manner as might in some cases make the entrance difficult or even dangerous of access, and the berthing within unsafe. There are many instances of harbours being materially injured by the erection of a quay wall across a beach where the waves were formerly allowed to expend their force.

It may be observed that when there is an inner harbour or stilling basin, the elliptical form seems to be the most promising. Let one focus be supposed to be on the middle line of the entrance and to coincide with the point from which the waves in expanding into the interior radiate as from a centre (which they do approximately), and if the other focus is situated inland of high-water mark, the waves will tend to reassemble at the landward focus, and on their way will be destroyed by breaking on the beach. This appears from the well-known property of the ellipse, that if two radii vectors be drawn from the two foci to any point in the curve they will make equal angles with the tangent at that point; and as the angles of incidence and reflection of a wave from any obstacle are practically equal, each wave will be nearly concentrated at the focus opposite to that from which it emanated.

Another cause of disturbance in harbours, which is often not sufficiently considered, is the indiscriminate deepening of the entrance without a proportionate enlargement of the internal area, or the execution of other works for counteracting the effect. As the depth of the water is more and more increased, waves of greater height become possible at the entrance, so that larger waves gain admission to the interior. The writer has had repeated proofs of this in the course of his practice. At the port of Sunderland Mr D. Stevenson recommended the removal of nearly the whole of the south stone pier, and the substitution of works of open framework in order to tranquillize the interior. These works, which have been quite successful, were rendered necessary by the frequent dredging of the channel at and near the entrance.

The preservation of the depth of harbours where there is a tendency to deposit is often attended with great difficulty and expense. Where the deposit of salt is confined to the space between high and low water marks, the scouring by means of salt or fresh water is in general comparatively easy, but where there is a bar outside of the entrance the case becomes most materially changed. The efficacy of the scour, so long as it is not impeded by encountering stagnant water, is kept up for great distances but soon comes to an end on its meeting the sea. Probably the only way in which this difficulty might to some extent be obviated would be by conducting the water in iron pipes to the bar, a plan which the author proposed in 1843 for Hynish harbour, but the expense was considerable and the success doubtful. The same plan was proposed by Mr Alexander Swan for Kirkcaldy some years later. When the volume of water liberated is great compared with the alveus or channel through which it has to pass, the objection based on the stagnancy of the water originally occupying the channel does not hold to the same extent as when the scouring is to be produced by a sudden finite momentum. In the one case the scouring power depends simply on the quantity liberated in a given space of time, while in the other it depends on the propelling head and the direction in which the water leaves the sluice. Mr Rendel's scheme for Birkenhead was on the former principle. The first example of artificial scouring in this country seems to be due to Smeaton who used it effectually at Ramsgate in 1779.

At Bute Docks, Cardiff, designed by Sir W. Cubitt, the access to the outer basin is kept open most successfully by means of artificial scouring on a gigantic scale. The entrance was cut through mud banks for a distance of about three-fourths of a mile seaward of high-water mark. The initial discharge when the reservoir is full, is stated to be 2500 tons per minute. The writer has known even so limited a discharge for an hour to two as one ton a minute, produce very useful effects in keeping a small tidal harbour clear of sand.

Many proposals have from time to time been made for floating mooring in the open sea floating frameworks of timber with breakwaters. The view of sheltering the space inclosed by them is obvious, objections to floating breakwaters are so great and obvious that there seems little chance of their ever being much used. From what was stated on the subject of booms, it will be recollected that it is a requisite that they should fit closely to a sill piece at the bottom, otherwise the run is found to extend into the harbour. From what will be afterwards stated regarding the liability of timber to speedy destruction from the marine worm, and to iron by chemical action, it is obvious that floating structures of wood, connected by iron and moored by iron chains, cannot possibly be of long duration. If to all these sources of evil we add the risk of their being broken by the sea we think the case may be almost regarded as hopeless. No doubt green-heart might be employed so as to resist the ravages of the worm, but its high specific gravity and its great expense would prove bars to its employment.

