, a period of 37 years. Of the remaining dates, that only of the 4th year of his two successors is available; all the others, which are on the west or left bank of the river, have been moved from their original place by the rapid floods which have overthrown and carried forward vast masses of rock. One singlemark only, that of the 9th year of Amenemha, has been preserved in its original place on one of the building stones, but somewhat below the principal rapid.[53]
"We have now to consider the relation which these—the most ancient of all existing marks of the risings of the Nile—bear to the levels of the river in our own time. We have here presented to us the remarkable facts, that the highest of the records now legible;viz., that of the 30th year of the reign of Amenemha, according to exact measurements which I made, is 8·17 metres (26 feet 8 inches) higher than the highest level to which the Nile rises in years of the greatest floods; and further, that the lowest mark, which is on the east bank, and indicated the 15th year of the same king, is still 4·14 metres (13 feet 6½ inches); and the single mark on the west bank, indicating the 9th year, is 2·77 metres (9 feet) above the same highest level.
“The mean rise of the river, recorded by the marks on the east bank, during the reign of Mœris, is 19·14 metres (62 feet 6 inches) above the lowest level of the water in the present day, which, according to the statements of the most experienced boatmen, does not change from year to year, and therefore represents the actual level of the Nile, independently of its increase by the falls of rain, in the mountains in which its sources are situated. The mean rise above the lowest level, at the present time, is 11·84 metres (38 feet 8 inches); and, therefore, in the time of Mœris, or about 2200 years before Christ, the mean height of the river, at the cataract or rapid of Semne, during the inundation, was 7·30 metres (23 feet 10 inches) above the mean level in the present day.”
Such are the facts recorded by Dr Lepsius; and then follow, in the same letter, his views as to the cause of the remarkable lowering of the level of the river.
“There is certainly no reason for believing,”he says, "that there has been any diminution in the general volume of water coming from the south. The great change in the level can, therefore, only be accounted for by some changes in the land, and these must also have altered the whole nature of the Nile Valley. There seems to be but one cause for the very considerable lowering of the Nile; namely, the washing out and excavations of the catacombs (Answaschen und Aushölen der Katakomben); and this is quite possible from the nature of the rocks themselves, which, it is true, are of a quality that could not well be rent asunder, and carried away by the mere force of the water, but might be acted upon directly by the rising of the water-level, and the consequent effects of the sun and air on the places left dry, causing cracks, into which earth and sand would penetrate, which would then give rise to still greater rents, until, at last, the rocks would of themselves fallin, by having been hollowed out, a process that would be hastened in those parts of the hills where softer and earthy beds existed, and which would be more easily washed away. But that, in historical times, within a period of about 4000 years, so great an alteration should take place in the hardest rocks, is a fact of the most remarkable kind,—one which may afford ground for many other important considerations.
"The elevation of the water-level at Semne must necessarily have affected all the lands above; and, it is to be presumed, that the level of the province of Dongola was at one time higher, as Semne cannot be the only place in the long tract of cliffs where the bed of rock has been hollowed out. It is to be conceived, therefore, that not only the widely-extended tracts in Dongola, but those of all the higher country in Meroë, and as far up as Fasogle, which, in the present day, are dry and barren on both sides of the river, and are with difficulty irrigated by artificial contrivances, must then have presented a very different aspect, when the Nile overflowed them, and yearly deposited its fertile mud to the limits of the sandy desert.
"Lower Nubia also, between Wadi Haifa and Assuan, is now arid almost throughout its whole extent. The present land of the valley, which is only partly irrigated by water-wheels, is, on an average, from 6 to 12 feet higher than the level to which the Nile now rises; and although the rise at Semne might have no immediate influence upon it, yet what has occurred there makes it more than probable, that at Assuan there was formerly a very different level of the river, and that the cataracts there, even in the historical period, have been considerably worn down. The continued impoverishment of Nubia is a proof of this. I have no manner of doubt that the land in this lower part of the valley, which, as already stated, is at present about 10 feet above the highest rise of the Nile, was inundated by it within historical time. Many marks are also met with here, that leave no doubt regarding the condition of the Nile Valley antecedent to history, when the river must have risen much higher; for it has left an alluvial soil in almost all the considerable bays, at an average height of 10 metres (32 feet 9 inches) above the present mean rise of the river. That alluvial soil, since that period, has doubtless been considerably diminished in extent by the action of rain. On the 17th of AugustHr.Erbkam and I measured the nearest alluvial hillock in the neighbourhood of Korusko, and found it 6·91 metres (22 feet 7 inches) above the general level of the valley, and 10·26 metres (33 feet 7 inches) above the present mean rise of the river. That rise, which at Semne, on account of the greater confinement of the stream between the rocks, varies as much as 2·40 metres (7 feet 10 inches) in different years, varies at Korusko less than 1 metre (3 feet 3 inches).
