ces holes through
it and escapes through the discharging pipe, carrying with it all the
liquid and thin material in the tanks. The cost of working the
Millwall Dock dredger is given by Mr Duckham at 1.75d. per cub. yd. of
mud lifted, conveyed and deposited on land 450 ft. from the
water-side, for working expenses only. This dredger is believed to be
the first machine constructed with a traversing ladder, as suggested
by Captain Gibson when dock-master of the Millwall Docks.

_Blasting combined with Dredging._--In some cases it has been found that
the bottom is too hard to be dredged until it has been to some extent
loosened and broken up. Thus at Newry, John Rennie, after blasting the
bottom in a depth of from 6 to 8 ft. at low water, removed the material
by dredging at an expense of from 4s. to 5s. per cub. yd. The same
process was adopted by Messrs Stevenson at the bar of the Erne at
Ballyshannon, where, in a situation exposed to a heavy sea, large
quantities of boulder stones were blasted, and afterwards raised by a
dredger worked by hand at a cost of 10s. 6d. per cub. yd. Sir William
Cubitt also largely employed blasting in connexion with dredging on the
Severn (see _Proc. Inst. C.E._ vol. iv. p. 362). The cost of blasting
and dredging the marl beds is given as being 4s. per cub. yd. A
combination of blasting and dredging was employed in 1875 by John Fowler
of Stockton at the river Tees. The chief novelty was in the barge upon
which the machinery was fixed. It was 58 ft. by 28 ft. by 4 ft., and had
eight legs which were let down when the barge was in position. The legs
were then fixed to the barge, so that on the tide falling it became a
fixed platform from which the drilling was done. Holes were bored and
charged, and when the tide rose the legs were heaved up and the barge
removed, after which the shots were discharged. There were 24 boring
tubes on the barge, and that was the limit which could at any time be
done in one tide. The area over which the blasting was done measured 500
yds. in length by 200 in breadth, a small part being uncovered at low
water. The depth obtained in mid-channel was 14 ft. at low water, the
average depth of rock blasted being about 4 ft. 6 in. The holes, which
were bored with the diamond drill, varied in depth from 7 to 9 ft., the
distance between them being 10 ft. Dynamite in tin canisters fired by
patent fuse was used as the explosive, the charges being 2 lb. and
under.