ing oil works similar to those at Bathgate, I suppose with the view of obtaining a supply when this field fails, or becomes too expensive to work, when all the thicker portions have been exhausted.

It is not my purpose to occupy your time with a description of the products; but as it may interest the members, I exhibit a small piece of the paraffine from Boghead gas-coal. I understand, also, that the naphtha got from using this coal for making oil gives hardly any benzole-a substance from which many of our fashionable colours, such as Magenta, Solferino, &c., are obtained; whereas, when used at a high temperature for gas, the naptha yields more of it. I have to apologise for the length of this paper, which I have found it impossible to condense further consistently with rendering the subject. intelligible; and if I have failed to make it generally interesting to your Society, I must conclude by begging the members to forgive me for bringing a matter so purely geological before them.

A Description of the Reclamation Embankment for the New Dry Dock at Leith. By GEORGE ROBERTSON, C.E., F.R.S.E., Resident Engineer. *

I have two special reasons for giving the following description of this bank:

1st, Because it was designed under somewhat peculiar engineering difficulties, and it answers perfectly the object for which it was intended-viz., to exclude the tide from a portion of the shore so effectually as to form a cofferdam for the construction of a dock built in deep sand, within a few yards of the back of the bank.

2d, Because this is the first decided, or at least active step, towards what appears to me a matter of immense importance, and a source of great future profit to Leith; that is, the reclamation (for dock, warehouse, building, and other purposes), of the hundreds of acres of shore now lying perfectly useless,

* Read before the Society and drawings exhibited, 25th Sept. 1861.

from which the tide daily retires in a most tantalizing manner, inviting the good people of Leith to come and take possession, to relieve their narrow streets and crowded docks. There has been a passive species of reclamation going on for many years, and much valuable land has been gained by the rubbish brought down in carts from Edinburgh, at a rate, however, which will certainly require four centuries to shift the coast line out to the Black Rocks. But as this kind of reclamation cannot be called active, a bank by which even six acres were gained in one tide, becomes a matter of some importance and interest; more real interest, I think, than even the fine Dry Dock for which the land has been reclaimed.

The question of gaining ground from the east sands for dock purposes is by no means new; the wonder is it has not been done before. So long ago as 1799, when Rennie was consulted as to the position of proposed wet docks, he took into consideration the advisability of making use of the east side of the harbour for that purpose. He gave in designs for at least one dock of 62 acres, with 18 feet of water on the cill, to be constructed on land reclaimed for the purpose by a seawall parallel to the shore. As this wall was to form a cofferdam during the construction of the works, in the way the reclamation embankment does now, he considered it would be necessary to convey it through the deep sand down to the natural clay. The level of the clay on the side of the harbour being below the level of low water, this could only be done by laying the foundations within cofferdams, and pumping out the water by a steam-engine. The cost of this reclaiming wall Rennie estimated at L.35,000, of which L.14,000 was for dams alone, with the candid but unpleasant proviso that it might not be sufficient, for the item was very uncertain. From my experience of the situation, and the running nature of the sand, I believe this sum would have proved totally inadequate. The higher and more favourable level of the clay on the west side settled the question against the east sands; and the old Wet Docks were built by Mr Rennie, in the beginning of the present century.

In 1828, the east side had another chance of being made useful, for Telford suggested that a rubble wall might be ad

vantageously thrown out from the back of the old stone pier, parallel to the shore, and the land within reclaimed, by depositing behind the wall the mud obtained from the old harbour. In this way, he said, enough of valuable land might be obtained to defray the expense of the wall.

In 1848, when the Victoria Dock was required to accommodate the increasing number and size of vessels, the situation of the existing docks weighed too heavily in favour of the west side, which again gained the day. Some years after, when it was proposed to build a dry dock for the repairs of large vessels, the west side was first thought of. In 1854, plans were got out, in Mr Rendel's office in London, for a dry dock, entering from the west side of the Victoria Dock. However, the limited amount of quayage in the Victoria Dock made the interference with a berth 300 feet long so serious, that Mr Rendel transferred his plans to the east side of the harbour, where the dock is now being built. The sanction of the Treasury was obtained in 1858, and the works commenced accordingly.

