CONTRIBUTIONS ON ENGINEERING AND SCIENTIFIC SUBJECTS.
Contributions to _Encyclopædia Britannica_ and _Edinburgh
Encyclopædia_--The alveus or bed of the German Ocean--Sectio
planography--Wasting effects of the sea at the Mersey and
Dee--Density of fresh and salt water--The Hydrophore.
We have seen that Mr. Stevenson’s college education was mainly, if not
altogether, due to his own thirst for knowledge, and his education
being voluntarily undertaken, could hardly fail to issue in good
results. That his early studies were of incalculable value to him
no one can doubt; and his own conviction of this may explain the
solicitude with which, in after life, he impressed on his sons the
extreme importance of being properly grounded in every branch of study,
_scientific_ and _practical_, which a well trained engineer has to call
to his aid in the practice of his profession.
Fortified by this valuable training, Mr. Stevenson had also that
unselfish love of his profession which alone can move a man to give
the results of his experience freely to others, and this he did to the
_Edinburgh Encyclopædia_ and the _Encyclopædia Britannica_, in articles
on “Roads,” “Lighthouses,” “Railways,” “Dredging,” “Blasting,” and
other engineering subjects.
But he did not confine his literary labours to matters purely
professional. His love for nature in all its aspects led him also to
make communications to the Scientific Journals of the day on subjects
of more general interest. Of these his papers “On the Alveus or Bed of
the German Ocean,” in which by an investigation of many evidences he
is led to the conclusion that the sea is gradually encroaching on the
land, may be quoted as an example.
Mr. Stevenson’s first communication on this subject was published in
1816, in vol. ii. of the Wernerian Transactions, in which he gives
examples, from actual observation, of the wasting effects of the
sea on various parts of the coasts of the British Isles. His second
communication was made to the Wernerian Society in March 1820, and
published in the _Edinburgh Philosophical Journal_ of that year.
In the fifth edition of Baron Cuvier’s “Essay on the Theory of the
Earth,” reference is made to Mr. Stevenson’s theory. His papers are
several times quoted in Lyell’s _Principles of Geology_, and the
General Committee of the British Association at York in 1834 passed
a resolution, “that Mr. Stevenson be requested to report to the next
meeting upon the waste and extension of the land on the east coast of
Britain, and upon the general question of the permanence of the level
of the sea and land, and that individuals who may be able to supply
information upon the subject be requested to correspond with him.”
Without discussing in how far Mr. Stevenson’s theory may be sound
(for on such questions it is notorious that the views of geologists
do not always coincide), it cannot be denied that his mode of dealing
with the subject is original and interesting, and as the papers are
not now accessible to the general reader, it may be excusable to give
one of them _in extenso_. I also notice another feature which gives
interest to the subject. In his illustrations he adopted a mode of
representation which was peculiarly suitable for the object in view. It
will be seen from Plate XII. that the sections are laid down on what is
now known by engineers as _sectio planography_, which it is believed
was used for the first time in illustrating this paper.
“ON THE BED OF THE GERMAN OCEAN, OR NORTH SEA. (Read before the
Wernerian Natural History Society, 25th March 1820.)”
“The efforts of man in exploring the more occult processes of nature
are necessarily much circumscribed, especially when his attempts are
directed to the investigation of regions which his senses cannot
penetrate. It has accordingly been with the utmost difficulty that
his exertions have been rendered in any degree successful in prying
into the bowels of the earth, or in his endeavours to ascend to the
aërial regions. In proof of this, the limited excavations even of the
most extensive mining works, have required the lapse of ages, and the
powerful stimulus of commercial enterprise, for their accomplishment.
From these the philosopher has not hitherto derived much light,
to enable him to compare the theories which have been assigned by
geologists to account for the various and discordant appearances of
the structure of the globe. It has also been with much difficulty,
and at no small personal hazard, that the philosophical inquirer has
ventured to climb the highest mountains, to examine into the phenomena
of the atmosphere. The balloon has indeed enabled us to attain still
higher points of elevation; but as yet we do not seem to have made
proportional progress in knowledge. In all such attempts to ascend
the greatest heights or penetrate the deepest excavations, we still
breathe in our own element, though under different modifications. If,
however, we would explore the depths of the Ocean, we immediately
encounter an element to which the organisation of our lungs is not at
all adapted; the density of air, compared with water on a level with
the surface of the sea, being in the ratio of one to about 850; and our
difficulties must consequently increase in a very rapid proportion.
Here therefore we are unavoidably left to conjecture on many points
of our inquiries regarding this highly interesting subject. Even the
ingenious contrivance of the diving bell contributes but little towards
our investigations for ascertaining the nature of the bottom of the
sea, at least to any considerable depth, on account of the difficulty
of its application in situations exposed to stormy weather, and also of
the increasing ratio of the pressure of the fluid as we descend. This
curious machine, it is believed, was invented and employed, about the
year 1720, by a Captain Rowe for raising the wreck of ships upon the
coast of Scotland; and in the year 1778, the active mind of Smeaton
first applied it to the operations of the engineer.
[Illustration:
_PLATE XII._
CHART of the
NORTH SEA OR GERMAN OCEAN
with SECTIONS of the
_DEPTHS of WATER_
_Illustrative of Observations_
by
_ROBERT STEVENSON_
_Civil Engineer_
1820.
_W. & A. K. Johnston, Edinburgh._
]
“Our knowledge of the bottom of the ocean, therefore, remains still
very imperfect, and, with little exception, the simple apparatus of the
mariner, consisting of a plummet and line, continues to be chiefly in
use for ascertaining the depth of the sea and the nature of the ground.
