development of ocean leviathans in recent years, the need of wide entrances is returning. In 1857 the Canada Lock was constructed at Liverpool, 100 feet wide. It was not until the year 1902 that another entrance of the same width was opened for traffic. During the interval the width considered requisite had fallen to 65 feet, from which it has gradually risen to its former dimension. No doubt a width of 100 feet is in excess of presentday requirements, the maximum breadth of a ship being as yet 70 feet, but another decade will probably see a large increase, so that the margin provided is no more than prudent foresight would warrant.

Another factor involved in the determination of width is the ratio between the sectional area of the entrance and the cubic capacity of the dock, or, what is the same thing, between the width of the entrance and the area of the dock. If a dock entrance remains open for any length of time after high water, a gradually increasing current is generated owing to the fall of the tidal level outside, and the consequent discharge of the water from within the dock through a narrow passage. If allowed to continue too long the current may become so rapid as to render the closing of the gates a hazardous proceeding. The limit of safety may be considered reached when the velocity is 3 feet per second. When the dock is of considerable area it may be necessary to provide two or more entrances, as much for facilities of traffic as for the reason given above.

As regards depth, the sill of the old Canada Lock was such as to afford a depth of 26 feet 6 inches of water at high water of ordinary spring tides at Liverpool, and 19 feet 4 inches at high water of ordinary neaps. The latest entrances constructed at that port provide for 39 feet 2 inches and 32 feet respectively. The loaded draught of modern vessels, it is true, does not exceed about 32 feet as yet, but the greatest length consistent with that draught has now been reached, and an increment in length will necessitate a corresponding increase in depth. The obstacle to this development in depth has been the limited draught of water obtainable at the ports which the vessels frequent, and there can be no doubt that with increased depth of water there will come increased depth of ships. The following abridged remarks of Dr. Francis Elgar,* made in 1893, are equally applicable at the present date:

"The deep draught of water is a most important element of speed at sea, and it is now strictly limited by the depth of water in the ports and docks used by the fast passenger steamships on both sides of the Atlantic. The result is that it is only a question of time, and not of a very long time with our present materials of construction and type of propulsive machinery, to find an absolute limit of speed imposed by the restriction of draught of water. The Atlantic trade is increasing at such a rapid rate that larger and swifter ships are certain to be soon called for; but much deeper harbours and docks will be required if further great increases of speed at sea are to be obtained without excessive difficulty and cost."

*

Elgar on "Fast Ocean Steamships," Min. Proc. Inst. N.A., 1893.

ARRANGEMENT AND TYPES OF ENTRANCES.

235

Commenting on and emphasising this statement in 1898, Dr. Elmer Corthell* added the following rider :

"It may be stated as a fact, palpable and undoubted, that no port of the world will, in the near future, be classed or used as a first-class port which will not readily admit steamers drawing at least 30 feet of water. This means 35 feet in the entrance channels through sea-bars, 32 feet in river channels and other entrance approaches, and 31 feet in harbours, basins, and along the quays and wharves."

The length to be given to an entrance will depend upon its arrangement, either as a lock with two pairs of gates, or as a simple entrance with one pair. In the latter case, apart from the wing walls adopted for entrances pointing amid stream, the length need not be more than will accommodate the gates and their side recesses. The length to be given to a lock entrance will, of course, be governed by the length of vessel which the lock is intended to receive. The largest lock on the Thames is the Tilbury entrance lock, 700 feet long, followed by the northern entrance lock of the Albert Dock, 550 feet long. The largest lock at Liverpool is 602 feet long; at Barry there is a lock 647 feet in length; at Barrow, 700 feet; and at Cardiff, 800 feet. This last represents the maximum length yet obtained. The new lock at Bremerhaven is 705 feet long. Swansea has an 800-feet lock in hand.

Arrangement and Types of Entrances.--Following local dispositions and requirements, there are three varieties of dock entrance, which are used either singly or in combination, viz

(1) A simple entrance, provided with one pair of ebb-gates.