"Near Abusimbel, on the west bank, I found the ground of the temple 6·50 metres (21 feet 2 inches) above the highest water-level. This temple, it is well known, was built under Rameses the Great, between 1388 and 1322 years before Christ. Near Ibrim there are, on the east bank, four grottoes excavated in the vertical rock that bounds the river, which belong partly to the 18th and partly to the 19th dynasties; the last, under Rameses the Great,is also the lowest, and only 2·50 metres (8 feet 1 inch) above the highest inundation; the next in height is 2·70 metres (8 feet 9½ inches) above the former, and was made 250 years earlier, under Tutmes III. Although I only measured the present level of the valley near Korusko, nevertheless it appears to me that, during the whole of the new kingdom, that is, from about 1700 years before Christ to this time, the Nile has not reached to the full height of the low land of the valley.
“It is, however, conceivable that, at the time when the present low land of the Nubian Valley was formed, the cataracts at Assuan were in a totally different state; one that would, in some degree, justify the overcharged descriptions of the ancients, according to whom they made so great a noise that the dwellers near them became deaf. The damming up of the inundation at Assuan could have no material influence on Egypt, any more than that at Semne, or the land from thence to Assuan.”
It appears therefore, from the above statements, that at the time mentioned, the Nile, during the inundations, stood 26 feet 8 inches higher than the highest level to which it now rises in years of the greatest floods; and that, to account for this, Professor Lepsius conceives that, between the time of Mœsis and the present day, the bed of the Nile, from a considerable distance above Semne to Assuan, must have been worn down to that extent. In the index to the volume of the Berlin Monatsbericht, in which the letters of Professor Lepsius are inserted, there is the following line:—
“Nil,senkung seines Bettes um 25 Fuss seit 4000 Jahren.”
“Nile, sinking of its bed about 25 feet (Paris) within the last 4000 years.”
Rivers are, undoubtedly, among the most active agents of change that are operating on the earth's surface; the solid matter which renders their waters turbid, and which they unceasingly carry to the sea, afford indisputable proof of this agency. But the power of rivers to abrade and wear down the rocks over which they flow, and to form and deepen their own bed, depends upon a variety of circumstances not always taken into account; and although the great extent of that power, in both respects, is shewn in the case of many rivers, to conclude, as some have done, from these instances, that all rivers have excavated the channels in which they flow, is a generalization that cannot be safely assented to. The excavation of the bed of a river is one of those problems in geological dynamics which can only be rightly solved by each particular case being subjected to the rigorous examination of the mathematician and the physicist. The solid matter which rivers carry forward is in part only the produce of their own abrading power; and the amount of it must be proportional to that power, which is mainly dependent on their velocity; they are the recipients of the waste of the adjoining lands by other combined agencies, and the carriers of it to the lower districts and to the sea. They often afford the strongest evidence ofthe vast lapse of time that must be included between the beginning and close of a geological period; and, when they flow through countries whose remote political history is known to us, they supply a scale by which we may measure and estimate that lapse of time. This is especially so in the case of the Nile.
When so startling an hypothesis as that now referred to,viz., that the entire bed of so vast a river as the Nile, for more than 250 miles, from Semne to Assuan, has been excavated, within historical time, to a depth of 27 feet, is made by a person whose name carries so much weight in one department of philosophical inquiry, the statement involves such important geological considerations, that it becomes the duty of the geologist to examine, and thoroughly test the soundness of the explanation, in order that the authority of Professor Lepsius, for the accuracy of the facts observed, may not be too readily admitted as conclusive for the correctness of his theory of the cause to which they owe their existence. That there has been such an undoubting admission, appears from the following passage in the work of one of the latest writers on Nubia:—
“The translation of the name of this town (Aswán) is 'the opening;' and a great opening this once was, before the Nile had changed its character in Ethiopia, and when the more ancient races made this rock (at the first cataract) their watch-tower on the frontier between Egypt and the south. That the Nile has changed its character, south of the first cataract, has been made clear by some recent examinations of the shores and monuments of Nubia. Dr Lepsius has discovered water-marks so high on the rocks and edifices, and so placed as to compel the conviction that the bed of the Nile has sunk extraordinarily by some great natural process, either of convulsion or wear. The apparent exaggerations of some old writers about the cataracts at Syene may thus be in some measure accounted for. If there really was once a cataract here, instead of the rapids of the present day, there is some excuse for the reports given from hearsay by Cicero and Seneca. Cicero says, that 'the river throws itself headlong from the loftiest mountains, so that those who live nearest are deprived of the sense of hearing, from the greatness of the noise.' Seneca's account is: 'When some people were stationed there by the Persians, their ears were so stunned with the constant roar, that it was found necessary to remove them to a more quiet place.'”[54]
Note.—The learned author of an article on Egyptian Chronology and History in the“Prospective Review”for May 1850, in referring to the contributions of Professor Lepsius to Egyptian history, says,“He has discovered undescribed pyramids, equal in number to those known before; has traced the Labyrinth, and ascertained its founder.He has detected inscriptions on the banks of the Nile, which show that its bed has subsided many feet in historic times.”9th June 1850.