With these, and the addition of a little grease applied to the lower
extremity of the plummet, which strikes against the bottom, we learn
the quality of the soil, though imperfectly, by the particles which
adhere to the grease. What the navigator has yet been able to discover
regarding the depth and the nature of the bottom of the German Ocean, I
shall now endeavour to notice, being myself enabled to offer the result
of a pretty extensive acquaintance with this field of inquiry.
“It may be necessary to premise, in treating of a subject so extensive,
and in comparing great things with small, that we are obliged to speak
of the North Sea as a bay or basin, and of the immense collection of
débris which we meet with, extending over a great proportion of its
bottom, under the common appellation of sand banks. We must also be
allowed to consider the undulating line, or the irregularities of the
bottom, to arise chiefly from the accumulation of deposited matters;
and in most of the situations connected with these banks, we are
supported and borne out in this conclusion, by their local positions
relatively to the openings of firths, and the line of their direction
in regard to the set or current of the ebb tide.
“The accompanying map (Plate XII.) of the eastern coast of Great
Britain, with the opposite Continent, though upon a small scale,
exhibits numerous soundings of the depth of the German Ocean; and
the sections delineated on it will perhaps be found to give a pretty
distinct view of the subject. This chart extends from the coast of
France, in latitude 50° 57´ to 61° N. On the east, this great basin is
bounded by Denmark and Norway, on the west by the British Isles, on the
south by Germany, Holland, and France, and on the north by the Shetland
Islands and the Great Northern or Arctic Ocean. The term _German
Ocean_, though in very common use, is certainly not so comprehensive in
its application to this great basin as that of _North Sea_, now more
generally used by the navigator. The extent of this sea from south to
north, between the parallels of latitude quoted above, is 233 leagues,
and its greatest breadth from west to east, reckoning from St. Abb’s
Head, on the coast of Scotland, to Ringhjoöbing Fiord, on the opposite
shore of Denmark, is 135 leagues. The greatest depth of the water in
this basin seems to be upon the Norwegian side, where the soundings
give 190 fathoms; but the mean depth of the whole may be stated at only
about 31 fathoms.
“To be more particular with regard to the depth of the German Ocean,
or North Sea, it will be observed by the sections and soundings marked
upon the chart, that the water gradually deepens as we sail from south
to north. The first of these sections which we shall notice is on the
parallel of three degrees of east longitude, running from Ostend to the
latitude of the northmost of the Shetland Islands, being an extent of
227 leagues. The depth, as will be seen from this section (which, to
avoid confusion in the body of the chart, is traced along the western
side of it), varies rather after an irregular progression, from 120
fathoms towards the northern extremity of this sectional line, to 58,
38, 24, and 18 fathoms, as we proceed southwards, to within five miles
of the shore, nearer which we do not approach in our remarks regarding
the soundings. Notwithstanding the irregularity of the depth from the
occurrence of numerous sandbanks, it is curious to observe the increase
upon the whole as we proceed from south to north, by which this sea
exhibits all the characteristic features of a great bay, encumbered
with numerous sandbanks.
“In the same manner, though not strictly connected with our present
purpose, we may observe that the English Channel deepens progressively
from Dover to its entrance, formed by the Land’s End of England and
the Isle of Ushant, on the coast of France; so that the Strait between
Dover and Calais may be said to form a point of partition between two
great inclined planes, forming the bottom of these seas.
“Besides the longitudinal, or north and south sectional line described
above, we have also six other sections delineated in an easterly
and westerly direction, across the accompanying chart, which are as
follow. One between the Shetland Islands and the coast of Norway; a
second between Tarbetness in Ross-shire and the Naze of Norway; a third
extends from the Firth of Forth to the coast of Denmark; a fourth
from the mouth of the river Tyne to Sylt Island, also in Denmark; a
fifth from Flamborough Head, in Yorkshire, to the mouth of the River
Elbe; and the sixth is from Yarmouth to Egmond-op-Zee, on the coast of
Holland. Other sections of this sea have also been made, which include
the general elevation of the land, as, for example, one of these
extends from Holland across the German Ocean to the Thames, and through
the interior of the country to the Bristol Channel; then crossing St.
George’s Channel, this sectional line passes through the southern
extremity of Ireland, and falls into the Atlantic Ocean; but this will
be more particularly noticed, when I come to speak of the bed of the
English Channel, in a future paper.
“On examining the accompanying cross sections of the depths of water
on the same parallel they will be found to vary considerably. It may,
however, be stated as a general conclusion, that there is a greater
depth of water on the eastern and western sides of the German Ocean
than in its central parts, and that, upon the whole, it is deeper
on the British than on the continental shores, the coast of Norway
excepted.
“We have already observed, that this sea is much encumbered with
sandbanks, or great accumulations of débris, especially in the middle
or central parts, and also along the shores towards what may be termed
the apex of the bay, extending from the river Thames along the shores
of Holland, etc., to the Baltic. One of these great central banks,
delineated on the chart, and known to mariners as the Long Forties,
trends north-east in the direction of the ebb tide from the entrance
of the Firth of Forth no less than 110 miles, while the Denmark and
Jutland banks may also be traced on the chart from the entrance of
the Baltic, upwards of 105 miles in a north-western direction. Besides
these, we have also another great central range of banks, which is
crossed by no fewer than four of our sectional lines. These are known
under the common appellation of the Dogger Bank, which is subdivided by
the navigator into the Long Bank, the White Bank, and the Well Bank,
including an extent of upwards of 354 miles from north to south. There
are also a vast number of shoals and sandbanks, lying wholly to the
southward of our section, between Flamborough Head and Heligoland.
