The main difficulty in drainage work is to provide a good footing for the machine which must necessarily be very heavy. Broad caterpillar tread surfaces are about the only means of locomotion. In very soft swampy regions dredges have to be used. Dredges are merely floating excavators. In them the problem of support and locomotion disappear and there are no limitations of size and weight to be considered. Floating excavators are therefore much more powerful than excavators that run on land. The ordinary dipper dredge is merely a floating steam shovel with a much longer dipper handle and much larger dipper. When work on the slides of the Panama Canal had proceeded far enough to admit of using dredges in place of steam shovels material progress was made. Two enormous dipper dredges were built, each provided with a dipper that had a capacity of fifteen cubic yards or about twenty tons at a single lift. The dipper was big enough to hold thirty men. The dipper handle was seventy-two feet long and it could reach down to a depth of fifty feet. A smaller dipper of ten cubic yards capacity was also provided. The power of these huge dredges was illustrated when one of them, while using its ten-yard bucket, picked up an enormous bowlder weighing forty tons. It was much bigger than the dipper that had raised it and it was far too big to be placed on the mud scow that was receiving the spoil brought up by the dredge. Had it been rolled off on the scow it would have crashed right through the bottom of the boat and so the big rock had to be drilled and broken up on the dipper before it could be dumped into the scow. For work on a soft bottom, grab buckets are used in place of dippers. These may be made of two scoops as in the clam-shell bucket or of four leaves as in the orange-peel bucket. No rigid handle is provided for these scoops. They are spread open as they are lowered into the water and on hauling in the hoisting cable, the scoops or leaves come together biting into the bottom and lifting up a load of mud. Grab buckets are largely used for picking up coal, ore, gravel, etc. The equivalent of the chain trench-digging machine is found in the ladder dredge. A ladder is hung over the stern of the dredge and carries a chain of buckets which dig into the bottom and bring up the mud or sand. GOLD MINING WITH A DREDGE It is a dredge of this type that is put to the peculiar task of gold mining. The dredge eats its way into gold-bearing sands, the material passing through a system of separators which extract the gold and then being discharged at the rear of the dredge. The dredge floats in a pool of water that it carries with it, for as it excavates ahead it builds up sand banks behind. In this way it may travel far from the river from which it first started. [Illustration: DIGGING A TRENCH WITH A CHAIN TYPE MACHINE] [Illustration: MULTIPLE PLOWING ON A WESTERN WHEAT FIELD--TURNING FIFTY FURROWS AT ONCE] SUCTION DREDGES