In the assumption of an excavation of the bed of the river, we have no small amount of wear to deal with, for the distance from Semne to Assuan, following the course of the river, is not less than 250 miles; and if, as Professor Lepsius supposes, the excavation extended to Meroë, we have a distance, between that place and Assuan, of not less than 600 miles.
Although these records of a former high level of the Nile at Semne had not been noticed by any traveller prior to Professor Lepsius, we may rest fully assured of the accuracy of his statements, from the habitual care and diligence, and the established character for fidelity, of the observer. The silence of other travellers may be readily accounted for by this, that none of them appear to have remained more than a very short time at this spot—not even the diligent Russegger—whereas we have seen that Professor Lepsius passed twelve days in the examination of this gorge in the Nile Valley.
The theory of a lowering of the bed of the river by wearing, involves two main considerations,viz., the power of the stream, and the degree of hardness of the rocks acted upon. The power depends upon the volume and velocity of the river—the velocity on its depth, and the degree of inclination of the bed: the hardness of the rocks we can form a tolerable estimate of when we know their nature. To judge, therefore, of the probability of the hypothesis of Professor Lepsius, we must inquire into the physical and geological features of the Nile Valley, in Nubia.
In the observations I have now to offer, my information has been derived of course entirely from the works of other travellers, particularly those of Burckhardt, Rüppell, and Russegger,[55]and especially the latter, who travelled in Nubia in 1837; for he not only enters far more into the details of the natural history of the country, but he is the only traveller in Nubia who appears, from previous acquirements, to have been competent to describe its natural history with any degree of accuracy—I refer more particularly to the physical and geological features of the country. Besides full descriptions in his volumes, he has given a geological map of Nubia, and also several sections, or what may more properly be calledvertical sketches—a term that would, perhaps, be a more appropriate designation for all sections that are not drawn to a true scale, or at least when the proportion of height to horizontal distance is not stated.
[51]Bericht über die zur Bekantmachung geeigneten Verhandlungen der Königl. Preuss. Akademie der Wissenshaften zu Berlin. Aus dem Jahre 1844.
[52]The breadth of the river itself. See Letter toHr.Böckh,p.27.
[53]See PlateI.
[54]Miss Martineau's Eastern Life,vol. i., p.99.
[55]Reisen in Europa Asien und Afrika, in der Jahren 1835, bis 1841.—Stuttgart1841-1846.
Russegger informs us,[57]that he believes he was the first travellerwho had succeeded in making a series of barometrical measurements along the Nile Valley, from the Mediterranean to Sennaar and Kordofan, and thence to the 10th degree of north latitude. He gives the following altitudes, above the sea:—
I shall now give the length of the Nile along its course from Abu Hammed to the island of Philæ, at the head of the cataract of Assuan. I employ for this purpose the map in the atlas which accompanies the work of Russegger, which bears the date of 1846, and which, doubtless, was constructed on the best authorities. He mentions a map of General von Prokesch with great praise.[58]It flows:—
Ascending the river, we have, between Philæ and Korusko, a distance of 24 German, or 115½ English miles, and without any rapid, except one near Kalabsche. Korusko being 115 feet above the head of the cataract of Assuan, at Philæ, we have an average fall of the river between these two places of a foot in a mile.
Between Korusko and Wadi-Halfa there is no rapid. The distance being 20 German, or 96⅓ English miles, and the difference of altitude being 42½ feet, we have an average fall throughout that part of the river's course of not more than 5·3 inches in a mile.