Altogether, therefore, the superficies of these extensive banks is
found to occupy no inconsiderable portion of the whole area of the
German Ocean; the surface of which, in making these investigations,
has been estimated to contain about 153,709 square miles, while the
aggregate superficial contents of the sandbanks alone amount to no less
than 27,443 square miles, or include an area of about 5¾ of the whole
surface of the North Sea.
“But to render these dimensions a little more familiar by comparison,
we may notice, that the Island of Great Britain contains about 77,244
square miles, being not quite one half of the area of the North Sea; so
that the area of the sandbanks bears a proportion equal to about one
third of the whole _terra firma_ of England and Scotland; and they are,
therefore, perhaps, far more considerable in their extent than has been
generally imagined.
“In speaking of the dimensions of sandbanks situate in the middle of
the ocean, we are aware that great allowance must be made in forming
a proper estimate of their extent, especially in speaking of their
cubical contents. From a vast number of observations and comparisons
relative to this subject, I have, however, been enabled to determine,
that the average height of these banks measures about seventy-eight
feet, from a mean taken of the whole. In ascertaining their height
above the surrounding bottom, the measurement has been taken from the
general depth around each respectively. Now, upon taking the aggregate
cubical contents of the whole of these immense collections of débris,
supposing the mass to be uniformly the same throughout, it is found to
amount to no less a quantity than 2,241,248,563,110 of cubic yards,
being equal to about fourteen feet of the depth of the whole German
Ocean, or to a portion of the firm ground of Great Britain, on a level
with the sea, taken twenty-eight feet in perpendicular height or depth,
supposing the surface to be a level plane.
“These calculations at least tend to show that an immense body of water
must be displaced, in consequence of these banks occupying so very
considerable a proportion of the bed of the North Sea, the unavoidable
effect of which must give a direct tendency to the tidal waters, and
the flux produced by storms in the Atlantic, to overflow the bed of
the German Ocean, in the same manner as if stones or other matter were
thrown into a vessel already nearly brimful of water. This may further
be illustrated by considering the actual state of any of the great
inland lakes, as those of Geneva, Lochness, Lochlomond, etc., which for
ages past have been receiving the débris of the surrounding mountains.
We must doubtless allow that they contain a smaller portion of water,
or are actually of a less depth than they were at an earlier period of
the history of the globe. Accordingly, from inquiries, which, in the
prosecution of this subject, I have been led to make regarding the two
last mentioned lakes, it has satisfactorily appeared that their waters
are subject to overflow or rise upon their banks. On Lochlomond, in
particular, the site of a house at the village of Luss was pointed
out to me, which is now permanently under _the summer water mark_,
while the gable of another house in its neighbourhood is in danger of
being washed down by the increase of the waters of the loch. Whether
this striking appearance is to be attributed wholly to natural causes,
or partly to artificial operations upon the bed of the river Leven,
flowing from the loch, I have had no opportunity of inquiring. But the
great bench or flat space round the margin of the loch, which is left
partly dry during summer, forms altogether such a receptacle for débris
as to be sufficient to affect the surface of the loch, and indeed
permanently to raise its waters. We also infer, though by a different
process, that the constant deposition going forward in the bed of
the German Ocean must likewise displace its waters, and give them a
tendency to enlarge their bed and to overflow their banks or boundary.
“In this view of the subject, it will appear that we have not only to
account for the supply of an immense quantity of débris, but we must
also dispose of the water displaced by the process of deposition which
is continually going forward at the bottom of the ocean.
“With regard, then, to the supply of the débris of which these banks
are composed.--We find that a very great portion of it consists of
siliceous matters in the form of sand, varying in size from the finest
grains to coarse bulky particles, mixed with coral and pounded shells,
the quantity of these calcareous matters being altogether astonishingly
great; and being specifically lighter than the particles of sand, the
shells generally cover the surface of these sunken banks. With regard
to the vast collection of siliceous particles connected with the banks,
our surprise ceases when we consider the receptacle which the North Sea
forms, to an almost unlimited extent of drainage from the surrounding
countries, on which the change of the seasons, and the succession of
rain and of drought upon the surface of the earth, are unceasingly
producing their destructive effects. All have remarked the quantity of
mud and débris with which every rill and river is charged, even after
the gentlest shower; especially wherever the hand of the agriculturist
is to be found. His labours in keeping up the fertilising quality of
the ground consist in a great measure in preparing a fresh matrix for
the chemical process or the germination of the seeds of the earth, in
lieu of that portion of the finely pulverised soil which the rains are
perpetually carrying to the sea, as the grand receptacle and storehouse
of nature for these exuviæ of the globe. From the effect of rills
and rivulets, we should, perhaps, rather be apt to expect a greater
deposition in the bed of sheltered bays and arms of the sea than we
really observe. So that we can readily believe that the quantity of
débris, even for a single year, along such an extent of coast, may
bear some consideration in respect to the bed of the German Ocean;
what, then, must these effects produce in the lapse of ages?
