Ladder The ladder dredge oper ates an endless chain to which many buckets are attached, the chain traveling around a ladder-shaped frame, hinged to the super structure of the vessel at the top. The lower end of the ladder may be dropped down until it touches the bottom, and the chain of buckets is then run at high speed, scooping up the material of the bottom as the dredge moves forward —usually under its own motive power. The buckets have a capacity of about one cubic yard and are raised at a speed of 16 per minute. The dredged material is carried to the top of the ladder and is there dumped into chutes which discharge it into scows alongside; or it may go into hoppers constructed in the hull of the boat itself. Another modification for canal work provides lon* discharge troughs by which the material is delivered on the canal banks. The ladder dredge is extensively used in mining operations in the streams of Alaska, California, South America, Siberia and South Africa, digging up the gold-bearing sand and gravel from the creek bottoms and landing it on the dredge where the gold is recovered and the waste returned to the stream. Tin and platinum are recovered in the same way.
Hydraulic Dredge.— The hydraulic dredge sucks up the material of the bottom by means of a powerful centrifugal pump. Where the bottom is hard a revolving cutter head is at tached to the inlet pipe of the pump and this breaks up the bottom and mixes it with the water so that it will be of the right degree of fluidity to flow readily through the pump about 15 parts of water to one part of solid material. The cutter head may be pushed ahead of the vessel on a boom dragged by a flexible suction pipe. The hydraulic dredge works well in sand, gravel, alluvial deposits and earth free from stones larger than eight inches diameter and from stumps, piling and similar obstruc tions. It has a special field in bottoms so light that the dipper and bucket dredges simply stir them up without lifting them. The discharge of the hydraulic dredge being in large part water, requires a particular method of disposal.
Sometimes a long pipe line is laid from the dredge to a point on shore where the spoil may spread out and settle into solid land as the water drains away. In this way much marsh land is filled in and made available for use, and the cost of the dredging not only recovered but a substantial profit secured. In some cases the dredge float has hoppers into which the discharge is poured, the solid matter settling and the water running out at the scuttle holes. Sea-going dredges of this type are used for harbor work. When their hoppers are full they sail 8 or 10 miles out to sea and dump them.
Universal Dredge.— This title is given to large sea-going dredges which are fitted with both the ladder with its endless chain of buckets and a great hydraulic pump and pipe line. Some of these vessels are of the dimen sions of small steamships, with hoppers capable of carrying 2,500 cubic yards of dredged material.
Scouring The scouring dredges are simply submersible harrows, scrapers and agitators used to stir up light silty bottoms and bars so that the natural current of the stream may carry the material to a point where it is not objectionable. The same result has been attained by the use of water jets and aeration with compressed air.
Dredges are employed in some cases in land excavations, a pool sufficient to float the dredge being artifically formed, water being supplied to preserve the pool level as the digging pro ceeds. This method has been successfully used in digging irrigation canals in the arid regions of the Western States. Along the Mississippi and Ohio rivers dredges are used to secure sand and gravel for commercial pur poses, the hydraulic dredge being used for sand, and the ladder dredge for gravel. Inci dentally the river channels are improved, as the operations are conducted under the direc tions of government engineers. Consult Fowler, C. E.