(2) A lock, with at least two pairs of ebb-gates.

(3) A half-tide basin, intervening between the river and the dock and separated from each by a pair of gates.

Referring to these seriatim, it may be remarked that (1) a simple entrance is only available for navigation at or about the time of high water. Where the rise and fall in the tide is sufficient to necessitate the use of gates, the working period will generally be confined to a period of three hours, or less, in each tide. Furthermore, a single pair of gates is but inadequate provision against contingencies. Should an accident by any means happen to the gates so that they could not be closed, a very grave risk would be incurred. The unexpected running dry of a dock would probably cause irreparable damage to the shipping berthed within it.

(2) A lock offers additional facilities for the docking of vessels, since it can discharge its functions for some time after the water within the dock has been impounded; to be precise, as long as there is sufficient depth of water on the outer sill to admit of boats entering the lock. It is a particularly useful arrangement when the dock is frequented by barges, lighters, and other small craft; and its value is enhanced by dividing the lock, by

* Corthell on "Maritime Commerce," Min. Proc. American Association for the Advancement of Science, vol. xlvii.

means of a pair of intermediate gates, into two sections or lengths, so that it can be accommodated to the reception of large or small vessels, as the case may be, with the minimum expenditure of water during the process. The quantity of water withdrawn from the dock will be a matter for consideration if the operation of locking be very prolonged. The following table, modified from one in Rankine's work on Civil Engineering, shows the results of lockage under various conditions.

Let L denote a lockful of water-that is, the volume contained in the lock chamber, between the upper and lower water levels; let B denote the volume displaced by a boat.

Against the advantages afforded by the use of a lock have to be set the greatly increased cost of construction over that of a simple entrance and the additional space required. The projection of the inner end of a lock into the dock itself is a plan which, though often adopted, is attended by a decrease in the utilisable length of quay and in the convenience of berthing.

In large ports, the combination of a simple entrance with one or more locks is no uncommon feature. The former is used for docking large ships during the period in which there is free communication between the dock and the river; the latter, which are often in two widths, are brought into active service when the entrance is closed, or they may be utilised contemporaneously as subsidiary entrances. At Barry there is a single entrance, 80 feet in width, and a lock adjoining, 647 feet by 60 feet. The recently constructed entrances, at the north end of the Liverpool dock system, comprise an entrance,* 100 feet wide, an 80-foot lock, 130 feet long, and a 40-foot lock, 165 feet long. These are all parallel in direction, pointing up-stream, but at Kidderpur docks, advantage has been taken of a bend in the waterway to arrange a lock, 400 feet by 60 feet, in an up-stream

*This entrance is, strictly speaking, a lock, being provided with two pairs of ebbgates; but it is rarely, if ever, used as such, the chamber being only 130 feet long, and the provision of two pairs of gates is really a safeguard against the contingencies previously referred to.

MAINTENANCE OF FAIRWAY.

237

direction, and an 80-foot entrance pointing down-stream. Ships docked before high water, anchor above the upper entrance, and, when the gates can be opened, are breasted in alongside the jetty-head. The lower entrance is intended for the use of vessels which cannot arrive before high water; it is also required during freshets in the rainy season, when the current in the river is always down-stream.

In connection with parallel entrances it has been noted, in the Mersey, that during the time in which they are open, a circulating current has been set up, the water entering through one passage and making its exit by the other, and this quite regardless of any change in the tide.

At the entrance to the Manchester Ship Canal there are three parallel locks-30 feet by 150 feet, 50 feet by 350 feet, and 80 feet by 600 feet respectively.