“Whatever be the cause, the fact is certain, that on almost every
part of the shores of Great Britain and Ireland, and their connecting
islands, from the northernmost of the Shetland to the southernmost of
the Scilly Islands, and also upon the shores of Holland, and part of
France, particularly in the neighbourhood of Cherbourg, this wasting
effect is going forward. These shores I have myself examined. But my
inquiries have not been confined to the coasts which I have personally
visited, having also, through the kind attentions of some nautical
friends, been enabled to extend my investigations even to the remotest
parts of the globe. The general result has been, that equally in the
most sheltered seas, such as the Baltic and Mediterranean, and on the
most exposed points and promontories of the coasts of North and South
America, and the West India Islands, abundant proofs occur, all tending
to show the general waste of the land by the encroachments of the sea.
Such wasting effects are quite familiar to those locally acquainted
with particular portions of the shores; and I have often received
their testimony to these facts, as the sad experience of the removal
of buildings, and the inundation of extensive tracts of land by the
encroachment of the sea.
“Indeed, by a closer inquiry into this department of the subject, we
shall, perhaps, find ourselves rather at a loss to account for the
_smallness_ of the quantity of this deposition, considering the waste
which is constantly going forward in the process of nature, and even
be led to seek for its wider distribution over the whole expanse of the
bed of the ocean, as has been supposed in that theory of the globe, so
beautifully and so ably defended by our late illustrious countryman
Professor Playfair.
“One of the most striking and general examples of this kind may perhaps
be found in the abrupt and precipitous headlands and shores which we
everywhere observe along the coast, and which we suppose to have once
been of the same sloping form and declining aspect with the contiguous
land. In the production of these effects alone, an immense quantity of
débris must have been thrown into the bed of the ocean. The channels
which are cut by the sea in the separation of parts of the mainland,
and the formation of islands, no doubt make way for a considerable
portion of the displaced fluid; but still these channels, when filled
with water, come far short, in point of bulk, when compared with the
portions of the elevated land which are thus removed. Now, it has been
alleged by some, that while the land is wasting at certain points,
it is also gaining in others; and this is a state of things which is
freely admitted to take place in various quarters; yet these apparent
acquisitions are no more to be compared with the waste alluded to,
than the drop is to the water of the bucket. But accurate observations
regarding the formation of extensive sandbanks, and the accumulation
of the débris, of which they are formed, are not to be made in a few
years, perhaps not in a century, nor indeed in several centuries; for
although the short period of the life of man is sufficient to afford
the most incontrovertible proofs of the waste of the land where we
become observers, yet when we extend our views to the depths of the
ocean, and speak of the events and changes which are there going
forward, we must not be supposed to set limits to time.
“We have many convincing proofs in the natural history of the globe,
that the sea has at one time occupied a much higher elevation than at
present. On the banks of the Firth of Forth, near Borrowstounness, for
example, I have seen a bed of marine shells, which is several feet in
thickness, and has been found to extend about three miles in length,
and which is now situate many feet above the present level of the
waters of the Forth. A recent illustration of this subject occurred
also in the remarkable discovery of the skeleton of a large whale,
found in the lands of Airthrey, near Stirling,--the present surface of
the ground where the remains of this huge animal were deposited, having
been ascertained (by my assistants, when lately in that neighbourhood)
to be no less than twenty-four feet nine inches above the present level
of the Firth of Forth at high water of spring tides. Now, whether
we are to consider these as proofs of the higher elevation of the
waters of the ocean in the most general acceptation of the word, at
a former period, I will not here attempt to inquire. But aside from
these anomalous appearances, there is reason for thinking that the
waters of the higher parts of the Firth of Forth, like those of the
Moray Firth, may, at one time, have formed a succession of lakes, with
distinct barriers, as we find in the case of Lochness, and the other
lakes forming the track of the Caledonian Canal. My object on the
present occasion, however, is simply to notice the wasting effects of
the North Sea upon the surrounding land, its deposition in the bottom
of the sea, and the consequent production of surplus waters at the
surface, and to endeavour to account for these appearances consistently
with the laws of nature. The opinion accordingly which I have formed,
and the theory which I have humbly to suggest (for I am not aware that
this subject has been before particularly noticed) is, that the silting
up of the great basin of the North Sea has a direct tendency to cause
its waters to overflow their banks.
“Referring to the chart, we find that the North Sea is surrounded with
land, excepting at two inlets or apertures, the one extending about
100 leagues, between the Orkney Islands and the Norwegian coast, and
the other between Dover and Calais, which is of the width of seven
leagues. The aggregate _waterway_ of these two passages forms the track
for the tidal waters, and also for the surplus waters produced during
storms which affect the Atlantic and Arctic Oceans. It is also obvious
that this waterway must remain nearly the same, and admit a constant
quantity; or, to speak more correctly, by allowing these inlets to
follow the general law, they must be enlarged by the waste or wearing
of their sides, in a ratio perhaps greater than the silting up of
the bottom in those particular parts, while the interior and central
portions of the German Ocean are continually acquiring additional
quantities of débris, along with the drainage water of the widely
surrounding countries. If therefore the same, or a greater quantity of
tidal and surplus waters continue to be admitted from the Atlantic and
Arctic Seas into this great basin, where the process of deposition is
constantly going forward, it is evident that the surface of the German
Ocean must be elevated in a temporary and proportionate degree, and
hence the production of those wasting and destructive effects which are
everywhere observable upon its shores.
“This reasoning is also applicable, in a greater or less degree, to
all parts of the world; for as the same cause everywhere exists, the
same effects, when narrowly examined, must everywhere be produced.
