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DOCK ENGINEERING.
The placing of pumps upon the framework of dams, though sometimes
unavoidable, is always to be deprecated. The vibration set up by the
machinery inevitably causes settlement and induces leakage.
Crânes for constructive work are mostly of the locomotive type, and
the power usually ranges from 3 to 10 tons lifting weight. The heavier
machines are fitted with two gearings, by which a light load can be lifted
speedily, or the full working load at a more moderate rate. There are
four motions—travelling, jibbing, lifting, and slewing. For raising and
depositing heavy loads within a short distance, derrick cranes may be
employed. Owing to the broader base afforded by their outlying arms,
these cranes have greater stability than the locomotive cranes, but they
lack the rapid travelling movement of the latter.
Overhead travellers, or gantries, are useful appliances for dealing with
excavation in trenches. They are built on the same principle as the
Goliath, illustrated in fig. 40, but are generally much lighter, and the
lifting power, in the generality of cases, does not exceed about 15 tons.
Skips are buckets of various forms, used for the transfer of material by
means of cranes or travellers. They hold from J to 1^ cubic yards, and