This very inconsiderable fall need not surprise us; for the averagefall of the Nile in Lower Egypt, at the lowest water, is little more than one-third of that now stated. At the time of the highest water the surface of the Nile, at Boulak, near Cairo; that is, about 116 miles in a direct line from the coast is only 43·437 English feet above the level of the Mediterranean, and at the time of the lowest water, only 17·33 feet. Thus, in the first case, there is an average fall of about 5·00 inches; in the second, of not more than 1·80 inches in a mile.[59]
Between Wadi Halfa and Dale, a distance of about 94 miles, six cataracts, or schellals, as they are called in the language of the country, are marked in Russegger's map. And here, it may be as well to notice, that there are no cataracts, in the ordinary sense of the term, on the Nile; no fall of the river over a precipice; all the so-called cataracts are rapids, where the river rushes through rocks in its bed; the rapids varying in their length and degrees of inclination. We have no measurements of their lengths or of their falls, except as regards the first and second cataracts. The former, according to Russegger, has a fall of about 85 English feet in a distance of about 8 miles; and he describes the latter as extending from 5 to 6stunden; that is, from 12 to 14½ miles, but he does not give the height. Speaking of the schellals above Semne, Russegger says, that all may be passed in boats without difficulty for about six weeks, or two months in the year. This is the case also, at the cataract or rapid of Assuan. But between Wadi-Halfa and Dale, with some inconsiderable spaces of free navigable water, in the ordinary state of the river, there is an almost uninterrupted series of rapids. We have no measurement of the height of Dale above Wadi-Halfa, near to which the second great cataract of the Nile occurs; but this is the part of the river's course where the fall is greatest, and from Semne to Dale there are about 45 miles of this more rapid fall.
From Dale to New Dongola, a distance of 35 German, or about 168 English miles, only three rapids are marked on Russegger's map—the highest being at Hannek, about 26 English miles below New Dongola. New Dongola being 806 English feet above the sea, and the distance from that place to the rapid of Hannek being 26 miles only, we may with probability estimate the surface of the river at the rapid of Hannek at 780 feet above the sea. Now, Wadi-Halfa being 522 feet, we have a difference of height, between these two last-named places, of 258 feet; and the length of the river's course between them being 236 miles, we have an average fall of 13·12 inches in a mile; that is, in the part of the river's course where nine rapids occur, in the provinces of Batn-el-Hadjar,Sukkot, and Dar-el-Mahass, where the river flows over granite and other plutonic rocks; gneiss, mica-schist, and other hard rocks, which Russegger considers to be metamorphic. But between Semne and the head of the second cataract at Wadi-Halfa, there is not a continuous rapid stream; for Hoskins says, that about two miles above that cataract, the river has a width of a third of a mile, and, when he passed it the water was scarcely ruffled.[60]
From the rapid of Hannek to Abu Hammed, the distance is 329 English miles, and the difference of altitude is 246 English feet. We have thus an average fall in that distance of 9·00 inches in a mile.
Thus, in the 776 miles between Abu Hammed and Philæ, we have an average fall of the Nile
[56]With reference to the object of this paper.
[57]Reisen, Bd.ii., 545.
[58]“Über den Stromlauf und das zunächst liegende Uferland des Nils, von der zweiten Katarakte bis Assuan, besitzen wir eine vortreffliche Karte namlich:”“Land zwischen der kleinen und grossen Katarakten des Nil. Astronomisch bestimmt und aufgenommen in J. 1827, durch v. Prokesch. Nil Grundrisse der Monumente. Wien, 1831.”—Reisen Bd.ii., Thl.iii.86.
[59]Russegger, Reisen, Bd.i., 258.
[60]Travels in Ethiopia,p.272.