In the Southern or Pacific Ocean we have wonderful examples of great
masses of land formed by madrepores and extensive coral banks, which in
time assume all the characteristic features of islands. These occupy
considerable portions of the watery bed of the ocean, and displace
corresponding portions of the fluid. Immense quantities of mud are
also said to be deposited in the Yellow Sea of China, in the great
deltas formed at the mouths of the Ganges, the Plate, the Amazon, the
Mississippi, the St. Lawrence, the Nile, the Rhine, and other large
rivers, whose joint operations, both at the surface and bottom of the
ocean, are continually carrying forward the same great process of
displacing the waters of the ocean; for it matters not to this question
whether the débris of the higher country which is carried down by the
rains and rivers, or is occasioned by the direct waste produced by the
ocean itself on the margin of the land, be deposited at the bottom or
surface of the ocean, it must still be allowed to displace an equal
or greater bulk of the fluid, and has therefore a direct tendency to
produce the derangement which we are here endeavouring to describe.
“A striking illustration of this doctrine may be drawn from M.
Girard’s able and ingenious observations on the delta of Egypt, made
in 1799, and published in the _Mem. de l’Acad._ for 1817, in a memoir
_Observations sur la Vallée d’Égypte, et sur l’exhaussement séculaire
du sol qui la recouvre_. It appears that the whole soil of the “Valley
of the Nile” is very considerably increased by the alluvium deposited
annually by the inundations of the Nile, as ascertained by the marks
on some ancient Nilometers and statues, the dates of which have been
traced and compared by Girard, with the corresponding historical
periods. In the quarter of Thebes, where the statue of Memnon is
erected, the increase of the soil since the commencement of the
Christian era is lm. 924 (6 feet 3·7 inches), or this process may be
stated as going forward at the rate of 0m. 106 (4·17 inches) in the
course of each century. The magnitude of the deposits at the mouths
of the Nile, in the bed of the Mediterranean, appears to be no less
surprising. It is remarked that the Isle of Pharos, which in the time
of Homer was a day’s journey from the coast of Egypt, is now united to
the continent.
“If, then, we compare these effects with the same process, going
forward in a certain proportionate rate over all parts of the globe,
and where the same facilities for these depositions being made on firm
ground are not afforded, we shall find that the quantity of deposit in
the bottom of the ocean must be so considerable as to affect the level
of the waters of the ocean.
“In thus disposing of the waste of the surrounding land beyond the
accumulation of the sunken banks in the German Ocean, we are not left
at any loss for a distributing cause, as this is provided by the tides
and currents of the sea; and with regard to their action we have
many proofs, even at very considerable depths, by the breaking up of
the wrecks of ships, the occasional drift of seaweed, and also drift
timber, nuts, etc., into regions far distant from those in which they
are spontaneously produced. The dispersion of fishes, evinced by their
disappearance from the fishing grounds in stormy weather, tends to show
the disturbance of the waters of the ocean to the depth of thirty or
forty fathoms. This observation I have frequently had an opportunity of
making near the entrance of the Firth of Forth. Numerous proofs of the
sea being disturbed to a considerable depth have also occurred since
the erection of the Bell Rock Lighthouse, situate upon a sunken rock in
the sea, twelve miles off Arbroath, in Forfarshire. Some _drift stones_
of large dimensions, measuring upwards of thirty cubic feet, or more
than two tons weight, have, during storms, been often thrown upon the
rock from the deep water. These large boulder stones are so familiar to
the lightkeepers at this station as to be by them termed _travellers_.
It is therefore extremely probable, that a large portion of the débris
is carried down with the drainage water of the higher country, as
before noticed, and ultimately washed out of the North Sea into the
expanse of the ocean.
“The question which naturally arises as to the result of all this
waste or transposition of the solid matters of a large portion of the
globe, is to inquire what has become of the body of water displaced by
this wasting process. Without attempting to go into all the minutiæ
of this part of the subject, I shall here briefly observe, that there
seems to exist (if I may be allowed so to express myself) a kind of
compensating arrangement between the solid or earthy particles of the
globe in the one case, and the waters of the ocean in the other. Thus
by the process of evaporation, and the universal application of water,
which enters so largely, in its simple or chemical state, into the
whole animate and inanimate creation, the surface of the ocean may be
kept nearly at a uniform level. Phenomena of this description are, no
doubt, difficult in their solution upon the great scale, being met
by the process of _decomposition_, which resolves bodies into their
constituent parts, and also by our theory of the atmosphere, by which
its limits and operations are determined. But were we to abstract our
attention from the more general view of the subject, and confine our
inquiries to the German Ocean, the Baltic, the Mediterranean, the Red
Sea, or to any other inland and circumscribed parts of the ocean, this
difficulty seems to be lessened. Indeed, the probability is, and it is
a pretty generally received opinion, that a greater quantity of water
is actually admitted at the Straits of Gibraltar and of Babelmandel
than flows out of the Mediterranean and Red Seas. We consider water,
therefore, as the great _pabulum of nature_, which, as before noticed,
enters either simply or chemically into the constitution of all
bodies, and appears to be held, almost exclusively, in solution, in the
formation and maintenance of the whole animal and vegetable kingdoms,
and is found to exist largely in the composition of all mineral
substances. The quantity of water, consequently, that is required, and
is continually supplied from the ocean by the process of evaporation,
both for the support and reanimation of nature, must be immense, and
may of course be supposed permanently to absorb a very large proportion
of the surplus waters of these circumscribed seas, while the remaining
portion of surplus water, if not thus wholly accounted for, may be
distributed over the general expanse of the ocean.