In some situations where there is a long shallow beach, a harbour or pier of timber or masonry may be made at or near the low-water mark, which may be connected with the shore by means of a suspension bridge. The inducements to adopt the suspension principle are its economy, and the free passage it affords to the currents which in this way are prevented from forming accumulations of sand, silt, or gravel. These advantages are, however, much reduced by the great wear and tear consequent upon the perishable nature of the structure. The late Sir Samuel Brown erected two chain piers, the one at Brighton, and the other at Newhaven, near Edinburgh, both of which are still in existence.

In every situation where it is easily practicable to make two entrances to a harbour, it will be found well worth the extra expense, provided they can be so placed that the one shall be available when the other has become difficult of access. In harbours which have but one mouth, vessels are often detained for a great length of time by the continuance of the wind in the direction which throws a heavy sea into the entrance. Whereas if there are two entrances situated as we have supposed, vessels are at once able to take their departure by the sheltered side. At the port of Peterhead, the north and south harbours were some years ago united by a canal, according to the writer's plans, and there the advantage has been of the most marked description. Vessels can now clear out as soon as loaded, either by the north or south mouth, according to the state of the sea. Some caution is necessary, however, as the run is apt to extend from the one harbour to the other unless there be a considerable area.

There is generally much prudence required in the alteration or repairs of existing marine works. The risk of having the whole structure destroyed by a gale coming suddenly on while there is an open breach in the works, must be obvious; and in one instance, where the exposure of the place was great, and the evil was a hidden one, the writer could not recommend the facework being disturbed. The cause of failure in this instance was supposed to be the decay of the backing, which having deprived the face-stones of support allowed them to be driven inwards by the force of the waves. Instead of removing the facework, the only recommendation that could be given was to inject the whole pier with fluid cement, so as, if possible, to render the mass monolithic. An alternative of this kind is obviously of very doubtful success, and can be regarded as nothing short of a last resort, for there is but a small chance of getting the injected fluid to permeate the whole mass of the pier. The system of permeating the masonry with fluid matter could, however, be employed with more chance of success in the Harbours. formation of a pier, while each course lies open to view.

In 1844, at a harbour that had stood for very many years, two or three faulty stones had been incautiously taken out of the facework by a mason who intended to replace them by Harbours others, when a sudden gale came on, and nearly the whole of the work was levelled with the beach.

Register of Height of Waves for 1852 observed at Lybster, Caithness-shire.

As an example of the suddenness with which our eastern coast is visited by gales, and as indicating graphically the relative eligibility of the summer and winter months for carrying on harbour works, we give the accompanying diagram of the heights of waves, as observed for the writer, by Mr William Middlemiss, resident engineer at Lybster harbour.

In landlocked bays, where a deep-water landing-place is all that is required, and where the bottom is sandy or soft, timber may be employed with great advantage. Even in exposed situations, timber can also be used, but the fatal disadvantage attending its employment in most places where there is no admixture of fresh water, is the rapid destruction occasioned by marine worms.

The damage occasioned to harbours in this way is noticed by Semple in his treatise On Building in Water in 1776, and very probably by much earlier writers. Indeed, the ravages of the Teredo navalis are very ludicrously described by Hector Boece in his Croniklis of Scotland, printed at Edinburgh circa 1536. In the Atlantic Ocean the Teredo navalis, and at many places in the German Ocean the Limnoria terebra, are the animals which are found to destroy any structure of timber which is exposed to the water. They are found to eat most rapidly between the bottom and low-water mark, but above low-water the damage is not so great; and what is singular, they do not appear to exist at all below the bottom where the pile is covered with sand. These observations do not, however, quadrature with Mr Hartley's at Liverpool, for he found the parts which were alternately wet and dry to decay faster than the parts which were constantly immersed. Even solid limestone is often destroyed by the persevering efforts of another marine animal called the Pholas.

The late Mr R. Stevenson made several experiments on the ravages of the Limnoria terebra at the Bell Rock in 1814, 1821, 1837, and 1843, by fixing pieces of different kinds of timber to the rock, and getting regular reports on their decay. From those experiments it appeared that green-heart, beef-wood, and bullet-tree, were not attacked by the worms, while teak stood remarkably well, although suffering at last. The kyanizing fluid and other preparations have been tried, but were not found to be of permanent service. In addition to these experiments on timber, no fewer than 25 different kinds and combinations of iron were tried, including specimens of galvanized irons. Although separate specimens of each were tried in places where they were always under water, and also in places where they were alternately wet and dry, yet all the ungallvanized specimens were found to oxidize with much the same readiness. The galvanized specimens resisted oxidation for three or four years, after which the chemical action went on as quickly as in the others.

