er water, raising them to the surface and depositing them
in barges, or delivering them through a shoot, a longitudinal conveyor,
or pipes, to the place where it is desired to deposit them. It has long
been useful in works of marine and hydraulic engineering, and has been
brought in modern times to a state of high perfection.

The employment of dredging plant and the selection of special appliances
to be used in different localities and in varying circumstances require
the exercise of sound judgment on the part of the engineer. In rivers
and estuaries where the bottom is composed of light soils, and where the
scour of the tide can be governed by training walls and other works
constructed at reasonable expense, so as to keep the channel clear
without dredging, it is manifest that dredging machinery with its large
cost for working expenses and for annual upkeep should be as far as
possible avoided. On the other hand, where the bottom consists of clay,
rock or other hard substances, dredging must, in the first instance at
any rate, be employed to deepen and widen the channel which it is sought
to improve. In some instances, such as the river Mississippi, a deep
channel has for many years been maintained by jetties, with occasional
resort to dredging to preserve the required channel section and to
hasten its enlargement. The bar of the river Mersey is 11 m. from land,
and the cost of training works would be so great as to forbid their
construction; but, by a capital expenditure of L120,000 and an annual
expense of L20,000 for three years, the depth of water over the bar at
low tide has been increased by dredging from 11 ft. to 27 ft., the
channel being 1500 ft. wide.

_"Bag and Spoon" Dredger._--The first employment of machinery for
dredging is, like the discovery of the canal lock, claimed by Holland
and Italy, in both of which countries it is believed to have been in use
before it was introduced into Britain. The Dutch, at an early period,
used what is termed the "bag and spoon" dredger for cleansing their
canals. The "spoon" consisted of a ring of iron about 2 ft. in diameter
flattened and steeled for about a third of its circumference and having
a bag of strong leather attached to it by leathern thongs. The ring and
bag were fixed to a pole which was lowered to the bottom from the side
of a barge moored in the canal or river. The "spoon" was then dragged
along the bottom by a rope made fast to the iron ring actuated by