“But if we suppose with some, that in nature there is neither an excess
nor diminution of the waters of the globe, and that the united and
counterbalancing processes of evaporation, condensation, decomposition,
and regeneration, so completely equalise each other, that the surplus
waters, arising from the displacement of a portion of the solid surface
of the globe, must again be wholly distributed and intermixed with
the waters of the ocean, the portion of water remaining thus to be
accounted for becomes more considerable, and, upon the great scale,
must be permanently disposed of, independently of the process of
evaporation.
“Another view has been suggested as applicable to the distribution of
the surplus waters produced by the gradual filling up of the bed of the
ocean. These waters, in place of being elevated in any sensible degree,
may be naturally disposed to find their level in the great polar
basins, or oblate portions of the surface of the globe which are known
to exist next the poles. The oblate figure of the earth at the poles
makes these imaginary points the nearest to the centre of the earth,
and consequently, with regard to level, they are also the lowest. It
therefore appears to follow, that any filling up of the bed of the sea
near the equator, or at a distance from the poles, will have the effect
of promoting the retiring of the surplus waters to the polar regions
by their own gravity, while the centrifugal force occasioned by the
earth’s diurnal motion will prevent their being further removed from
the earth’s centre, without a corresponding elevation of the waters in
the great polar basins.
“In this manner, such an accumulation of water may, at a former period
of time, have taken place at the then poles of the globe, as to have
altered the position of these points, and given rise to the Flood, or
temporary general overflowing of the waters over the earth’s surface,
producing a change in the beds of the seas or oceans of former times.
In this way may have been produced many of the phenomena observable
in the crust of the earth, which are otherwise with much difficulty
accounted for.
“Of what has now been advanced, regarding the waste of the land by the
operations of the sea, it will be proper to notice that much consists
with my own personal observation. The consequences of this process must
be the deposition of débris, and a tendency to raise the bottom of the
ocean and produce a proportional elevation of the water. With regard,
however, to the distribution of the surplus waters that is produced,
what I have now said is offered with much deference, in the hope that
some one better qualified than myself will turn his attention to this
curious subject.”
* * * * *
In connection with this discussion I give the following interesting
account of observations on the estuary of the Mersey:--
“WASTING EFFECTS OF THE SEA ON THE SHORE OF CHESHIRE BETWEEN THE
RIVERS MERSEY AND DEE. (Read before the Wernerian Society, 8th
March 1828.)
“On a former occasion I had the honour to make a few observations which
appeared in the second volume of the Society’s Memoirs regarding the
encroachment of the sea upon the land generally. The present notice
refers only to that portion of the coast which lies between the rivers
Mersey and Dee, extending to about seven miles.
“To this quarter my attention, with that of Mr. Nimmo, Civil Engineer,
had been professionally directed in the course of last month. In our
preambulatory survey we were accompanied by Sir John Tobin and William
Laird, Esq., of Liverpool, along the Cheshire shore and its connecting
sandbanks between Wallasey Pool in the Mersey, and Dalpool in the river
Dee.
“Within these estuaries the shores may be described as abrupt,
consisting of red clay and marl, containing many land or boulder stones
of the cubic contents of several tons, and very many of much smaller
size, diminishing to coarse gravel. But the foreland or northern
shore between these rivers, which I am now to notice, is chiefly low
ground, and to a great extent is under the level of the highest tides.
The beach or ebb extends from 300 to 400 yards seaward, and toward
low water mark exposes a section of red clay; but toward high water
it consists of bluish coloured marl, with peat or moss overlaid by
sand. This beach, at about tide level, presents a curious and highly
interesting spectacle of the remains of a _submarine forest_. The
numerous roots of trees, which have not been washed away by the sea, or
carried off by the neighbouring inhabitants for firewood, are in a very
decayed state. The trees seem to have been cut off about two feet from
the ground, after the usual practice in felling timber, and the roots
are seen ramifying from their respective stumps in all directions, and
dipping towards the clay subsoil. They seem to have varied in size
from eighteen inches to perhaps thirty inches in diameter, and when
cut with a knife appear to be oak. Several of the boles or trunks have
also been left upon the ground, and being partly immersed in the sand
and clay, are now in such a decomposed state that, when dug into with a
common spade, great numbers of the shell fish called _Pholas candida_,
measuring about three fourths of an inch in length and two inches in
breadth, were found apparently in a healthy state. These proofs of the
former state of this ebb or shore--now upwards of twenty feet under
full tide--having been once dry land to a considerable extent beyond
the region of these large forest trees were rendered still more evident
by the occurrence of large masses of greenstone, which, at a former
period, had been embedded in the firm ground here, and especially on
the shore within the river Dee. It may further deserve notice that the
inhabitants of this district have a traditional rhyme expressive of the
former wooded state of this coast, where not a tree is now to be seen,
viz., “From Birkenhead to Helbre a squirrel may hop from tree to tree;”
that is, from the Dee to the Mersey, now presenting a submarine forest.
“As these evidences of great changes upon the state and former
appearances of the land were highly interesting to the party, and
intimately connected with the professional inquiries of myself and
colleague, it seemed desirable, if possible, to get them corroborated
by oral testimony. Sir John Tobin accordingly very obligingly took
measures for examining the oldest people in the neighbourhood as to
their recollection of the former state of these shores. In particular,
Thomas Barclay, aged ninety-three, “all but two months,” by profession
a mason and measurer of country work; Henry Youd, labourer, aged
eighty-six; and John Crooksan, labourer, aged eighty, were examined.
