TRANSACTIONS OF SECTION G. 643 
by Mr. Scamp, Deputy Director of the Admiralty Works. This caisson had a 
rectangular section, and consisted of six horizontal divisions or counterparts, of 
which two at the bottom formed the air chamber intended to float the caisson a few 
inches above the cill, so that it might be drawn into a recess out of the way of 
passing vessels. The next compartments were open to each other, and had a sluice 
valve on each side to admit water as ballast, to retain the structure in equilibrium, 
and to balance its floating power. The upper or sixth compartment formed a 
tank, capable of containing 70 tons of water, supplied from the main by a hose pipe, 
and was used for sinking the caisson into its place. 
A few years later (1858) another form of caisson on a new plan was adopted 
in the formation of the Victoria Docks, London, to act instead of a coffer-dam, 
which in the circumstances would have been costly, and have caused loss of time 
in construction. This caisson, which was made of wrought-iron plates, was rec- 
tangular in side elevation, the heel-posts being vertical, and shaped like those of 
gates, so as to fit into a hollow quoin, as into a kind of rebate, Its height was 3h 
feet, and its breadth 80 feet. Its curvature was not so great as that of the gates, 
having a rise, or versed sine, of only 8 feet. 
Caissons which slide into a cut in the wall of the entrances at right angles with 
the waterway have also been used successfully. Their chief advantage is that they 
can be moved in less time than floating caissons. 
There has been some controversy as to the relative advantage of caissons and 
gates for closing the entrance to docks. The former seem to be in favour in the 
Government docks; and at the Portsmouth Dockyard extension caissons were 
exclusively employed. Where a road has to be provided for, probably a caisson is 
not more expensive than a pair of gates and a swing-bridge; but it cannot be so easy 
or so quick to handle, especiaily since the introduction of hydraulic machinery for 
opening and closing dock gates. 
One of the most important operations in connection with shipping is the re- 
pairing, cleaning, and painting of ships. For this purpose graving docks, from 
which the water was removed after the vessel had entered, were and continue to 
be mostly employed. But during the lifting of the tubes of the Britannia Bridge 
into place with what were then called hydraulic presses, it occurred to Mr, Edwin 
Clark that similar means might be used to lift a vessel out of the water and place it 
in a position to be dealt with similarly to a construction on dry land. Floating 
docks consisting of pontoons which lifted the vessel out of the water have been 
used in this country, and more extensively in America, for this purpose ; and at 
San Francisco and Philadelphia a dock was constructed of pontoons in sections 
called ‘camels, any number of which might be used according to the size of the 
vessel to be docked. Mr. Clark’s plan is quite different from these. His hydraulic 
dock consists of a number of columns arranged in two parallel rows, in which 
columns are placed the hydraulic lifting power. Between these two rows of 
columns extends a frame or cradle, over which the ship is drawn in the water. 
When the ship is in position the hydraulic lifts are set to work, and they raise the 
cradle first to the bottom of the ship, which, being properly secured, is then lifted 
with the cradle clear of the water. There is no difficulty whatever in the manage- 
ment of this form of dock, and it has been perfectly successful ; its chief recom- 
mendation being that any area of shallow water can be made available for docking 
large vessels, and that it is especially valuable in tideless seas. 
Among the many mechanical appliances for saving labour on railways and 
docks, the machinery for shipping coal is remarkable; the bulk, weight, and low 
price of coal render every item of saving in transport relatively important. It is 
commercially important also that the coal in the different stages of transport from 
the pit to the distant consumer should be broken as little as possible, and a good 
deal of attention has been given to contrivances to secure these ends. On the 
Tyne, coals were brought down to the river on the tramways and put into small 
barges called keels, holding about twenty tons, from which they were shovelled into 
the colliers through a porthole; or where the collier could be brought to the river 
bank, the coal was turned through spouts direct from the colliery waggon into the 
ship, There was no arrangement for meeting the difference of level caused by the 
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