ses. A dredger of this kind and power would only work in ballast, mud
or other soft material, but the machine was gradually improved and
increased in capacity and power by different manufacturers until it
became a very efficient machine in skilful hands, excavating and raising
material from depths of 5 ft. to 60 ft. of water at a cost not very
different from, and in many cases less than, that at which the same work
could be performed on land. With the powerful dredgers now constructed,
almost all materials, except solid rock or very large boulders, can be
dredged with ease. Loose gravel is perhaps the most favourable material
to work in, but a powerful dredger will readily break up and raise
indurated beds of gravel, clay and boulders, and has even found its way
through the surface of soft rock, though it will not penetrate very far
into it. In some cases steel diggers alternating with the buckets on the
bucket frame have been successfully employed. The construction of large
steam dredgers is now carried on by many engineering firms. The main
feature of the machine is the bucket ladder which is hung at the top end
by eye straps to the frame of the vessel, and at the lower end by a
chain reived in purchase blocks and connected to the hoisting gear, so
that the ladder may be raised and lowered to suit the varying depths of
water in which the dredger works. The upper tumbler for working the
bucket chain is generally square or pentagonal in form and made of steel
with loose steel wearing pieces securely bolted to it. The tumbler is
securely keyed to the steel shaft which is connected by gearing and
shafting to the steam engine, a friction block being inserted at a
convenient point to prevent breakage should any hidden obstacle causing
unusual strain be met with in the path of the buckets. The lower tumbler
is similar in construction to the upper tumbler, but is usually
pentagonal or hexagonal in shape. The buckets are generally made with
steel backs to which the plating of the buckets is riveted; the cutting
edge of the buckets consists of a strong steel bar suitably shaped and
riveted to the body. The intermediate links are made of hammered iron or
steel with removable steel bushes to take the wear of the connecting
pins, which are also of steel. The hoisting gear may be driven either
from the main engine by frictional gearing or by an independent set of
engines. Six anchors and chains worked by powerful steam crabs are
prov