Barclay stated that he had been employed at the erection of the
Leasowe landward lighthouse in the year 1764; that there were then
two lighthouses near the shore, for a leading direction to shipping
through the proper channel to Liverpool; and that the seaward light
became uninhabitable from its being surrounded by the sea. A new light
was then built upon Bidstone Hill, and the present Leasowe Lighthouse,
formerly the landward light which he had assisted in building, became
the sea light. He could not condescend upon the distance between the
two original lights, but was certain that it must have been several
hundred yards; that he knows that in the course of thirty years the
shore of the Leasowe lost _by measurement_ eleven Cheshire roods or
eighty-eight yards; and verily believes that, since he knew this shore,
it has lost upwards of half a mile of firm ground. To the correctness
of these statements the other two aged men gave ample testimony, Henry
Youd having also worked at the lighthouse.
“As to the present state of things, the party alluded to were
eye-witnesses of the tides on the 16th, 17th, and 18th of February
1828, having exhibited a very alarming example of the encroachment
of the sea upon the Leasowe shore. At high water it came over the
bank, and ran in a stream of about half a mile in breadth surrounding
the lighthouse, and continued its course through the low grounds
toward Wallasey Pool on the Mersey, thereby forming a new channel,
and threatening to lay several thousands of acres of rich arable and
pasture lands into the state of a permanent salt lake. The present
Leasowe Lighthouse, which, in 1764, was considered far above the reach
of the sea, upon the 17th of February last was thus surrounded by salt
water, and must soon be abandoned unless some very extensive works be
undertaken for the defence of the beach, the whole of the interior
lands of the Leasowe being considerably under the level of high water
of spring tides.
“This coast, with its sandbanks in the offing, its submarine forest,
and the evidence of living witnesses as to the encroachment of the
sea upon the firm ground, is altogether highly interesting to the
geological and scientific inquirer. The remains of forests in the bed
of the ocean occur in several parts of the British coast, particularly
off Lincoln, on the banks of the Tay near Flisk, at Skail in the
mainland of Orkney, and in other places noticed in the Transactions of
this Society, and are strong proofs of the encroachments of the sea
upon the land. However difficult, therefore, it may be to reconcile
the varied appearances in nature regarding the sea having at one time
occupied a higher level than at present, yet its encroachment as a
general and almost universal principle seems to be beyond doubt in the
present day.
“Since I had last the honour of addressing the Society on this
subject, opportunities have been afforded me of making many additional
observations on the British shores, and of personally extending these
to almost every port on the Continent between the Texel and the
Garonne. I have also, through the obliging communications of friends,
been enabled to extend my inquiries to other quarters of the globe,
and I am now prepared to state that, with a few comparatively trifling
exceptions, the sea appears to be universally gaining upon the land,
tending to confirm the theory that débris arising from the general
degradation of the land, being deposited in the bed of the minor seas,
is the cause of their present tendency to overflow their banks.”
DENSITY OF SALT AND FRESH WATER.
Mr. Stevenson’s discovery that the salt water of the ocean flows up the
beds of rivers in a stream quite distinct from the outflowing fresh
water, was made in 1812, when investigating a question regarding
salmon fishings on the Dee. It is described in the following extract
from his Report:--
“The reporter observed in the course of his survey that the current
of the river continued to flow towards the sea with as much apparent
velocity during flood as during ebb tide, while the surface of the
river rose and fell in a regular manner with the waters of the ocean.
He was led from these observations to inquire more particularly into
this phenomenon, and he accordingly had an apparatus prepared under
his directions at Aberdeen, which, in the most satisfactory manner,
showed the existence of two distinct layers or strata of water; the
lower stratum consisting of salt or sea water, and the upper one of
the fresh water of the river, which, from its specific gravity being
less, floated on the top during the whole of flood as well as ebb tide.
This apparatus consisted of a bottle or glass jar, the mouth of which
measured about two and a half inches in diameter, and was carefully
stopped with a wooden plug, and luted with wax; a hole about half an
inch in diameter was then bored in the plug, and to this an iron peg
was fitted. To prevent accident in the event of the jar touching the
bottom, it was coated with flannel. The jar so prepared was fixed to
a spar of timber, which was graduated to feet and inches, for the
convenience of readily ascertaining the depth to which the instrument
was plunged, and from which the water was brought up. A small cord was
attached to the iron pin for the purpose of drawing it, at pleasure,
for the admission of the water. When an experiment was made the bottle
was plunged into the water; by drawing the cord at any depth within
the range of the rod to which it was attached, the iron peg was lifted
or drawn, and the bottle was by this means filled with water. The peg
was again dropped into its place, and the apparatus raised to the
surface, containing a specimen of water of the quality at the depth to
which it was plunged. In this manner the reporter ascertained that the
salt or tidal water of the ocean flowed up the channel of the river
Dee, and also up Footdee and Torryburn, in a distinct stratum next the
bottom and under the fresh water of the river, which, owing to the
specific gravity being less, floated upon it, continuing perfectly
fresh, and flowing in its usual course towards the sea, the only
change discoverable being in its level, which was raised by the salt
water forcing its way under it. The tidal water so forced up continued
salt; and when the specimens from the bottom, obtained in the manner
described, were compared with those taken at the surface by means of
the common hydrometer of the brewer (the only instrument to which the
reporter had access at the time), the lower stratum was always found to
possess the greater specific gravity due to salt over fresh water.”
