contributions to encyclop?dia britannica and edinburgh encyclop?dia—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 encyclop?dia and the encyclop?dia britannica, in articles on “roads,” “lighthouses,” “railways,” “dredging,” “blasting,” and other engineering subjects.
204 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 that205 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 a?rial 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 and206 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.
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.
207 “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, though208 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 ringhjo?bing 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 confusion209 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 the210 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 the211 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 5? 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 their212 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 doubtless213 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.
214 “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 exuvi? 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,215 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 constantly216 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 incontrovertible217 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,218 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 surplus219 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 therefore220 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.
221 “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 result222 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 minuti? 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 all223 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 polar224 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 is225 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, is226 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 ground227 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 been228 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 scientific229 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, when230 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 plunged231 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.
fig. 18.
fig. 18 is used for procuring specimens of water from moderate depths, drawn on a scale of one-tenth of the full232 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 is233 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.
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. on234 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 extended235 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.
