Life of Robert Stevenson, Civil Engineer

by David Stevenson

“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.