THE HYDROPHORE.
The instrument Mr. Stevenson then invented and used was that to which
the term _hydrophore_ has been applied. Figs. 18 and 19 show two forms
of hydrophores made under his directions.
[Illustration: FIG. 18.]
Fig. 18 is used for procuring specimens of water from moderate depths,
drawn on a scale of one-tenth of the full size. It consists of a tight
tin cylinder, _a_, having a conical valve in its top, _b_, which is
represented in the diagram as being raised for the admission of water.
The valve is fixed _dead_, or immoveable, on a rod working in guides,
the one resting between two uprights of brass above the cylinder,
and the other in its interior, as shown in faintly dotted lines. The
valve rod is by this means caused to move in a truly vertical line,
and the valve attached to it consequently fills or closes the hole in
the top of the cylinder with greater accuracy than if its motion was
undirected. A graduated pole or rod of iron, _c_, which in the diagram
is shown broken off, is attached to the instrument, its end being
inserted into the small tin cylinder at the side of the large water
cylinder, and there fixed by the clamp screws shown in the diagram;
the bottom of the water cylinder may be loaded with lead to any extent
required, for the purpose of causing the apparatus to sink; but this,
when an iron rod is used for lowering it, is hardly necessary. The
spindle carrying the valve has an eye in its upper extremity, to which
a cord is attached for the purpose of opening the valve when the water
is to be admitted, and on releasing the cord, it again closes by its
own weight. When the hydrophore is to be used, it is lowered to the
required depth by the pole which is fixed to its side, or, if the depth
be greater than the range of the pole, it is loaded with weights, and
let down by means of a rope so attached as to keep it in a vertical
position. When the apparatus has been lowered as far as is required,
the small cord is pulled, and the vessel is immediately filled with the
water which is to be found at that depth. The cord being then thrown
slack, the valve descends and closes the opening, and the instrument is
slowly raised to the surface by means of the rod or rope, as the case
may be, care being taken to preserve it in a vertical position.
[Illustration: FIG. 19.]
The form of hydrophore represented in Fig. 19 is used in deep water,
to which the small one just described is inapplicable. It consists of
an egg-shaped vessel _a_, made of thick lead to give the apparatus
weight, having two valves, _b_ and _c_, one in the top and another in
the bottom, both opening upwards; these valves (which are represented
as open in the diagram) are, to insure more perfect fitting, fixed on
separate spindles, which work in guides, in the same manner as in the
instrument shown in Fig. 18. The valves, however, in this instrument
are not opened by means of a cord, but by the impact of the projecting
part, _d_, of the lower spindle on the bottom, when the hydrophore is
sunk to that depth. By this means the lower valve is forced upwards,
and the upper spindle (the lower extremity of which is made nearly to
touch the upper extremity of the lower one, when the valves are shut)
is at the same time forced up, carrying along with it the upper valve,
which allows the air to escape, and the water rushing in fills the
vessel. On raising the instrument from the bottom, both valves again
shut by their own weight, and that of the mass of lead, _d_, which
forms part of the lower spindle. The mode of using this hydrophore is
sufficiently obvious; it is lowered by means of a rope, made fast to a
ring at the top, as shown in Fig. 19, until it strikes on the bottom,
when the valves are opened in the manner described, and the vessel is
filled; on raising it the valves close, and the vessel can be drawn to
the surface without its contents being mixed with the superincumbent
water through which it has to pass. This instrument, shown on a scale
of one twentieth of full size, weighs about half a hundredweight, and
has been easily used in from thirty to forty fathoms water.
* * * * *
Mr. Stevenson subsequently extended his experiments on the density of
salt and fresh water to several firths and tidal rivers, and gave the
results in a paper communicated to the Royal Society of Edinburgh in
May 1817, of which the following digest is given in Thomson’s _Annals
of Philosophy_:[14]--
“The waters of the Thames opposite the London Dock gates were found to
be perfectly fresh throughout; at Blackwall, even in spring tides, the
water was found to be only slightly saline; at Woolwich the proportion
of salt water increases, and so on to Gravesend. But the strata of salt
and fresh water are less distinctly marked in the Thames than in any of
those rivers on which Mr. Stevenson has hitherto had an opportunity of
making his observations. But these inquiries are meant to be extended
to most of the principal rivers in the kingdom, when an account of the
whole will be given.
“From the series of observations made at and below London Bridge,
compared with the river as far up as Kew and Oxford, Mr. Stevenson
is of opinion that the waters of the Thames seldom change, but are
probably carried up and down with the turn of the alternate tides for
an indefinite period, which, he is of opinion, may be one, if not the
principal cause of what is termed the extreme softness of the waters of
the Thames.
“Mr. Stevenson has made similar experiments on the rivers Forth and
Tay, and at Loch Eil, where the Caledonian Canal joins the Western Sea.
The aperture at Corran Ferry, for the tidal waters of that Loch, being
small compared with the surface of Loch Eil, which forms the drainage
of a great extent of country, it occurred to him that the waters of the
surface must have less saline particles than the waters of the bottom.
He accordingly lifted water from the surface at the anchorage off Fort
William, and found it to be 1008·2; at the depth of 9 fathoms 1025·5;
at the depth of 30 fathoms, in the central parts of the Loch, it was
1027·2; being the specific gravity of sea water.”
The hydrophore, which was originally devised and used by Mr. Stevenson,
in 1812, at Aberdeen, has now reached its height of excellence of
construction and scientific importance in the famous ‘Challenger’
Expedition.