The following Table shows the different kinds of Wood which were made the subject of experiment at the Bell Rock in 1814, 1821, 1837, 1843, with their relative durability.

| Kind of Timber | Decay observed | Unsound and quite decayed | Quite sound for Remarks | |----------------|---------------|--------------------------|------------------------| | Green-heart | | | | | Teak-wood | | | | | Do. | | | | | Do. | | | | | Treenail of locust | | | | | Beef-wood | | | | | Treenail of Bullet-wood | | | | | African Oak | | | | | Do. do. | | | | | English Oak | | | | | Do. | | | | | British Oak | | | | | Do. | | | | | English Oak, kyanized | | | | | American Oak | | | | | Do. do. | | | | | English Elm | | | | | Do. | | | | | Scotch Elm | | | | | American Elm | | | | | Canada Red Elm | | | | | Honduras Mahogany | | | | | Do. | | | | | teak treenails | | | | | Birch | | | | | Do. Payne's patent pro. | | | | | Cedar of Lebanon | | | | | Scotch Fir, teak treenails | | | | | Do. from Larnarkshire | | | | | Do. locust treenails | | | | | Memel Fir. | | | | | Riga Fir | | | | | Pantis Fir | | | | | Norway Fir | | | | | Baltic Red Pine | | | | | Do. | | | | | kyanized | | | | | Pitch Pine | | | | | Do. | | | | | Virginia Pine | | | | | Yellow Pine | | | | | Red Pine | | | | | Candle Pine | | | | | American Yellow Pine | | | | | Do. locust treenails | | | | | American Red Pine | | | | | Do. | | | | | kyanized | | | | | Larch | | | | | Polish Larch | | | | | Birch, Payne's patent pro. | | | |

Fig. 2

A little held at one end underneath. Green-heart timber is now generally had recourse to in places where the worms are destructive. It appears to have been first used by Mr J. Hartley of Liverpool, who published in the Minutes of Institution of Civil Engineers an account of its virtues in 1840, as ascertained at the Liverpool Docks. Its cost is considerably greater than memel or than most of the other timbers generally used. Memel logs for the inner piles of piers might perhaps, from their not being exposed to abrasion from ships, be clad with green-heart planking at those parts which are exposed to the worm. Copper sheathing and scupper nailing are often and successfully employed as protections for piles in exposed situations. Breaming or scouring the wood, and afterwards saturating it with train oil, also forms a partial protection.

It is much to be regretted that timber is so expensive in this country, and that some simple and economical specific against the worm has not been discovered for protecting memel and the cheaper kinds of pine. The grand desideratum in harbour works, which is the want of continuity in the structure, would then be supplied. It follows, from the known laws of fluids, that each individual stone in a pier which is equally exposed throughout its whole length, is subjected to a force which it can only resist by its own inertia, and the friction due to its contact with the adjoining stones. The stability of a whole hydraulic work may therefore be perilled by the use of small stones in one part of the fabric, while it is in no way increased by the introduction of heavier stones into other parts. By the use of long logs of timber carefully bolted together a new element of strength is obviously obtained. A pier could be erected almost free of sea risk if constructed of rectangular or other shaped prisms, consisting of logs of timber trenailed and bolted together, so as to form boxes, say 10 feet square and 30 or 40 feet long. The interior of the boxes would be filled with rubble or béton. The first layer would be arranged across the pier, so as to fit the irregularities of the bottom, and above that, they might be arranged lengthways of the pier, so as to form its outer and inner walls, the space between being filled with common rubble or béton.

