Chapter XVII: Scientific Intelligence (6)

Another important fact respecting this glacial draft consists in the universal absence of marine, as well as fresh-water fossils in its interior--a fact which strengthens the view that they have been accumulated by the agency of strictly terrestrial glaciers; such is, at least, the case everywhere far from the sea-shore. But we may conclude that these ancient glaciers reached, upon various points, the sea-shore at the time of their greatest extension, just as they do at present in Spitzbergen and other arctic shores; and that therefore, in such proximity, phenomena of contact should be observed, indicating the onward movement of glacial material into the ocean, such as the accumulation within these materials of marine fossil remains, and also the influence of the tidal movements upon them. And now such is really the case. Nearer the sea-shores we observe distinctly, in some accumulations of the drift, faint indications of the action of the tide, reaching the lower surface of glaciers, and the remodelling to some extent of the materials which these poured into the sea. A beautiful example of the kind may be observed near Cambridge, along Charles River, not far from Mount Auburn, where the unstratified glacial drift (_a_) presents in its upper masses strictly the characters of true terrestrial glacial accumulation, but shews underneath faint indications (_b_) of the action of tides. Above, regular tidal strata (_c_) are observed, formed probably after the masses below had subsided. The surface of this accumulation is covered with soil (_d_).

The period at which these phenomena took place cannot be fully determined, nor is it easy to ascertain whether all glacial 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 subsequently stratified 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 the northern 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]

Footnote 48: An interesting account of the natural terraces
around Lake Superior is given at p. 413-416 of "Lake Superior."

_Description of the Marine Telescope._
By JOHN 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) a plate of glass, which, when used, is to be kept under the surface of the water.

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 imperceptible to 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--the exact 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 gentleman already 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]

Footnote 49: Leslie's Elements of Nat. Phil., p. 19.

Footnote 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.

_Experimental Investigations to Discover the Cause of the Change
which takes place in the Standard Points of Thermometers._
By JOHN 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 undergo some 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 takes place; 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:--

+-------------------------------------------------------------------------+
| No.| Description | Value of | Observed | Rise after | | |
| | of |one Degree|rise, Jan. | having | Rise at | Total |
| |Thermometer. | of Fahr. | 1849. |been boiled |Jan. 1850.| rise. |
| | | | |for a week. | | |
|----+-------------+----------+-----------+------------+----------+-------|
| { | Sealed in | } | | | | |
|1.{ | vacuum, | } 0·166 | 0·24 | 0·48 | 0·12 | 0·84 |
| { | not boiled. | } | | | | |
|----+-------------+----------+-----------+------------+----------+-------|
| { | Sealed in | } | | | | |
|2.{ | vacuum and | } 0·168 | 0·24 | 0·41 | 0·18 | 0·83 |
| { | boiled. | } | | | | |
|----+-------------+----------+-----------+------------+----------+-------|
| { | Sealed with | } | | | | |
|3.{ | air, not | } 0·199 | 0·20 | 0·50 | 0·20 | 0·90 |
| { | boiled. | } | | | | |
|----+-------------+----------+-----------+------------+----------+-------|
| { |Sealed with | } | | | | |
|4.{ |air, boiled. | } 0·154 | 0·06 | 0·45 | 0·13 | 0·65 |
+----+-------------+----------+-----------+------------+----------+-------+

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, thereby requiring 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.

_Observations on the Discovery, by_ Professor LEPSIUS, _of Sculptured
Marks on Rocks in the Nile Valley in Nubia; indicating that,
within the historical period, the river had flowed at a higher
level than has been known in Modern Times_.
By LEONARD HORNER, Esq., F.R.S.S. L. & E., F.G.S., &c.
Communicated by the Author. With a Plate.

The recent archaeological 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 Lepsius is 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 Moeris, 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]

Footnote 51: Bericht über die zur Bekantmachung geeigneten
Verhandlungen der Königl. Preuss. Akademie der Wissenshaften zu
Berlin. Aus dem Jahre 1844.

"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 of Hr. 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.

Footnote 52: The breadth of the river itself. See Letter to Hr.
Böckh, p. 27.

"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 Tutmosis III., a king of
the 18th 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 Sesuatesen III., 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 Amenemha III., the Moeris of
the Greeks: he who accomplished the gigantic work of forming the
artificial lake of Moeris, 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 Moeris. 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 Moeris, 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--

"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 single mark 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]

Footnote 53: See Plate I.

"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 Moeris, 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 Moeris, 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 fall in, 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 August Hr. 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 Moesis 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 of the 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]

Footnote 54: Miss Martineau's Eastern Life, vol. i., p. 99.

_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 called _vertical 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.

Footnote 55: Reisen in Europa Asien und Afrika, in der Jahren
1835, bis 1841.--Stuttgart 1841-1846.

_The Physical Geography of Lower Nubia._[56]

Footnote 56: With reference to the object of this paper.

Russegger informs us,[57] that he believes he was the first traveller who 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:--

Paris Feet. English Feet.

The upper part of the Cataract of Assuan, 342 = 364·37
Korusko, on the right bank of the Nile, in
Nubia, 450 = 479·43
Wadi-Halfa, 490 = 522·00
New Dongola, 757 = 806·52
Abu Hammed, 963 = 1026·00

Footnote 57: Reisen, Bd. ii., 545.

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:--

German M. English M.

From NE. to SW., from Abu Hammed to
Meroë, about 31 = 150
It makes a curve between Meroë and Old
Dongola, of about 16 = 77
It flows between Old and New Dongola,
from SE. to NW., about 16 = 77
Then, with some short windings, nearly due
north to the island of Sais, for about 30 = 145
And from Sais to the island of Philæ, from
SW. to NE., about 68 = 327
---- ----
Making the whole length of the course, from
Abu Hammed to Philæ, about 161 = 776

Footnote 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.