Our information is very scanty respecting the breadth and depth of the river, either at the time of lowest water or during the inundations. About two miles above Philæ, it is stated by Jomard[61]to be 3000 metres, or nearly two English miles wide. At the second cataract, or rapid of Wadi-Halfa, it spreads over a rocky bed of nearly two miles and a-quarter in width (2000 klafter),[62]but contracts above the rapid to a third of a mile. Russegger also states, that the Nile, near Boulak, in Lower Egypt, is 2000 toises, nearly two-and-a-half English miles in breadth, and yet that it is considerably wider in some parts of Southern Nubia; but Burckhardt says, that the bed of the Nile in Nubia is, in general, much narrower than in any part of Egypt. Near Kalabsche, about 30 miles above Philæ, the river runs through a gorge not more than 300 paces wide, and its bed is full of granite blocks. It shortly afterwards again widens for some distance; but near Sialla, 78 miles above Philæ, it is contracted by the sandstone hills on both sides coming so near each other, that the river's bed is again not more than from 250 to 300 paces wide. It is about 600 yards broad about two miles above the second cataract near Wadi-Halfa, but is again very much contracted in the rocky region of Batn-el-Hadjar. At Aulike it is only 200 paces broad.[63]
I have not met with any measurements of the depth of the riverin any part of its course in Nubia; but Hoskins describes it as being so shallow at the island of Sais, 327 miles above Philæ, on the 9th of June, which would be before the commencement of the inundation, as only to reach the knees of the camels.[64]Near Derr, about 86 miles below the Cataract of Wadi-Halfa, Norden, in January, found the river so shallow that loaded camels waded through it, and his boat frequently struck the ground. In May, Burckhardt found the river fordable at Kostamne, 53 miles above Philæ; and Parthey states, that between Philæ and the island of Bageh, to the west of it, the river is so shallow before the commencement of the inundation, that it may be waded through.[65]Burckhardt says, that from March to June the Nile-water, in Nubia, is quite limpid.[66]Miss Martineau, who visited Nubia in December and January, speaking of the river above Philæ says, that it“was divided into streamlets and ponds by the black islets. Where it was overshadowed it was dark-gray or deep blue, but when the light caught it rushing between a wooded island and the shore, it was of the clearest green.”[67]At the second cataract she describes the river as“dashing and driving among its thousand islets, and then gathering its thousand currents into one, proceeds calmly in its course.”[68]
Although we have no accurate measurements of the velocity of the Nile in Nubia, we may arrive at an approximate estimate of it by comparing its fall with that of a river well known to us.
I have stated the fall of the Nile in different parts of its course to be 5·30, 9·00, 12·00, and 13·12 inches in a mile. The fall of the Thames from Wallingford to Teddington Lock, where the influence of the tide ends, is as follows:—
“In general, the velocity may be estimated at from half-a-mile to two miles and three-quarters per hour; but the mean velocity may be reckoned at two miles per hour. In the year 1794, the late Mr Rennie found the velocity of the Thames at Windsor two miles and a half per hour.”[69]
It will thus be seen that the velocity of the Nile is probably greatly inferior to that of the Thames; for it appears that, except during the inundation, for more than half the year the depth is inconsiderable. The average fall when greatest, that is, including the province of Batn-el-Hadjar, where the rapids chiefly occur, is considerably less than that of any part of the above course of the Thames; so that there must be long intervals between the rapids where the fall must be far less than 13 inches in a mile. The breadth of the Nile is vastly greater; but supposing the depth of the water to be the same as that of the Thames, on account of the friction of the bed, the greater breadth would add very little to the velocity. If we assume the average depth of the Thames in the above distance to be 5 feet, and that it flows with an average velocity of 2 miles in an hour, and if we assume the average depth of the Nile in that part of its course where the fall is 13·12 inches to be 10 feet, when not swollen by the rise, the velocity would be 2⅘ miles nearly in an hour,[70]if the fall were equal to that of the Thames. We shall probably come near the truth, by assuming the velocity of the Nile on this part at 2 miles in an hour. That it must be considerably less in the other divisions of the course I have named, and especially in that part immediately below the second cataract, where the average fall is only 5·30 inches for a distance of 96 miles, is quite evident.
The power of a river to abrade the soil over which it flows, so far as water is by itself capable of doing so, must depend upon its volume and velocity, and the degree of hardness of the material acted upon. The power is increased when the water has force enough to transport hard substances. But even transported gravel has little action on the rocks with which it comes in contact, when it is free to move in running water, unless the fall be considerable, and, consequently, the velocity and force of the stream great. When stones are firmly set in moving ice, they then acquire a great erosive power, cutting and wearing down the rocks they are forcibly rubbed against; but this condition never obtains in Lower Nubia, as ice is unknown there.
[61]Description de l'Égypte.—Separate Memoir entitled,“Description de Syène et des Cataractes.”
[62]Russegger, Bd. ii., 3 Thl.85.
[63]Russegger, Bd. ii., 3 Thl. 76.
[64]Travels,p.257.
[65]Wanderungen durch das Nilthal, von G. Parthey, Berlin1840. 378.
[66]Travels, pp. 9 and 11.
[67]Eastern Life, i. 10½.