In many ports the original depth has been decreased by the deposit of silt, sand, and gravel. This is, indeed, a great evil, and one which unfortunately is most difficult of cure. So obscure and apparently capricious are the causes which lead to the formation of shoals, that in the present state of our knowledge it would be little short of quackery to lay down any general rules for the guidance of the engineer. In fixing on the site for a harbour, all existing obstructions should be examined to ascertain whether there be a tendency to deposit, and the works should be kept as far as possible from places where the tendency is most strongly developed. The agents which occasion bars at the mouths of harbours are the waves, the tide currents, and land streams where they exist. Rivers are often more pernicious than beneficial in their effects, especially where they intersect a gravelly soil; but in some cases the descending gravel may be successfully intersected by the erection of weirs from which the accumulations must be from time to time removed. We agree with Sir H. De la Beche in believing that the bars at the mouths of rivers are most generally formed by the constant tendency of the waves to preserve the continuity of the beach profile. It is therefore not to be wondered at, that heavy gales should distort and fill up the narrow trench which the back waters cut in gravelly or sandy beaches. The erection of breakwaters on each side has undoubtedly a good effect in protecting the channel, but still a bar is very apt to form outside of the breakwaters. In some cases the depth of the track might probably be maintained by driving, on each side of the mid-channel, dwarf Harbours, piles to which continuous walings should be attached so as to confine the current at low-water. The timber framework should not project more than a foot or two above the bottom, which in some cases might be planked. This, however, is but a hint, and has, so far as the author is aware, never been tried. The principle on which the proposal is based is that of contracting the low-water channel to a smaller width than that of the high-water channel, and thus by fixing the low-water track, to prevent a tortuous channel. The same principle was adopted by the writer with success in controlling and fixing the meanderings of a gravelly river, which is subject to very sudden and heavy freshes.

The want of sufficient funds occasions a great national loss in the construction of our harbours. The history of large majority of those ports which have been erected by private or local enterprise, presents but a record of the building of piers at one period when the funds were small, and local funds of taking them down at another when the trade had increased and more room and accommodation were required. Want of funds often prevents the original works from being carried within deep water, and in consequence the most expensive part of the protecting breakwater is often put down just in the very place which has afterwards to be converted, at great expense, into a deep water access or berthingage. Sometimes, indeed, a whole line of pier is, from motives of economy, placed in such a manner as to interfere most materially with what might have been by far the best and safest berths for shipping, so that in the further extension of the works a great part of the old harbour has to be demolished. Want of a proper marine survey has also often led to very serious errors in the position of the piers.

To such an extent has this system prevailed, that were an engineer called on to value many of our works as they exist at present, his estimate, however fairly and fully made out, would fall lamentably short of the actual cost. This estimate would proceed on a measurement of what he sees, while the actual cost would include the building of piers and jetties which had long since ceased to exist. For these reasons we conceive there could hardly be a more advisable expenditure of the public money than by a system of grants for supplementing the local funds on a liberal scale. With such aid the authorities on the spot would be enabled to protect and improve the existing physical advantages which the shores possess, by preventing the construction of proposed improvements on too narrow a scale. But a comparatively slight increase of the means would, in instances of which the writer is aware, have included a great extra area, and secured a deeper access with superior internal tranquillity, the want of which now cripples the trade, and is the subject of lasting regret to all frequenting the harbours.

For other subjects connected with harbours see articles on DOCKS, and PORTS.

Reference may be made to Brit. Assoc. Rep. 1850 (Scoresby); Min. Inst. Civ. Eng. 1848 (Rankine); Do. 1847 (Scott Russell); Do. 1844 (Bremner); Smeaton's Reports, passim. Rep. Com. on Waves by Brit. Assoc., J. S. Russell, Lond. 1848. Researches on Hydrodynamics, J. S. Russell.—Trans. Roy. Soc. Edin., vol. xiv. 1837. Account of Experiments on Force of Waves of Atlantic and German Oceans, Thomas Stevenson.—Trans. Roy. Soc. Edin., vol. xvi. 1845. On Reduction of Height of Waves after passing into Harbours, T. Stevenson.—Edin. New Phil. Journ., 1852. Account of the Plymouth Breakwater, by Sir J. Rennie, London 1848. Belidor's Architecture Hydraulique, Paris. Serres' Treatise on Building in Water, Dublin, 1776. Report Total Harbour Committee (Reports Captain Washington), London 1845–6. The Article on Tides and Waters in the Encyclopædia Metropolitana, by G. B. Airy, Astronomer Royal. Report by Commissioners of Harbours of Refuge, with the Protest, by Sir Howard Douglas.