(3) Half-tide basins, which are practically locks on a very large scale, are said to be due to the initiative of the late Mr. Jesse Hartley. They differ only from locks in regard to their irregular shape and great size. The gates of the dock proper are closed at, or soon after, high water, whereas the gates of the half-tide basin are kept open, as the name implies, for several hours afterwards, so that belated vessels can enter as long as there is sufficient depth of water over the outer sill which, of course, is necessarily lower than that of the inner dock. Vessels may remain in the half-tide dock until the ensuing flood tide and discharge part of their cargo there, or, if it be desirable to establish immediate communication with the inner dock, this can be done by pumping water into the half tide dock from some external supply, usually the river itself. To equalise the level by running down the water in the inner dock would generally prove to be too wasteful of water, unless the latter were relatively much larger than the half-tide basin. This last condition may, of course, be fulfilled by grouping several inner docks together. The Sandon half-tide dock at Liverpool has an area of 14 acres, and is in direct communication with the Sandon Dock (10 acres), the Huskisson Dock and branches (36 acres), the Wellington Dock (8 acres), and the Bramley Moore Dock (10 acres)—64 acres in all.* The North Dock (13 acres) at Swansea is approached by two half-tide basins, one at each end, with areas of 2 and 1 acres, respectively. At Sunderland there is a half-tide basin of 2 acres, acting as a vestibule to the Hudson Docks, of over 40 acres in extent.

Maintenance of Fairway.-The absolute necessity for a sufficient and continuous depth of water in the channel leading to a dock entrance is selfevident. The tendency, which the channel has, to become silted up, must be checked by some corrective agency, either natural or artificial. The natural means would be the utilisation of some beneficial current. Where this is impracticable, recourse must be had to sluicing, scouring, scraping, or dredging. Sluicing. This method consists in forming an aqueduct or culvert in the side walls of an entrance, communicating with the dock at its inner end, * And indirectly with others, the total impounded area being over 100 acres.

and branching into a series of outlets, discharging as low as possible, at convenient intervals along the channel frontage. During the lowest period of ebb-tide, water from the dock is allowed to run off through these culverts and the velocity, which it possesses in consequence of the head of water within the dock, enables it to stir up and remove the mud in front of the outlets. The quantity of water run off is controlled by a penstock, or paddle, near the entrance of the culvert, and, in addition to this, other pe stocks are often provided, one at each outlet in order to regulate the number of exits, for it may often be desirable to concentrate the whole discharge at a few points in order to obtain the maximum effect. Where this system is adopted, it is very essential to provide a masonry or concrete apron in front of the wall, otherwise there will be a decided risk of the wall becoming undermined. For the same reason the discharge should be perfectly horizontal, as any downward inclination causes the water to act the part of an excavator. The ground in such cases is ploughed up, and the excavated material is deposited a short distance away as soon as the current slackens, in such a manner as to form a ridge, which, being out of range of the sluice, is very dangerous, and can only be removed by dredging.

This tendency to excavate below the toe of a wall is one of the drawbacks of a mural sluice; another is that its effective action is restricted to a very small area immediately in front of the opening, so that it lowers the sides of the channel at the expense of the middle of the bed. A third objection lies in the fact that the formation of numerous outlets at the base of the wall weakens the wall at the locus of greatest intensity of pressure. A fourth objection is the very serious loss of head due to friction and bends, whereby the force of the discharge is materially diminished.

Accordingly, it is not surprising that the alternative method of sluicing through apertures in the dock gates has been adopted in many cases. There is an absence of skin friction, there are no bends, and the only loss of head is that due to discharge through a thin orifice, which is much less than the loss due to friction in a long conduit. Furthermore, by this means a large body of water is discharged along the axis of the channel, the bed of which is thus kept clear without endangering the stability of the wing walls. On the other hand, the provision of sluice valves and gear adds considerably to the weight of the gates and entails greater strength in their structure.

Velocity of Efflux from Sluices.-The velocity of efflux, from which the scouring effect of a sluicing current can be gauged, is calculated from formulæ based upon the following principles :

The theoretical velocity of a liquid issuing from an outlet under a given head or charge, considered without reference to friction, is the same as that acquired by a solid particle in falling freely from a height equal to the head-i.e.,

v2 2 g'