The rock is oolite shale of variable hardness, and the average
time occupied in drilling holes 5 ft. deep was 12 minutes. The dredger
raised the blasted rock. The cost for blasting, lifting and discharging
at sea was about 4s. per cub. yd., including interest on dredging and
other plant employed. The dredger sometimes worked a face of blasted
material of from 7 to 8 ft. The quantity blasted was 110,000 cub. yds.,
and the contract for blasting so as to be lifted by the dredger was 3s.
1d. per cub. yd. A similar plan was adopted at Blyth Harbour (see _Proc.
Inst. C.E._ vol. 81, p. 302). The cost of the explosives per cub. yd.
was 1s. 4d., of boring 1s. 9d. per cub. yd., and of dredging 3s. per
cub. yd., including repairs, but nothing for the use of plant. The whole
cost worked out at 6s. 1d. per cub. yd. on the average.

_Sand-pump Dredgers._--Perhaps the most important development which has
taken place in dredging during recent years has been the employment of
sand-pump dredgers, which are very useful for removing sandy bars where
the particular object is to remove quickly a large quantity of sand or
other soft material. They are, however, apt to make large holes, and are
therefore not fitted for positions where it is necessary to finish off
the dredging work to a uniform flat bottom, for which purpose bucket
dredgers are better adapted. Pump dredgers are, however, admirable and
economical machines for carrying out the work for which they are
specially suited.

In the discussion upon Mr J. J. Webster's paper upon
"Dredging-Appliances" (_Proc. Inst. C.E._ vol. 89) at the Institution
of Civil Engineers in 1886, Sir John Coode stated that he had first
seen sand-pump dredgers at the mouth of the Maas in Holland. The
centrifugal pump was placed against the bulkheads in the after part of
the vessel, and the sand and water were delivered into a horizontal
breeches-piece leading into two pipes running along the full length of
the hopper. The difficulty of preventing the sand from running
overboard was entirely obviated by its being propelled by the pump
through these pipes, the bottoms of which were perforated by a series
of holes. In addition, there were a few small flap-doors fixed at
intervals, by means of which the men were able to regulate the
discharge. On being tested, the craft pumped into its hopper 400 tons
of sand in 22 minutes. The coamings round the well of the hoppers were
constructed