many situations, and for a variety of purposes, it is in fact quite useless; for I have always found that it was impossible, with this dam, to drive the piles straight, from there being only one waling-piece to direct them. But even although they could be driven, a farther source of inconvenience still remains, for, as the stuff is removed from the interior, there is nothing left but the single waling to resist the pressure from the outside, and the bottoms of the piles being speedily forced inwards, all attempts to carry the excavation farther must necessarily be abandoned.

At Hynish harbor, Argyllshire, in 1843, I had a talus-wall to found on sand, which covered a rocky beach to the depth of from two to three feet. At another place, the rock was not only to be bared, but a navigable channel, twenty feet wide, and in some places as deep as eight feet in the rock, together with a small tide-bason, were to be excavated to the level of the low-water springs. The shores also were frequently subject, even during the summer months, to a very heavy surf.

The excavation of the tide-bason, which formed the landward part of the work, was effected by means of a series of dams, consisting of walls, built of Pozzolano rubble. These were found to be quite watertight, and to answer remarkably well in every respect; but they required, for their protection against the waves, a considerable bulwark or breakwater of Pierres perdues to shelter them from the waves.

In the excavation, however, which had to be undertaken seaward of the breakwater of Pierres perdues, any attempt to exclude the water during the whole of the tide, was what I never considered practicable. A trial was accordingly made to effect the excavation by means of a low wall, composed of clay-rubble, resembling in its object those low dams consisting of logs of wood bedded in clay, which are often adopted in harbor works, and which are only intended to keep out the tide during the first part of the flood, and to be pumped dry before the operations of the next tide are begun. But after many attempts with this clay wall, it became quite evident that it would not be possible, with its assistance, to carry the excavations to near the level of low-water springs, which was due principally to two causes: First, because sand and shingle were, during almost every tide, washed in large quantities over the top of the wall into our excavation pit; and, secondly, because the waves washed out the clay from among the stones, so as to render the barrier no longer water-tight.

Being now compelled to set about some other way of carrying on the work, I had recourse to the simple method shortly to be explained, and which more than realized my expectations. Before giving a description of this method, however, it will be interesting, as well as still farther explanatory of the required objects, to quote a few lines relating to somewhat similar difficulties, from a Report upon the Harbor of Peterhead, which was drawn up in the year 1806 by the late Mr. John Rennie: "The next material object of consideration," says the Report," is that of deepening the harbor, which at present cannot well accommodate vessels drawing more than 12 feet of water in the spring tides, but in neaps is not sufficient. To render this harbor more ex

tensively useful, it would be advisable to have 17 or 18 feet of water over the greatest part of its bottom, and particularly along the west quay. The mode of performing this kind of work will be different, according to the difference of situation. Those places where the tide ebbs from the surface, and continues so for some time, may be done by blasting, or by loosening the stones with quarrying tools in the usual manner; but in those parts where the tide seldom leaves the bottom, and in others but for a short time, different methods must be resorted to. The best of all would be enclosing large spaces by cofferdams, and working at all times of tide by quarrying tools or blasting, as might best suit; but in some situations this would be inconvenient, as the dams would be in the way of vessels going into and coming out of the harbor. In such situations, perhaps the simplest and most expeditious mode would be to use cast iron cylinders, of 7 or 8 feet diameter, having strong canvas fixed to the lower flanch, which might be kept to the bottom by bags of sand in places where there was but little agitation; but where there is much, an outer cylinder might be sunk thereon, to keep them in their situations."

The cylinders proposed by Mr. Rennie were, no doubt, quite adequate to the special purpose and locality for which they were designed, and they unquestionably possess some advantages not to be gained by other means; but, on the other hand, they are attended with difficulties and disadvantages which precluded their adoption in the present case. Those objections were the limited area, the weight and unwieldiness of such cylinders, their inflexible nature and unalterable form, as affording no means in themselves of adaptation to the very irregular rocky bottom which was to be excavated, and what was of as much consequence, the difficulty which must have attended the removal of the partitions of rock, or those parts which would necessarily be left between the different compartments of the cutting. The last two objections, it may be remarked, refer equally to wooden caissons, or other contrivances on the same principle.

In the present case, then, the following requisites were to be provided for. In the founding of the talus-wall, all that was required was some method which would enable the found-stones to be laid as deep in the sand as possible, for which purpose the dam did not require to be absolutely water-tight, provided it were capable of excluding from the inside the sand which was so liable to replace what was removed from the interior. For the excavation of the rock, on the other hand, it was necessary that the dam should be water-tight, and suitable for taking out all the partitions; and both situations required piles for fitting close to the irregular bottom, and those piles needed some support other than the soil into which they were to be driven.

To effect such objects, it was clear that the means to be adopted must be at once easily managed and efficient. For although, where there is time for their employment, many complicated and troublesome refinements of construction are forced to answer purposes which might have been attained by simpler means, or by less cumbrous arrangements, yet I was well aware that in the hurry and bustle attending tidal operations and night work, nothing can be tolerated but what is in every respect easily managed and truly efficient.

In the accompanying Diagrams, A G (Plate II.) represents a frame of double waling-pieces connected at the angles by the uprights I I, and bound together by the long bolts L, with forelocks and washers, while E F shews similar double-framed walings for the inside of the dam, and of smaller dimensions, with their uprights D, and connecting bolts K. These frames being placed in the required position, the one frame inside of the other, the piles, C, are driven down between them with heavy malls.

The dam was 12 feet long by 10 feet broad inside, so that five men were able to work in the interior.* If it was to be fixed within low water-mark, the two frames being placed in the water, were guided to the spot by the men in charge, and whenever they were in the desired position, the men at once moored or fixed the frames to the bottom, by driving down a pile at each corner. After this was done, all the piles were placed between the frames and driven down, and keyed up by the small piles called "closers." Four iron jumpers, J, were then driven down to their proper places outside of the frames, and edge planks for retaining the clay were slipped down upon the jumpers through iron staples, which were fixed to the planks. After this, good clay, (which should have some gravel mixed with it, to protect it from the wash of the sea,) was punned hard between the planks and the cofferdam, after which the mast N was erected, and the water taken out by means of the iron scoop shewn in the drawing, which not only was used in taking out the stuff, but proved far more efficacious than any pump we ever had. Indeed, to get the dam pumped dry, was for long the greatest difficulty we had to contend with. But Mr. William Downie, to whom I gave the charge, soon removed this difficulty, by using the scoop instead of a pump. The capacity of the scoop was about 37 gallons, and they generally made nine deliveries a minute, so that we found this method greatly more expeditious than any other.

As the excavation proceeded, the piles were from time to time driven down; and when the rising tide began to come over the pile heads, or to rise above the clay, the men, before leaving their work, placed the flooring or "deck," as it was called, within the piling, with the ends of the planks resting upon the top of the inner frame. On this deck, ballast (consisting of stones of a convenient size) was deposited, to prevent the whole frame from being floated up,-the quantity so deposited varying with the height of tide, or appearance of the weather. As each compartment of the excavation was completed, and before the dam was removed, the rock below the two rows of piles which adjoined the next cuttings, was completely taken out, and the piles driven down to the bottom of the excavated pit, and left standing. When the dam was taken up, the frames were, for the next compartment of

* Since this paper was printed, a cofferdam on the same principle, and 30 feet square, has been made for the Forth Navigation works, Stirling, where, in the removal of the "fords," under my direction, much difficulty has hitherto been experienced, from the constant flow of the river.

+ Before lifting the cofferdam, the pit was filled with sand, to support the piles that were to remain, which, when the works were done, was cleared out by means of a water-scour provided for the purpose of keeping permanently open the navigable tract.