their strength and solidity has to be on a commensurate scale. A very expensive business, wallbuilding for a tidal dock, when a considerable rise and fall has to be reckoned with. This is in the case of the tidal dock as distinguished from tidal quays. When tidal or river quays have to be provided the cost of construction is much greater. For while the open dock can be excavated in the dry, and the river entrance cut only when the dock is finished, in the case of bank quays the foundations have to be dug and the lower part of the walls built under water. This means either working under compressed air or behind costly caissons. So that it is easy to understand the great cost of the river quays at Antwerp-three times, or even five times those of the closed docks.

Then, as Antwerp knows only too well, unless the soil be such as to give a solid bed to the foundations, there is always the possibility of the walls slipping into the river. Quite recently this has actually happened, 100 metres of new quay having thus slipped, whilst for the distance of 150 metres in another place the land behind the wall sank 3 feet.

It is sometimes urged that open quays can be constructed cheaply and efficiently of timber

instead of stone or cement, and that there is a future for such structures in the Thames. But the Belgians, who are not reputed either reckless or extravagant, employ stone and concrete for their Antwerp quays, and at Glasgow also the same materials are largely used.

M. Royers, the Antwerp municipal engineer, and Mr. Alston, Chief Engineer of the Clyde Navigation Trust, have both been kind enough to justify this preference. It is explained that while timber may answer where depths are moderate and the ground good, in other cases solid work is to be preferred-work which its own solidity will serve to keep in position. Wooden supports, apart from their inadequacy where cargo-sheds, cranes, and railway-trucks have to be carried, are always liable to damage by fire, by shipping, and by worms. Moreover, wood alternately dry and wet is apt to decay, and not only is the cost of repairs a serious item, but during repair the use of the quay is interfered with. To use timber good and solid enough to do the work of stone or concrete quays would cost nearly as much as the latter and be less durable. So that in the long run the best economy will doubtless be the more solid and expensive work.

To return, however, to our comparison. A tidal dock, as it would seem, is much more costly than a closed dock, and river quays are still more costly than a tidal dock, the degree of difference in each case depending on the rise and fall of the tide. In the case of tidal docks dredged below the bottom of the river, the cost of the mud removal is prohibitive. It was stated before the Royal Commission that such docks for London would involve a dredging bill of £300,000, as against the present £30,000.

As to the advantages on the score of convenience and safety, they are all in favour of the closed dock. It is true that with the lastmentioned system there is some little loss of time in the locking process-between thirty minutes and an hour-but this is of no serious importance, and is greatly outweighed by the advantage of a berth out of the flow and out of the rise and fall of a flowing stream.

As to the random assertion that a vessel can come alongside a tidal or open quay 'at any time,' we have already seen that in this respect the lock, the tidal entrance, and the open quay are all on the same footing, and that you can no more bring a ship drawing 20 feet alongside a tidal quay when

there are only 10 feet in the river than you can bring her up to the lock entrance. A vessel of light draft, a vessel drawing no more than the low-water depth of the river, can of course come alongside an open quay-or enter or leave dockat any time, but a vessel drawing more than this must obviously wait for the necessary rise of tide before she can approach either quay or dock. Whatever the system, its working is dependent absolutely and in the same degree on the depth of the navigable approach to the system.

A

CHAPTER III

RIVERS, TIDES, AND DREDGERS

T this point we may perhaps conveniently

devote a little consideration to the subject of rivers, from the point of view of their navigation, and to tides. In our brief reference to Glasgow we have already noticed what marvels may be brought about by training-banks and dredging. The effect of a training-bank is to force the stream to follow a certain channel. The flow of a river is itself a powerful excavator and obstruction-remover. If the constant dropping of water, as the old adage tells us, will wear away a stone, much more will the constant scour of a river, artificially contracted, wear away the shingle and ballast of its bed.

In a very few years the effect of narrowing the wide and shallow Clyde was to cut out and add to the channel 7 feet of the river's bed.

When the bed happens to be rock, other means