The site for the Dry Dock is somewhat peculiarly situated; being almost an island. The circumference is about half a mile; of this only one-eighth, or 330 feet, is land, and the remaining seven-eighths, or 2310 feet, are water. The neck of land is at the south-west corner of the area to be enclosed; and, as it happened, the place which was longest of drying after the tide was excluded was opposite the very corner where alone there was no water. The harbour side, where the depth of water is some twenty-four feet, is protected by the old stone pier (built a century and a half ago, at the end of Queen Anne's reign), and is therefore for the present tolerably safe. The pier is eventually to be removed to form the entrance to the Dry Dock.

The south side is threatened by the water in the timber pond; but, as the depth here is not great, and the distance from the dock is considerable, nothing has been done to guard this side. The real danger is on the north and east sides, bounded by the reclamation embankment of a quarter of a mile long; more especially as it was desirable for several reasons to build the dock as close to the bank as we dare go. The level of the natural clay over the whole area may be taken as twenty-four feet below high water of spring tides, the same

level as the cill of the caisson invert. At the south-west corner the clay is only 23 feet below high water, but it falls gently seawards, and rather to the east. The thickness of the overlying bed of sand is 14 feet at the bank and 24 feet at the old bulwark; the excavation for the dock being through an average depth of 18 feet of sand. The depth of water along the whole north side is 10 feet, the water on the east side shoaling of course as the beach rises to high-water mark. The depth of water is not great enough for very heavy waves; but they come rolling in over the long expanse of flat sands with considerable velocity and force. A wind from the north-east causes a swell sufficiently destructive to all unfinished or insufficient work. The rounded-off angle, where the north and east banks join, has therefore been put facing the north-east, so that the waves being broken and divided never fairly strike either half of the bank, but glide along it in an oblique direction. This has been a great protection to the work. As the wave glides along the north bank it meets the recoil wave from the East Pier, and the two clash together, throwing up the water to a height of twenty or even thirty feet. In a heavy ground swell, this corner presents a very tumultuous and foaming appearance; but the waves expend their force more on each other than on the bank.

The embankment has to do the double duty of reclaiming the sands from the sea and excluding the water from the excavation for the dock, the lowest point of which is thirty-three feet below high water. The back of the dock wall is ninety feet from the coping of the bank-as near as it was possible to put it, allowing for the necessary slopes and benches.

To have formed a masonry cofferdam, by carrying a solid wall down to the clay, as Rennie had intended, would have been a most difficult and expensive operation, in running sand with the clay level five feet below low water of spring tides. In a financial point of view, it would have been impracticable. Neither would it have been prudent to have built solid masonry of any description upon sand so near an open cutting; the risk of settlement would have been too great. When sand can be securely confined, it is by no means so bad a

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K

foundation practically as it is proverbially. In this situation, however, it could not be trusted, for there would be a constant run of water, more or less, through it into the cutting at the lower level. So the wall was designed of dry rubble deposit, in which settlement would not much matter, or at least would cause no unsightly cracks.

The whole success of the bank in excluding the tide from the enclosed area depended on the truth of the principle, that water, in traversing a certain distance through sand, will encounter so much friction in passing between the particles of sand that the initial pressure will at last be destroyed, and the watter throttled to such an extent as to be within easy control of pumps. That in short, within limits, a sufficiently water-tight cofferdam can be formed of sand itself, when left undisturbed. The correctness of the principle has been satisfactorily proved by the very small quantity of water which passes under the bank; so small (for a work of this kind) that it can be kept down by a 9" chain-pump working one hour in four. Indeed, were it not for some beds of open gravel unexpectedly met with at the north-west angle of the dock, there would not be half this quantity of water. A good deal of it also comes from the harbour and timber pond sides of the excavation.

The Reclamation Embankment of the New Dry Dock at Leith.