ead waves are not more than 3 ft. high and the cross waves not more
than 1-1/2 ft. high. The dredgings are taken 2 m. to sea, and the price
for dredging and depositing from 800,000 to 900,000 cub. metres in 5
or 6 years was 7.25d. per cub. yd. The contractor offered to do the
work at 4.625d. per cub. yd. on condition of being allowed to work
either at Calais or Boulogne, as the weather might permit. Sand-pump
dredging has also been extensively carried out at the mouth of the
ports of Amsterdam, Rotterdam and on the north coast of France by sand
dredgers constructed by Messrs L. Smit & Son and G. & K. Smit. The
largest dredger, the "Amsterdam," is 141 ft. by 27 ft. by 10 ft. 8
in., and has engines of 190 i.h.p. The hopper capacity is 10,600 cub.
ft., and the vessel can carry 600 tons of dredgings. The pump fan is 6
ft. 3 in. in diameter by 10 in. wide, the plates being of wrought
iron, and makes 130 revolutions a minute. The pump can raise 230 cub.
ft. a minute from a depth of 33 ft., which, taking the proportion of 1
of sand to 7 of water, gives a delivery of 29 cub. ft. of sand per
minute. The hopper containing 10,600 cub. ft. was under favourable
circumstances filled in 40 minutes. The vessels are excellent sea
boats.

_Combined Bucket-Ladder and Sand-Pump Dredgers._--Bucket ladders and
sand pumps have also been fitted to the same dredger. A successful
example of this practice is furnished by the hopper dredger "Percy
Sanderson" (Plate I. fig. 8), constructed under the direction of Sir C.
A. Hartley, engineer of the Danube Commission for the deepening of the
river Danube and the Sulina bar. This dredger is 220 ft. by 40 ft. by 17
ft. 2 in., and has a hopper capacity for 1250 tons of dredgings. The
buckets have each a capacity of 25 cub. ft., and are able to raise 1000
tons of ordinary material per hour. The suction pump, which is driven by
an independent set of triple expansion engines, is capable of raising
700 tons of sand per hour, and of dredging to a depth of 35 ft. below
the water-line. The lower end of the suction pipe is controlled by
special steam appliances by which the pipe can be brought entirely
inboard. The "Percy Sanderson" raises and deposits on an average 5000
tons of material per day.

_Grab Dredgers._--The grab dredger was stated by Sir Benjamin Baker
(_Proc. Inst. C.E._ vol. 113, p. 38) to have been invented by Gouffe in
1703, and was worked by two ropes and a b